Rs 485 provides data exchange between. Physical interfaces RS485 and RS422

Tariffs

It is easy to make designs using RS-485 lose weight if you understand how to maintain good communication quality at the same time. This article covers facts, myths, and cruel jokes that you should be aware of in order to achieve this goal.

In industrial and building automation systems, a number of remote data collection devices are used that transmit and receive information through a central module that provides access to the data to users and other processors. Data loggers and readers are typical for these applications. A near-ideal data line for this is defined by the RS-485 standard, which links data acquisition devices with a twisted pair cable.

Since many of the data acquisition and storage devices in RS-485 networks are compact, self-contained battery-powered devices, measures to reduce their power consumption are necessary to control their heat generation and increase battery life. Likewise, energy savings are important for wearable devices and other applications where RS-485 is used to download data to a central processing unit.

The next section is primarily intended for those who are not familiar with RS-485.

RS-485: history and description

The RS-485 standard was jointly developed by two manufacturers' associations: the Electronics Industries Association (EIA) and the Telecommunications Industry Associastion (TIA). The EIA once labeled all of its standards with the prefix "RS" (Recommended Standard). Many engineers continue to use this designation, but the EIA / TIA has formally replaced "RS" with "EIA / TIA" to make it easier to identify the origin of their standards. Today, various extensions to the RS-485 standard cover a wide variety of applications.

RS-485 and RS-422 have a lot in common and are therefore often confused. Table 1 compares them. RS-485, which defines bi-directional half-duplex communication, is the only EIA / TIA standard to allow multiple receivers and drivers in bus configurations. EIA / TIA-422, on the other hand, defines a single, multi-receiver, unidirectional driver. RS-485 elements are backward compatible and interchangeable with their RS-422 counterparts, however RS-422 drivers should not be used in RS-485 based systems as they cannot relinquish bus control.

Table 1. Standards RS-485 and RS-422

	RS-422	RS-485
Working hours	Differential	Differential
Tx and Rx allowed	1 Tx, 10 Rx	32 Tx, 32 Rx
Maximum cable length	1200 m	1200 m
Maximum baud rate	10 Mbps	10 Mbps
Driver minimum output range	± 2V	± 1.5V
Driver maximum output range	± 5V	± 5V
Maximum driver short-circuit current	150 mA	250 mA
Load resistance Tx	100 ohm	54 Ohm
Rx input sensitivity	± 200 mV	± 200 mV
Maximum input impedance Rx	4 kΩ	12 kΩ
Rx input voltage range	± 7V	-7 V to +12 V
Logic-one level Rx	\u003e 200 mV	\u003e 200 mV
Rx logic zero level	< 200 мВ	< 200 мВ

ESD protection

Differential signal transmission in RS-485 and RS-422 based systems ensures reliable data transmission in the presence of noise, and the differential inputs of their receivers can also suppress significant common-mode voltages. However, additional measures must be taken to protect against the significantly higher voltage levels commonly associated with electrostatic discharge (ESD).

The charged capacity of the human body allows a person to destroy an integrated circuit by simply touching it. Such contact can easily occur when laying and connecting the interface cable. To protect against such damaging influences, MAXIM interface chips include "ESD structures". These structures protect transmitter outputs and receiver inputs in RS-485 transceivers from ESD levels up to ± 15kV.

To ensure the stated ESD protection, Maxim repeatedly tests the positive and negative power pins in 200V steps to verify the consistency of levels up to ± 15kV. Devices in this class (meeting the Human Body Model or IEC 1000-4-2 specifications) are marked with an additional "E" suffix in the product designation.

The load carrying capacity of an RS-485 / RS-422 driver is quantified in terms of a unit load, which in turn is defined as the input impedance of one standard RS-485 receiver (12kΩ). Thus, the standard RS-485 driver can drive 32 unit loads (32 parallel 12kΩ loads). However, for some RS-485 receivers, the input impedance is higher - 48 kOhm (1/4 of a unit load) or even 96 kOhm (1/8 of a unit load) - and, accordingly, 128 or 256 of these receivers can be connected to one bus at once ... You can connect any combination of receiver types as long as their parallel impedance does not exceed 32 unit loads (i.e., the total resistance is not less than 375 ohms).

Consequences of high speeds

Faster transmissions require higher slew rates at the driver output, and these in turn produce higher levels of electromagnetic interference (EMI). Some RS-485 transceivers keep EMI to a minimum by limiting their slew rates. Slower slew rates also help control reflections caused by fast transients, high data rates, or long links. The key to minimizing reflections is to use terminating resistors with a rating that matches the characteristic impedance of the cable. For conventional RS-485 cables (24AWG twisted pair), this means placing 120 ohm resistors at both ends of the communication line.

Where does all the power go?

An obvious source of power loss is the transceiver quiescent current (IQ), which is significantly reduced in modern devices. Table 2 compares the quiescent currents of low-power CMOS transceivers to the industry standard 75176.

Table 2. Comparison of leakage currents for different RS-485 transceivers

Another characteristic of the power consumption of RS-485 transceivers occurs when there is no load, the driver output is enabled, and the presence of a periodic input signal. Since open lines in RS-485 should always be avoided, the drivers “hammer” their output structures with each output switch. This short turn on of both output transistors immediately causes a surge in supply current. A large enough input capacitor dampens these surges, producing an RMS current that rises with the baud rate to its maximum value. For MAX1483 transceivers, this maximum is approximately 15 mA.

Connecting a standard RS-485 transceiver to the minimum load (one more transceiver, two terminating resistors and two protection resistors) allows you to measure the dependence of the supply current on the baud rate under more realistic conditions. Figure 2 shows ICC versus baud rate for the MAX1483 under the following conditions: standard 560 ohm, 120 ohm, and 560 ohm resistors, VCC \u003d 5V, DE \u003d / RE \\ \u003d VCC, and 300m cable.

As you can see from Figure 2, the current consumption rises to approximately 37mA even at extremely low baud rates; this is primarily caused by the addition of terminating and bias resistors. For low power applications, this should demonstrate the importance of the type of negotiation used, as well as the way to achieve fault tolerance. Fault tolerance is discussed in the next section, and a detailed description of reconciliation is available in the Bad Jokes of Negotiation.

fault tolerance

With voltages at the inputs of RS-485 receivers in the range from -200mV to + 200mV, the output state remains undefined. In other words, if the differential voltage on the RS-485 side in half-duplex configuration is 0V and none of the transceivers are leading the line (or the connection is broken), then a logical one and a logical zero at the output are equally probable. To ensure a certain state at the output under such conditions, most modern RS-485 transceivers require the installation of bias resistors: an initial high level (pullup) resistor on one line (A) and a low level (pulldown) on the other (B), like this shown in Figure 1. Historically, bias resistors in most circuits have been specified as 560 ohms, but to reduce power loss (when terminating only at one end of the link) this value can be increased to approximately 1.1 kΩ. Some designers install resistors at both ends with ratings from 1.1k to 2.2k. Here you have to find a compromise between noise immunity and power consumption.

Figure 1. Three external resistors form a termination and bias circuit for this RS-485 transceiver.

Figure 2. MAX1483 Transceiver Supply Current vs. Baud Rate.

Manufacturers of RS-485 transceivers previously eliminated the need for external bias resistors by providing internal positive bias resistors at the receiver inputs, but this approach was only effective in solving the open circuit problem. The positive bias resistors used in these pseudo failsafe receivers were too weak to level the receiver output on the matched bus. Other attempts to avoid the use of external resistors by changing the receiver thresholds to 0V and -0.5V violated the RS-485 specification.

Maxim's MAX3080 and MAX3471 family of transceivers solved both of these problems by defining a precise threshold range of -50mV to -200mV, thus eliminating the need for bias resistors while maintaining full compliance with the RS-485 standard. These ICs ensure that a 0V input to the receiver will cause the output to go high. Moreover, this design guarantees a known output state of the receiver for open and closed line conditions.

As shown in Table 2, transceivers differ greatly in their quiescent current values. Therefore, the first step in conserving power should be choosing a low power device such as the MAX3471 (2.8 μA with driver disabled, up to 64 Kbps). Since power consumption increases significantly during data transmission, another goal is to minimize driver runtime by transmitting short telegrams with long waiting periods in between. Table 3 shows the structure of a typical serial telegram.

Table 3. Serial telegram

An RS-485 based system using receivers in one unit load (up to 32 addressable devices) can, for example, have the following bits: 5 address bits, 8 data bits, start bits (all frames), stop bits (all frames), parity bits (optional), and CRC bits (optional). The minimum telegram length for this configuration is 20 bits. For secure transmissions, you must send additional information such as data size, sender address and direction, which will increase the telegram length to 255 bytes (2040 bits).

This change in telegram length, with a structure defined by standards such as X.25, provides data reliability at the expense of increased bus time and power consumption. For example, transferring 20 bits at 200 kbps would take 100 µs. Using the MAX1483 to send data at 200Kbps per second, the average current is

(100 μs * 53 mA + (1 s - 100 μs) * 20 μA) / 1 s \u003d 25.3 μA

When the transceiver is in idle mode, its driver should be disabled to minimize power consumption. Table 4 shows the effect of telegram length on the power consumption of a single MAX1483 driver that operates with specific interruptions between transmissions. Using shutdown mode can further reduce power consumption in a system that uses polling technology at fixed intervals or longer, deterministic gaps between transmissions.

Table 4. Relationship between telegram length and current consumption when using the MAX1483 driver

In addition to these software considerations, hardware offers many places for power improvement. Figure 3 compares the currents consumed by various transceivers when transmitting a square wave signal over a 300-meter cable with active drivers and receivers. The 75ALS176 and MAX1483 use a standard 560Ω / 120Ω / 560Ω termination network at both ends of the link, while the "true failsafe" devices (MAX3088 and MAX3471) have only 120Ω termination resistors at both ends of the bus ... At 20 Kbps, the consumption currents range from 12.2mA (MAX3471 with VCC \u003d 3.3V) to 70mA (75ALS176). Thus, a significant reduction in power consumption occurs immediately when you select a low power device with a "true fail-safe" feature, which also eliminates the need to install bias resistors (to ground and to VCC). Ensure that the receiver of the RS-485 transceiver you choose is outputting the correct logic levels for both open and closed circuit conditions.

Figure 3. The transceiver microcircuits differ greatly in the dependence of the current consumption on the data transfer rate.

Bad jokes of alignment

As noted above, terminating resistors eliminate reflections caused by impedance mismatch, but the disadvantage is additional power dissipation. Their effect is shown in Table 5, which shows the consumption currents for various transceivers (with an active driver) for conditions without resistors, using only terminating resistors, and also a combination of terminating resistors and protective bias resistors.

Table 5. The use of terminating resistors and bias resistors increases the current consumption

	MAX1483	MAX3088	MAX3471	SN75ALS176
I VCC (no RT)	60 μA	517 μA	74 μA	22 μA
I VCC (RT \u003d 120)	24 μA	22.5 μA	19.5 μA	48 μA
I VCC (RT \u003d 560-120-560)	42 μA	N / A	N / A	70 μA

Eliminate matching

The first way to reduce power consumption is to eliminate termination resistors altogether. This option is only possible for short links and low data rates, which allow reflections to calm down even before the data is processed by the receiver. As practice shows, matching is not necessary if the signal rise time is at least four times the delay time of one-way signal propagation through the cable. The following steps use this rule to calculate the maximum allowable length of unmatched cable:

Step 1. For the cable in question, find the one-way speed of the signal, typically provided by the cable manufacturer as a percentage of the speed of light in free space (c \u003d 3x10 8 m / s). Typical value for standard PVC insulated cable (consisting of # 24 AWG twisted pair) is 203mm / ns.
Step 2... From the specification of the RS-485 transceiver, find its minimum rise time (t r min). For example, for the MAX3471 it is 750ns.
Step 3... Divide this minimum rise time by 4. For the MAX3471, we get t r min / 4 \u003d 750ns / 4 \u003d 187.5ns.
Step 4... Calculate the maximum cable length that does not require matching: 187.5ns (230mm / ns) \u003d 38m.

Thus, the MAX3471 can provide decent signal quality when transmitting and receiving at 64Kbps over a 38m cable without terminating resistors. Figure 4 demonstrates the dramatic reduction in consumption of the MAX3471 achieved when 30 meters of cable without terminating resistor is used instead of 300 meters of cable and 120 terminating resistors.

Figure 4. Terminating resistors - the main power consumer.

RC matching

At first glance, the ability of RC termination to block DC current is very promising. You will find, however, that this technique imposes certain conditions. The termination consists of a series RC network in parallel with the differential receiver inputs (A and B), as shown in Figure 5. Although R is always equal to the cable impedance (Z 0), choosing C requires some consideration. A large value of C provides a good match, allowing any signal to see an R that corresponds to Z0, but a large value also increases the driver's peak output current. Unfortunately, longer cables require higher C values. Entire articles have been devoted to determining the C rating to achieve this tradeoff. You can find detailed equations on this topic in the tutorials linked at the end of this article.

Figure 5. RC matching reduces consumption, but requires careful selection of the C value.

Average signal voltage is another important factor that is often overlooked. Unless the average signal voltage is DC balanced, the DC stair-stepping effect causes significant jitter due to an effect known as "intersymbol interference." In short, RC termination is effective at reducing power consumption, but it tends to degrade signal quality. Since RC negotiation imposes so many restrictions on its use, the best alternative in many cases is no negotiation at all.

Schottky diode matching

Schottky diodes offer an alternative matching method when high power consumption is a concern. Unlike other types of termination, Schottky diodes do not attempt to match the impedance of the bus. Instead, they simply suppress the positive and negative peaks caused by reflection. As a result, voltage changes are limited to a positive threshold voltage and zero.

The Schottky matching circuit wastes little energy because they only conduct in the presence of positive and negative surges. On the other hand, standard resistive termination (with or without bias resistors) constantly dissipates power. Figure 6 illustrates the use of Schottky diodes to combat reflections. Schottky diodes do not provide fail-safe operation, however, the threshold voltage levels chosen in the MAX308X and MAX3471 transceivers allow for fail-safe operation with this type of termination.

Figure 6. Despite the high cost, the Schottky diode matching circuit has many advantages.

The Schottky diode, the best available approximation to an ideal diode (zero forward voltage Vf, zero turn-on time tON, and zero reverse recovery time trr), is of great interest as a replacement for power-hungry terminating resistors. The disadvantage of this matching in RS-485 / RS-422 based systems is that Schottky diodes cannot suppress all reflections. Once the reflected signal decays below the forward voltage of the Schottky diode, its energy will remain unaffected by the matching diodes and will remain until it is dissipated by the cable. Whether or not this lingering disturbance is significant depends on the magnitude of the signal at the receiver inputs.

The main disadvantage of the Schottky terminator is its cost. One termination point requires two diodes. Since the RS-485 / RS-422 bus is differential, this number is multiplied by two again (Figure 6). The use of multidimensional Schottky terminators on the bus is not unusual.

Schottky diode terminators offer many advantages for RS-485 / RS-422 based systems, and energy savings are key (Figure 7). Nothing needs to be calculated as the specified cable length and data rate limits will be reached before any Schottky terminator limits. Another advantage is that multiple Schottky terminators in different taps and at the receiver inputs improve signal quality without loading the communication bus.

Figure 7. The current consumption in RS-485 systems is highly dependent on the baud rate and the type of termination.

Summarizing

When the baud rate is high and the cable is long, it is difficult to provide ultra-low power consumption in the RS-485 system (in the original "flea power" - Approx. Per.), Since it becomes necessary to install matching devices on the communication line ( terminators). In this case, true-to-noise transceivers at the receiver outputs can save energy even with terminators by eliminating the need for bias resistors. Software communication can also help reduce power consumption by placing the transceiver in a disabled state or disabling the driver when not in use.

For lower speeds and shorter cables, the difference in power consumption is huge: Transmitting data at 60 Kbps over a 30 meter cable using a standard SN75ALS176 transceiver with 120 ohm termination resistors will require 70mA of power from the power system. On the other hand, using the MAX3471 under the same conditions would require only 2.5mA from the power supply.

RS-485 and RS-422 interfaces are described in the standards ANSI EIA / TIA -485-A and EIA / TIA-422.The RS-485 interface is the most common in industrial automation. It is used by industrial Modbus networks, Profibus DP, ARCNET, BitBus, WorldFip, LON, Interbus and many non-standard networks. This is due to the fact that according to all main indicators, this interface is the best of all possible at the current level of technology development. Its main advantages are:

two-way data exchange using just one twisted pair of wires;
work with several transceivers connected to the same line, that is, the ability to organize a network;
long communication line length;
sufficiently high transmission speed.

2.3.1. Construction principles

Differential signal transmission

The RS-485 interface is based on differential transmission signal, when the voltage corresponding to the level of a logical unit or zero is measured not from "ground", but is measured as the potential difference between two transmission lines: Data + and Data - (Fig. 2.1). In this case, the voltage of each line relative to the "ground" can be arbitrary, but should not go beyond the range of -7 ... + 12 V [- TIA].

Signal receivers are differential, i.e. perceive only the difference between the voltages on the Data + and Data - lines. With a voltage difference of more than 200 mV, up to +12 V, it is considered that the value of a logical unit is set on the line, with a voltage less than -200 mV, up to -7 V - a logical zero. The differential voltage at the output of the transmitter in accordance with the standard must be at least 1.5 V, therefore, at a receiver response threshold of 200 mV, interference (including the voltage drop across the ohmic resistance of the line) can swing 1.3 V above the 200 mV level. Such a large margin is required to operate on long lines with high ohmic resistance. In fact, it is this voltage margin that determines the maximum communication line length (1200 m) at low transmission rates (less than 100 kbps).

Due to the symmetry of the lines relative to the "ground", interference is induced in them, which are close in shape and magnitude. In a receiver with a differential input, the signal is extracted by subtracting the line voltages, so that after subtraction, the interference voltage is zero. In real conditions, when there is a slight asymmetry of lines and loads, the interference is not completely suppressed, but is attenuated significantly.

Twisted pair wires are used to minimize the sensitivity of the transmission line to electromagnetic interference. The currents induced in adjacent loops due to the phenomenon of electromagnetic induction, according to the "gimbal rule", are directed towards each other and mutually compensate. The degree of compensation is determined by the quality of cable manufacture and the number of turns per unit length.

"Third" state of outputs

Figure: 2.1. Two-wire connection of three devices with RS-485 interface

The second feature of the transmitter D (D - "Driver") of the RS-485 interface is the ability to transfer the output stages to the "third" (high-resistance) state by a signal (Driver Enable) (Fig. 2.1). For this, both transistors of the transmitter output stage are turned off. The presence of the third state allows for half-duplex exchange between any two devices connected to the line, using just two wires. If in fig. 2.1 transmission is performed by the device, and reception is performed by the device, then the outputs of the transmitters are also transferred to a high-impedance state, that is, in fact, only the receivers are connected to the line, while the output resistance of the transmitters does not bypass the line.

The transfer of the interface transmitter to the third state is usually carried out by a signal RTS (Request To Send) COM port.

Four-wire interface

The RS-485 interface has two versions: two-wire and four-wire... Two-wire is used for half duplex transmission (Fig. 2.1), when information can be transmitted in both directions, but at different times. For full duplex (duplex) transmissions use four communication lines: information is transmitted in two in one direction, in the other two - in the opposite direction (Fig. 2.2).

The disadvantage of a four-wire (Fig. 2.2) circuit is the need for strict indication of the master and slaves at the system design stage, while in a two-wire circuit, any device can be both a master and a slave. The advantage of the four-wire circuit is the ability to simultaneously transmit and receive data, which is necessary when implementing some complex exchange protocols.

Echo Receive Mode

Figure: 2.2. Four-wire connection of devices with RS-485 interface

If the receiver of the transmitting node is turned on during transmission, then the transmitting node receives its own signals. This mode is called "echo pickup" and is usually set using a microswitch on the interface board. Receiving an echo sometimes used in complex transmission protocols, but more often this mode is disabled.

Grounding, galvanic isolation and lightning protection

If the RS-485 ports connected to the transmission line are located at a great distance from each other, then the potentials of their "grounds" can be very different. In this case, to eliminate the breakdown of the output stages of microcircuits transceivers (transceivers) interface should use galvanic isolation between the RS-485 port and ground. With a small potential difference of the "ground" for potential equalization, in principle, you can use a conductor, however, this method is not used in practice, since almost all commercial RS-485 interfaces are galvanically isolated (see, for example, an NL-232C converter or an interface repeater NL-485C by RealLab!).

The interface is protected against lightning by means of gas-discharge and semiconductor protection devices, see the section "Protection against interference".

2.3.2. Standard parameters

Recently, there have appeared many microcircuits for transceivers of the RS-485 interface, which have wider capabilities than those established by the standard. However, to ensure compatibility of devices with each other, it is necessary to know the parameters described in the standard (see table 2.2).

2.3.5. Eliminate line ambiguity

When the transmitters of all devices connected to the line are in the third (high impedance) state, the logical state of the line and inputs of all receivers is undefined. To eliminate this ambiguity, the receiver's non-inverting input is connected through a resistor to the power rail, and the inverting input to the ground rail. The resistor values \u200b\u200bare selected so that the voltage between the inputs becomes greater than the receiver response threshold (+200 mV).

Since these resistors turn out to be connected in parallel with the transmission line, it is necessary for the equivalent resistance at the line input to be equal to 120 ohms to ensure line matching with the interface.

For example, if the resistors used to eliminate line uncertainty have a resistance of 450 ohms each, then the line termination resistor should be 130 ohms, then the equivalent circuit resistance will be 114120 ohms. In order to find the differential line voltage in the third state of all transmitters (see Fig. 2.6), it is necessary to take into account that another 120 Ohm resistor and up to 32 receivers with an input differential resistance of 12 kOhm are connected to the opposite end of the line in the standard configuration. Then at the supply voltage (Fig. 2.6), the differential line voltage will be +272 mV, which meets the requirements of the standard.

2.3.6. Through currents

In a network based on the RS-485 interface, there may be a situation when two transmitters are turned on simultaneously. If at the same time one of them is in the state of a logical unit, and the second is in a state of logical zero, then a large "through" current flows from the power source to the ground, limited only by the low resistance of two open transistor switches. This current can damage the transistors of the output stage of the transmitter or trigger their protection circuitry.

This situation is possible not only in the event of gross errors in the software, but also in the event that the delay between the moment when one transmitter is turned off and the other is turned on is incorrectly set. The slave device should not transmit data until the transmitting device has finished transmitting. Interface repeaters must determine the beginning and end of data transmission and, in accordance with them, put the transmitter in an active or third state.

2.3.7. Cable selection

Depending on the baud rate and the required cable length, either a specially designed cable for the RS-485 interface or almost any pair of wires can be used. The cable specially designed for the RS-485 interface is twisted pair with a characteristic impedance of 120 ohms.

For good suppression of radiated and received interference, a large number of turns per unit of cable length is important, as well as the identity of the parameters of all wires.

When using non-isolated interface transceivers, in addition to the signal wires in the cable, it is necessary to provide one more twisted pair for connecting the ground circuits of the interfaces to be connected. If the interfaces are galvanically isolated, this is not necessary.

Cables can be shielded or not. Without experiment, it is very difficult to decide if a screen is needed. However, given that the cost of a shielded cable is not much higher, it is best to always use a shielded cable.

At low transmission rates and with DC current, the voltage drop across the ohmic resistance of the cable plays an important role. So, a standard cable for the RS-485 interface with a cross section of 0.35 sq. Mm has an ohmic resistance of 48.5 * 2 \u003d 97 Ohm with a length of 1 km. With a 120 Ohm terminal resistor, the cable will act as a voltage divider with a division factor of 0.55, i.e., the voltage at the cable output will be approximately 2 times less than at its input. This limits the permissible cable length for transmission rates below 100 kbps.

At higher frequencies, the allowable cable length decreases with increasing frequency (Figure 2.7) and is limited by cable loss and effect front jitter impulses. The losses are the sum of the voltage drop across the ohmic resistance of the conductors, which at high frequencies increases due to the displacement of the current to the surface (skin effect) and losses in the dielectric. For example, Belden 9501PVC has 10 dB (3.2 times) attenuation at 20 MHz and 0.4 dB (4.7%) at 100 kHz for 100 m cable lengths.

2.3.8. Expanding the limit

RS-485 standard allows connecting up to 32 receivers to one transmitter. This value is limited by the power of the output stage of the transmitter with a standard input impedance of the receiver of 12 kΩ. The number of loads (receivers) can be increased by using more powerful transmitters, receivers with higher input impedance, and intermediate signal repeaters (interface repeaters). All of these methods are used in practice when necessary, although they are outside the scope of the standard.

In some cases, it is required to connect devices at a distance of more than 1200 m or connect more than 32 devices to one network. This can be done using repeaters ( repeaters , repeaters) interface. The repeater is installed between two segments of the transmission line, receives the signal of one segment, restores the edges of the pulses and transmits it using a standard transmitter to the second segment (Fig. 2.5). Such repeaters are usually bi-directional and galvanically isolated. An example is the NL-485C repeater from RealLab! ... Each repeater allows you to add 31 standard devices to the line and increase the line length by 1200 m.

A common method for increasing the number of line loads is to use receivers with higher impedance inputs than the EIA / TIA-485 standard (12 kΩ). For example, with a receiver input impedance of 24 kΩ, 64 receivers can be connected to a standard transmitter. Transceiver microcircuits for the RS-485 interface are already being produced with the ability to connect 64, 128 and 256 receivers in one network segment (www.analog.com/RS485). Note that an increase in the number of loads by increasing the input impedance of the receivers leads to a decrease in the power of the signal transmitted through the line, and, as a consequence, to a decrease in noise immunity.

2.3.9. RS-232 and RS-422 interfaces

The RS-422 interface is used much less frequently than RS-485 and, as a rule, not to create a network, but to connect two devices over a long distance (up to 1200 m), since the interface RS Fig. 2.9. Connection of two RS-232 / RS-422 interface converter modules Differential

Differential

Maximum number of receivers

Maximum cable length

Maximum transfer rate

30 Mbps **

Output common mode voltage

Line voltage under load

Load impedance

Leakage current in "third" state

Acceptable range of signals at the receiver input

Receiver sensitivity

Receiver input impedance

Note... ** Transfer rate 30 Mbit / s is provided by modern element base, but not standard.

* EIA - The Electronic Industries Association is an association for the electronics industry. TIA - Telecommunications Industry Association - Telecommunications Industry Association. Both organizations are developing standards.

For industrial applications, wireless data lines can never completely replace wired... Among the latter, the most common and reliable is still serial interface RS -485 ... And the manufacturer of the most protected from external influences and various in configuration and degree of integration of transceivers for him, in turn, remains the companyMaxim Integrated .

Despite the growing popularity of wireless networks, the most reliable and stable communication, especially in harsh operating conditions, is provided by wired ones. Properly designed wired networks enable efficient communication in industrial applications and industrial process control systems, while providing immunity to interference, electrostatic discharge and surges. The distinctive features of the RS-485 interface have led to its widespread use in the industry.

Comparison of RS-485 and RS-422 interfaces

The RS-485 transceiver is the most common physical layer interface for implementing serial data networks for harsh environments in industrial and building management systems. This serial interface standard enables high-speed communication over a relatively long distance over a single differential line (twisted pair). The main problem of using RS-485 in industry and in automated building management systems is that electrical transients arising from fast switching of inductive loads, electrostatic discharges, as well as surge voltages, acting on the networks of automated control systems, can distort the transmitted data or lead to their failure.

Currently, there are several types of data transfer interfaces, each of which is designed for specific applications, taking into account the required set of parameters and protocol structure. Serial communication interfaces include CAN, RS-232, RS-485 / RS-422, I 2 C, I 2 S, LIN, SPI and SMBus, however RS-485 and RS-422 are still the most reliable. especially in harsh operating conditions.

RS-485 and RS-422 interfaces are similar in many ways, however, they have some significant differences that must be taken into account when designing data transmission systems. In accordance with the TIA / EIA-422 standard, the RS-422 interface is specified for industrial applications with one data bus master to which up to 10 slaves can be connected (Figure 1). It provides transmission at speeds up to 10 Mbps using twisted-pair cable, which improves noise immunity and achieves the highest possible range and data transfer rate. Typical applications for the RS-422 are process automation (chemical manufacturing, food processing, paper mills), integrated manufacturing automation (automotive and metalworking industries), ventilation and air conditioning systems, security systems, motor control and object movement control.

RS-485 provides greater flexibility by allowing multiple masters on a common bus and increasing the maximum number of devices on the bus from 10 to 32. According to the TIA / EIA-485 standard, RS-485 has more a wide common-mode voltage range (-7 ... 12 V instead of ± 7 V) and a slightly smaller differential voltage range (± 1.5 V instead of ± 2 V), which ensures a sufficient receiver signal level at maximum line load. Using the enhanced capabilities of the multidrop data bus, you can create networks of devices connected to a single RS-485 serial port. Due to its high noise immunity and multi-drop capability, RS-485 is the best serial interface for use in industrial distributed systems connected to a programmable logic controller (PLC), graphics controller (HMI) or other data acquisition controllers. Since RS-485 is an extended version of RS-422, all RS-422 devices can be connected to a bus controlled by an RS-485 master. Typical applications for RS-485 are similar to those listed above for RS-422, with the increased use of RS-485 due to its enhanced capabilities.

RS-485 is the most popular industrial interface

The TIA / EIA-485 standard allows the use of RS-485 at a distance of up to 1200 m. At shorter distances, the data transfer rate is over 40 Mbps. Using a differential signal provides the RS-485 interface with a longer range, but the baud rate decreases as the line length increases. The baud rate is also affected by the cross-sectional area of \u200b\u200bthe line wires and the number of devices connected to it. It is recommended to use RS-485 transceivers with a built-in high frequency correction function, such as the MAX3291, if you need to obtain both long range and high data transfer rates. The RS-485 interface can be used in half duplex mode using one twisted pair of wires or in full duplex mode with simultaneous transmission and reception of data, which is provided using two twisted pairs (four wires). In a multidrop configuration in half duplex, RS-485 is capable of supporting up to 32 transmitters and up to 32 receivers. However, the new generation transceiver ICs have a higher input impedance, which can reduce the receiver line load from 1/4 to 1/8 of the standard value. For example, using the MAX13448E transceiver, the number of receivers connected to the RS-485 bus can be increased to 256. With the enhanced RS-485 multidrop interface, it is possible to network multiple devices connected to a single serial port, as shown in Figure 2.

The receiver sensitivity is ± 200 mV. Therefore, to recognize one data bit, the signal levels at the receiver connection point must be greater than +200 mV for zero and less than -200 mV for unity (Figure 3). In this case, the receiver will suppress interference, the level of which is in the range of ± 200 mV. The differential line also provides effective common mode rejection. The minimum input impedance of the receiver is 12 kΩ, the output voltage of the transmitter is in the range of ± 1.5 ... ± 5 V.

Serial problems in an industrial environment

Industrial system designers face the daunting challenge of ensuring reliable operation in an electromagnetic environment that can damage equipment or disrupt digital data transmission systems. One example of such systems is the automatic control of technological equipment in an automated industrial enterprise. The controller that controls the process measures its parameters, as well as environmental parameters, and transmits commands to executive devices or generates emergency notifications. Industrial controllers are, as a rule, microprocessor devices, the architecture of which is optimized for solving the problems of a given industrial enterprise. Point-to-point data lines in such systems are subject to strong electromagnetic interference from the environment.

DC / DC converters used in industrial production operate with high input voltages and provide isolated voltages from the input to power the load. To power devices of a distributed system that do not have their own mains power supply, voltages of 24 or 48 V DC are used. The terminal load is supplied with 12 or 5 V, obtained by converting the input voltage. Systems that communicate with remote sensors or actuators require protection against transients, EMI, and ground potential.

Many companies, such as Maxim Integrated, go to great lengths to ensure that ICs for industrial applications are highly reliable and resilient to harsh electromagnetic environments. Maxim's RS-485 transceivers have built-in high-voltage ESD and surge protection and are hot-swappable without data loss on the line.

Protection of data transmission systems from adverse external influences

Enhanced ESD protection

Electrostatic discharge (ESD) occurs when two oppositely charged materials come into contact, thereby transferring static charges and generating a spark discharge. ESD often occurs when people come into contact with their surroundings. Spark discharges arising from careless handling of semiconductor devices can significantly degrade their characteristics or lead to complete destruction of the semiconductor structure. ESD can occur, for example, when replacing a cable or simply touching an I / O port and cause the port to be disabled due to the failure of one or more interface chips (Figure 4).

Such accidents can lead to significant losses, as they increase the cost of warranty repairs and are perceived by consumers as a consequence of the poor quality of the product. In industrial production, ESD is a serious problem that can cause billions of dollars in losses annually. Under real-world conditions, ESD can lead to failure of individual components, and sometimes the entire system. External diodes can be used to protect data interfaces, but some interface chips contain built-in ESD protection components and do not require additional external protection circuits. Figure 5 shows a simplified block diagram of a typical embedded ESD protection circuit. Signal line impulses are limited by diode protection at the supply voltage V CC and ground, and thus protect the interior of the circuit from damage. Currently manufactured interface chips and analog switches with built-in ESD protection generally comply with IEC 61000-4-2.

Maxim Integrated has invested heavily in the development of chips with robust built-in ESD protection and is currently the leader in RS-232 to RS-485 transceivers. These devices are designed to withstand IEC 61000-4-2 and JEDEC JS-001 ESD test pulses directly on the I / O ports. Maxim's ESD solutions are reliable, affordable, have no additional external components, and are less expensive than most peers. All interface microcircuits manufactured by this company contain built-in elements that ensure protection of each output from ESD arising during production and operation. The MAX3483AE / MAX3485AE family of transceivers protect transmitter and receiver outputs from high-voltage pulses up to ± 20 kV. At the same time, the normal mode of operation of the products is maintained, there is no need to turn off and turn on the power again. In addition, built-in ESD protections provide power-on / power-down and low power standby operation.

Overvoltage protection

In industrial applications, the inputs and outputs of RS-485 drivers are prone to failures due to surge voltages. Surge voltage parameters differ from ESD - while ESD duration is usually in the range up to 100 ns, the duration of surge voltages can be 200 μs or more. Surges can be caused by wiring errors, poor connections, damaged or defective cables, and solder droplets that can form a conductive connection between power and signal lines on a PCB or connector. Since industrial power systems use voltages in excess of 24 V, exposing standard RS-485 transceivers that are not surge protected to such voltages will damage them within minutes or even seconds. To protect against surge voltages, conventional RS-485 interface chips require expensive external devices based on discrete components. RS-485 transceivers with built-in surge protection can handle up to ± 40, ± 60, and ± 80 V common-mode data line noise. Maxim manufactures a line of RS-485 / RS-422 transceivers MAX13442E… MAX13444E that withstands DC input voltages and outputs up to ± 80 V with respect to ground. The protection elements operate regardless of the current state of the chip - whether it is on, off, or in standby mode - which makes these transceivers the most reliable in the industry, ideal for industrial applications. Maxim's transceivers survive overvoltages caused by shorted power and signal lines, wiring errors, improper plug connections, faulty cables, and improper operation.

Tolerance of receivers to undefined line conditions

An important characteristic of the RS-485 interface microcircuits is the immunity of receivers to undefined line states, which guarantees the setting of a high logic level at the receiver output when the inputs are open or closed, as well as when all transmitters connected to the line go into inactive mode (high impedance state of outputs). The problem of correct perception by the receiver of closed data line signals is solved by shifting the input signal thresholds to negative voltages of -50 and -200 mV. If the input differential voltage of the receiver V A - V B is greater than or equal to -50 mV, the output R 0 is set to a high level. If V A - V B is less than or equal to -200 mV, the output R 0 is set to a low level. When all transmitters go to an inactive state and the line is terminated, the differential input voltage of the receiver is close to zero, as a result of which the output of the receiver goes high. In this case, the margin of noise immunity at the input is 50 mV. Unlike previous generation transceivers, the -50 and -200 mV thresholds correspond to the ± 200 mV values \u200b\u200bset by the EIA / TIA-485 standard.

Hot swappable

Literature

Application note 4491, “Damage from a Lightning Bolt or a Spark – It Depends on How Tall You Are!”;
Application note 5260, “Design Considerations for a Harsh Industrial Environment”;
Application note 639, "Maxim Leads the Way in ESD Protection".

RS-485 Is the number of the standard first adopted by the Electronic Industries Association (EIA). This standard is now called TIA / EIA-485 Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems (Electrical characteristics of transmitters and receivers used in balanced digital multipoint systems).
Among the people RS-485 is the name of a popular interface used in industrial control systems to connect controllers and other equipment. The main difference between RS-485 and also widespread RS-232 is the ability to combine multiple devices.

Description of the RS-485 interface

The RS-485 interface provides data exchange between several devices over one two-wire communication line in half-duplex mode. It is widely used in industry to create process control systems.

Speed \u200b\u200band range

RS-485 provides data transfer rates up to 10 Mbps. The maximum range depends on the speed: at a speed of 10 Mbit / s, the maximum line length is 120 m, at a speed of 100 kbit / s - 1200 m.

Number of connected devices

The number of devices connected to one interface line depends on the type of transceivers used in the device. One transmitter is designed to control 32 standard receivers. Receivers with input impedance 1/2, 1/4, 1/8 of the standard are available. When using such receivers, the total number of devices can be increased by an appropriate number of times.

Protocols and connectors

The standard does not standardize the format of information frames and the exchange protocol. Most often, the same frames are used to transfer data bytes as in the RS-232 interface: start bit, data bits, parity bit (if necessary), stop bit.
Exchange protocols in most systems work on the "master" - "slave" principle. One device on the trunk is the master and initiates the exchange by sending requests to the slaves, which differ in logical addresses. One of the popular protocols is the Modbus RTU protocol.
The type of connectors and wiring are also not specified by the standard. There are DB9 connectors, terminal connectors, etc.

Connection

Connection diagram

The figure shows a local network based on the RS-485 interface, which unites several transceivers.
When connecting, you must correctly connect the signal circuits, usually called A and B. The polarity reversal is not terrible, but the device will not work. How to define nets by levels, see below.

The signal transmission medium is a cable based twisted pair.
Cable ends must be plugged terminal resistors (usually 120 ohms).
The network must be laid according to the bus topology, without branches.
Devices should be connected to the cable with wires minimum length.

Signal levels

The RS-485 interface uses a balanced (differential) signal transmission scheme. This means that the voltage levels on signal circuits A and B change in antiphase, as shown in the figure below:

The transmitter should provide a signal level of 1.5 V at maximum load (32 standard inputs and 2 terminal resistors) and no more than 6 V at idle. Voltage levels are measured differentially, one signal wire relative to the other.
In the absence of a signal, there is a small offset on the signal circuits, in the order of 200 mV, to protect the receivers from false alarms. In this case, circuit B has a positive potential relative to circuit A, which can serve as a guide when connecting a new device to a cable with unmarked wires.
On the side of the RS-485 receiver, the minimum level of the received signal must be at least 200 mV.

Distortion due to incorrect network wiring

Compliance with the above recommendations guarantees normal transmission of electrical signals to any point in the network based on the RS-485 interface. If at least one of the requirements is not met, the signal is distorted. For example, here are the oscillograms of the signal taken at the point of connection of the receiver, located 15 meters from the transmitter and 30 meters from the end of the line, with the terminating resistor turned on and off:

The following oscillogram shows the signal distortion that occurs when connected to the main terminated cable with a long 3-meter tap:

The presented oscillograms are typical for high exchange rates (1 Mbit / s and higher). However, even at lower speeds, these recommendations should not be neglected, even if "it works anyway".

When programming applications for controllers using the RS-485 interface for communication, there are a few things to consider:

Before starting the delivery of the parcel, you must turn on the transmitter. Although some sources state that transmission can be started immediately after being turned on, we recommend that you wait for a period equal to or longer than the transmission time of one frame (including start and stop bits). In this case, the receiver has time to normalize and prepare to receive the first byte of data.
After the last byte of data has been issued, you should also pause before turning off the RS-485 transmitter. This is because a serial port controller usually has two registers: a parallel input for receiving data and a shift output for serial output. The controller generates a transmission interrupt when the input register is empty, when the data has already been laid out in the shift register, but not yet issued! Therefore, there must be a pause from the moment of interruption until the transmitter is turned off. The approximate duration of the pause is 0.5 bits longer than the frame; for an accurate calculation, you should carefully study the documentation for the serial port controller.
Since the transmitter and receiver of the RS-485 interface are connected to the same line, its own receiver will "hear" the transmission of its own transmitter. Sometimes, in systems with random access to the line, this property is used to check that there are no "collisions" between two transmitters. In master-slave systems, it is best to simply close interrupts from the receiver during transmission.

Let's consider how to control a frequency converter using the rs 485 protocol. Let's make the spindle control automatic. For this we have:

Lathe with ET65A-800W spindle.
Frequency converter from Schneider Electric Altivar 71 plant.
RS232 / RS485 interface modernizer.
Mach3 v.3.042.029.

First, we make the configuration of the mach:

We allow work on ModBus by checking the appropriate box.

In the spindle settings in the submenu, remove unnecessary checkboxes.

Add an initialization line to the general conf menu.
To work, you need two registers in the frequency converter - this is the CMD control and the installation with a register. To make it more convenient, we select the engine speed with a setpoint.

We make a polling configuration:

Connecting elements 19200 8-N-1. Scanning at a frequency of 10 hertz in a dimensional chart. Polling is needed so that a self-diagnosis occurs in the connection, and the frequency is converted. If the exchange of the network has stopped for the size of the given break, then the frequency converter issues an error.

We fix VBA scripts:

M3
SetModOutput (0, & H0006)
SetModOutput (1,0)
DoSpinCW ()
SetModOutput (0, & H000F)

M4
SetModOutput (0, & H0006)
SetModOutput (1,0)
DoSpinCCW ()
SetModOutput (0, & H000F)

SetModOutput (0, & H0006)
SetModOutput (1, & H0000)
DoSpinStop ()

rpm \u003d GetRPM ()
SetSpinSpeed \u200b\u200b(rpm)
SetModOutput (1, rpm)

Correcting the postprocessor:

@start_tool
if only_xyz eq false
if tool_direction eq CW then
mcode \u003d 4
else; CCW
mcode \u003d 3
endif

call @gen_nb
; (‘S’spin: integer_def_f,‘ M’mcode)
(‘M’mcode)
call @gen_nb
(‘S’spin: integer_def_f)
call @gen_nb
(‘M8’)
endif
endp
We work with SolidWorks / SolidCAM.
This control method has advantages and differs from PWM converters:
- if the spindle speed is zero, then the motor is guaranteed to be turned off;
- the control program has the ability to exchange information with a frequency converter;
- real engine speed is interpreted with the frequency setting;
- good adaptability to interference (up to one kilometer) is allocated over a long distance of the communication line.

How to wire the RS-485 networks correctly?

RS-485 transfers information in digital form between objects. Data can be transmitted at 10 Mbps. RS-485 is used to send a signal over an increased length. The length and speed of data for RS-485 depends on various factors.

Cable.

RS-485 is designed as a balance system. This means that there are two wires used to transfer data.

Figure: 1. The balance system uses two wires for signal transmission.

This system is balanced as the signal on the two wires at both ends is exactly opposite. See Fig. 2.

Figure: 2. Data differing on both sides of the wires.

RS-485 must be used with twisted pair wiring.

Why use twisted pair wiring?

It is a simple pair of wires that are the same length. They are retinues together. The twisted-pair cable transmitter reduces two problems for high-speed network builders, EMI and induced interference.

Electromagnetic Radiated Interference.

The figure shows that when using pulses with large edges, there are high frequency components in the signal. Such edges are necessary for higher speeds than RS-485 can provide.

Figure: 3. Rectangular impulses.

The high frequency components of these edges with large wires lead to the emission of electromagnetic interference. The balance system uses twisted pair communication lines, reduces the effect, the emitter becomes unnecessary. The data on the wires are the same, inverse, the signals will also be equal and inverse. This makes the effect of reducing one signal due to another. This means that there is no electromagnetic radiation. But this is only a guess. The alignment of the wires provides a neutralization of radiation due to the length between the wires.

Electromagnetic induced interference.

This is the same problem, just the opposite. The connections in the RS-485 based system act as an antenna. These signals distort the desired signals, which lead to data problems. It can also reduce interference dependence. The noise of one wire is the same as that of the second wire. It is called in-phase. They suppress the noise of both wires.

Wave resistance of twisted pair.

A twisted pair has wave properties as specified by the manufacturer. RS-485 dictates that the resistor size is 120 ohms. This impedance recommendation is needed to calculate the worst load in the common mode voltage range in RS-485. The specification does not give such impedance for flexibility. If a 120 ohm cable cannot be used, then the worst case load and worst voltage range must be re-calculated to ensure that the system is working. The transmitter can only control one twisted pair, the other is not provided by the specification.

Terminating resistors.

The terminating resistor is a common resistor at one end of the cable. The size of the matching resistor is equal to the resistance of the wave cable.

Figure: 4. The matching resistors have the same resistance as the twisted pair.

If the value of the two resistors is different from the wave cable, then there will be reflection, the signal will be screwed back. Discrepancies cause reflection to make data errors.

Figure: 5. Signal received from MAX3485. The signal on the right is obtained by matching with a resistor.

It is necessary to match the greater approximation of the size of the matching resistor and waves. It does not matter where to install the terminating resistor, at both ends of the cable.

As a rule of thumb, termination resistors are placed at the ends of the cable, although it is best to make termination of both ends critical for many system designs. In one case, only one resistor is needed. This case exists in a system with a transmitter. It is located at the other end of the cable. There is no need to put a resistor at the end of the cable along with the transmitter, as the signal comes from it.

The largest number of receivers and transmitters in the network.

A typical RS-485 network consists of a receiver and a transmitter. RS-485 gives flexibility, allowing more transmitters and receivers per pair. The maximum number depends on the system load.

Ideally, transmitters and receivers will have high impedance and will not load the system. In reality, this cannot be so. The connected receiver increases the load. To help the designer of the RS-485 network find out how many devices will be added to the network, we created a load unit. Such structures are characterized by multipliers or loads.

Receiver and transmitter one at a time.

Resistor matched on the wire at the transmitter side. You can move the transmitter to the near edges of the wire, and add the transmitters to the network.

Figure: 6. RS-485 has one receiver and one transmitter.

Multiple receivers and one transmitter.

It is very important here that the length from the twisted pair is as short as possible.

Figure: 7. Network with multiple receivers and one transmitter.

Wrong networks. Inconsistent network.

Let us compare the formulation of data from the wrong network of the developed system. It was measured at points A and B. Here at the ends of a pair of matching resistors. The signal comes from the source, collides with the circuit on the cable. This leads to destruction of impedances, reflection. In an open circuit, energy goes backwards, causing distortion of the signal.

Figure: 8. The network is inconsistent. The waveform is not correct.

Terminator location is wrong.

The resistor is matched, but placed differently from the other end of the cable. The signal collides with impedance and its mismatch, is connected across a resistor. The resistance has been matched to the cable resistance. The extra cable gives misalignment and reflects the screen. Another mismatch is the other end of the cable.

Figure: 9. Network with a resistor that is incorrectly placed, its signal.

Composite cables.

The problem is with drivers that are designed to drive one twisted pair. Not every transmitter can handle 4 twisted parallel pairs. Logical minimum levels are not guaranteed. Along with a heavy load, there is a difference in impedance at the point where the cables are connected. Difference in impedance means signal reflection and distortion.

Figure: 10. Incorrect network with multiple pairs.

Long taps.

The cable is matched, the transmitter is loaded on one twisted pair. The wired segment at the connection point of the receiver is too long. Large taps have large impedance mismatches and reflect the signal. The taps are made to the shortest length.

Figure: 11. Network with a three meter coupler and the signal in total versus the signal with a small coupler.

What steps are needed to understand the rs485 control?

Search for design documentation. It is attached in printed form to the frequency manager and is relevant to him. Documents can be attached electronically on disk. You can find documentation online.
We find out the numbers of the revision, version. Our goal is the version of the program.
Study of documents for specific words.
Find the connecting cable diagram and the connector pinout.
Search for the description of Modbus registers. This is a memory stick. Registers are called variables.
Determining the type of variable addressing. Modbus has two types of different addressing, logical and physical.
Indicates a search in a direction. This is a crucial step.