In modern technology, the exchange of information between various devices is becoming increasingly important. And for this it is required to transmit data both over short distances and over significant ones, of the order of kilometers. One of these types of data transmission is communication between devices via the RS-485 interface.

Where it is necessary to transfer data via RS 485.

One of the most common use cases for communication devices is. Electricity meters connected into a single network are dispersed among cabinets, switchgear cubicles and even substations located at a considerable distance from each other. In this case, the interface is used to send data from one or more metering devices.

The system "one meter - one modem" is being actively implemented to transfer data to the services of energy sales companies from metering centers of private houses, small enterprises.

Another example: receiving data from microprocessor relay protection terminals in real time, as well as centralized access to them for the purpose of making changes. For this, the terminals are tied through the communication interface in the same way, and the data from it goes to the computer installed by the dispatcher. In the event of a protection actuation, operating personnel have the opportunity to immediately receive information about the place of action and the nature of the damage to power circuits.

But the most difficult task solved by the communication interfaces is the systems of centralized control of complex production processes - APCS. The operator of an industrial installation has a computer on the table, on the display of which he sees the current state of the process: temperatures, productivity, units turned on and off, their operating mode. And it has the ability to manage all this with an easy click of the mouse.

The computer, on the other hand, exchanges data with controllers - devices that convert commands from sensors into a language that the machine can understand, and the reverse transformation: from the language of the machine into control commands. Communication with the controller, as well as between different controllers, is carried out via communication interfaces.

The RS-232 interface is the little brother of RS 485.

It is impossible not to at least briefly mention the RS-232 interface, which is also called serial. Some laptops have a connector for the corresponding port, and some digital devices (the same relay protection terminals) are equipped with outputs for communication using RS-232.

In order to exchange information, you need to be able to transmit and receive it. For this we have a transmitter and a signal receiver. They are included in every device. Moreover, the output of the transmitter of one device (TX) is connected to the input of the receiver of another device (RX). And, accordingly, the signal moves in the opposite direction along the other conductor in a similar way.

This provides a half-duplex communication mode, that is, the receiver and transmitter can work simultaneously. Data on the RS-232 cable can move in one direction or the other at the same time.

The disadvantage of this interface is low noise immunity. This is due to the fact that the signal to the connecting cable for both reception and transmission is formed relative to the common wire - ground. Any interference, even in a shielded cable, can lead to a communication failure, loss of individual bits of information. And this is unacceptable when managing complex and expensive mechanisms, where any mistake is an accident, and a loss of communication is a long downtime.

Therefore, it is mainly used for small temporary connections of a laptop to a digital device, for example, for setting up an initial configuration or correcting errors.

Organization of the RS-485 interface.

The main difference between RS-458 and RS-232 is that all receivers and transmitters operate on one pair of wires, which is a communication line. In this case, the ground wire is not used, and the signal in the line is formed by a differential method. It is transmitted simultaneously over two wires ("A" and "B") in reverse form.

If the output of the transmitter is a logical "0", then a zero potential is given to the conductor "A". On the “B” conductor, a “not 0” signal is generated, that is, “1”. If the transmitter broadcasts "1", the opposite is true.

As a result, we get a change in the signal voltage between the two wires, which are twisted pair. Any pickup entering the cable changes the voltage relative to ground equally on both wires of the pair. But the voltage of the useful signal is formed between the wires, and therefore does not suffer at all from the potentials on them.

The order of data exchange between devices via RS-485.

All devices connected by the RS-485 interface have only two terminals: "A" and "B". For connection to a common network, these terminals are connected in a parallel circuit. For this, a chain of cables is laid from one device to another.

In this case, it becomes necessary to streamline the exchange of data between devices, setting the sequence of transmission and reception, as well as the format of the transmitted data. This is done with a special instruction called a protocol.

There are many RS-485 communication protocols, the most commonly used is Modbas. Let's briefly consider how the simplest protocol works, and what other problems have to be solved with its help.

As an example, let's take a look at a network in which one device collects data from multiple data sources. It can be a modem and a group of electricity meters. In order to know from which counter the data will go, each transceiver is assigned a number that is unique for the given network. The number is also assigned to the modem transceiver.

When it's time to collect data on power consumption, the modem generates a request. First, a start pulse is transmitted, according to which all devices understand that a code word will come now - a message from a sequence of zeros and ones. In it, the first bits will correspond to the subscriber's number in the network, the rest will be data, for example, a command to transmit the required information.

All devices receive the message and compare the called party's number with their own. If they match, the command sent as part of the request is executed. If not, the device ignores its text and does nothing.

At the same time, in many protocols, a confirmation is sent back that the command is accepted for execution or completed. If there is no answer, the transmitting device can repeat the request a certain number of times. If there is no reaction, information about the error is generated, associated with a malfunction of the communication channel with the silent subscriber.

The answer may not follow, not only in the event of a breakdown. In the presence of strong interference in the communication channel, which nevertheless penetrates there, the teams may not reach their destination. They are also distorted and not recognized correctly.

Incorrect execution of the command cannot be allowed, therefore, deliberately redundant information is introduced into these messages - a checksum. It is calculated according to a certain law prescribed in the protocol on the transmitting side. At the reception, the checksum is calculated according to the same principle and compared with the transmitted one. If they match, the reception is considered successful and the command is executed. If not, the device sends an error message to the sending side.

Requirements for cable connections.

Twisted pair cables are used to connect devices with RS-485 interface. Although one pair of wires is enough to transmit this data, cables with at least two are usually used to provide a reserve.

For better protection against interference, the cables are shielded, and the shields along the entire line are connected together. For this purpose, in addition to the “A” and “B” pins, the devices to be combined have a “COM” terminal. The line is grounded at only one point, usually at the location of the controller, modem, or computer. It is forbidden to do this at two points in order to avoid interference, which will inevitably go across the screen due to the potential difference at the ground points.

Cables are connected only in series with each other, branches cannot be made. To match the line, a 120 Ohm resistor is connected at its end (this is the characteristic impedance of the cable).

In general, the installation of interface cable lines is a simple task. It will be much more difficult to set up the equipment, which will require people with special knowledge.

For a better understanding of the operation of the RS-485 interface, we suggest you watch the following video:

For industrial applicationswireless data lines can never completely replace wired ones. Among the latter, the most widespread and reliable is still the serial RS-485 interface. 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 Maxim 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 interfaces include CAN, RS-232, RS-485 / RS-422, I2 C, I2 S, LIN, SPI, and SMBus, but RS-485 and RS-422 are still the most reliable, especially in harsh operating conditions.

In many ways they are similar, but 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 a single master of the data bus to which up to 10 slaves can be connected (Figure 1). It provides transmission at speeds up to 10 Mbps using a twisted pair cable, which improves noise immunity and achieves the highest possible data transmission range and speed. 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.

Figure: 1. RS-422 interface with connection of several receiving devices to a common two-wire communication line

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, data rates are more than 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 mode, 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 extended RS-485 multidrop interface, you can network multiple devices connected to the same serial port, as shown in Figure 1. 2.

Figure: 2. Multipoint half-duplex transceiver system used in industrial applications

The receiver sensitivity is ± 200 mV. Therefore, to recognize one data bit, the signal levels at the point of connection of the receiver 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.

Figure: 3. Minimum signal levels in the RS-485 line

Serial problems in industrial environments

Industrial system designers face the daunting challenges 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 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 (Fig. 4).

Figure: 4. The result of exposure to electrostatic discharge on a crystal of a microcircuit with an insufficient level of protection

Figure: 5. Simplified diagram of the built-in I / O port ESD protection circuit

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. In fig. 5 shows a simplified functional diagram of a typical embedded ESD protection circuit. Signal line transients are limited by diode protection at the VCC and ground levels 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 provide 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 operating mode 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 MAX13442E / MAX13444E transceivers that tolerate 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 will survive overvoltages caused by shorted power and signal lines, wiring errors, improper plug connections, defective cables, and misuse.

Robustness 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 receiver differential input voltage VA - VB is greater than or equal to -50 mV, the R0 output is set high. If VA - VB is less than or equal to -200 mV, the R0 output goes low. When all transmitters go to an inactive state and there is a termination on the line, 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

Figure: 6. Simplified block diagram of DE input hot-swap protection

Tell in:
The EIA RS232C interface is designed for serial communication of two
devices. It is generally accepted and widely used in hardware systems with
connecting external equipment to a personal computer. Interface
RS / 232C provides for the use of "single-ended" transmitters and
receivers, while data transmission is carried out using an "asymmetric"
signal on two lines - TxD and RxD, and the signal amplitude is measured relative to the line
GND ("zero"). A logical unit corresponds to a range of amplitude values
signal (voltage) from –12 to –3 V, logical zero - from +3 to +12 V. Range from
–3 to +3 V corresponds to the deadband that determines the receiver hysteresis.
The asymmetry of the signal causes the low noise immunity of this
interface, especially with industrial interference. Receiving (RxD) and transmitting lines
(TxD) data allows you to support full-duplex data transmission, i.e.
simultaneously, information can be both transmitted and received.

The advantages are simplicity.

Disadvantages - only one device is connected to one port, the signal transmission range without additional gadgets is only a few meters

The hardware method is most widely used for data flow control.
management. For correct data transmission, it is necessary that the receiver is in
state of readiness to receive information. With hardware control
the RTS / CTS signal is used to stop data transmission if
the receiver is not ready to receive them. Hardware flow control provides the most
fast response of the transmitter to the state of the receiver.
When designing industrial automation systems, the greatest
information networks based on the interface of the standard
EIA RS485. Unlike RS / 232, this interface provides data transmission from
using a "balanced" (differential) signal on two lines (A and B)
(see figure) and the use of an additional potential equalization line
grounding of devices connected to the RS / 485 network. Logic signal level
is determined by the voltage difference on the lines (A - B), while the logical unit
corresponds to the range of voltage values \u200b\u200bfrom +0.2 to +5 V, and logical zero corresponds to the range
values \u200b\u200bfrom –0.2 to –5 V. The range from –0.2 to +0.2 V corresponds to the dead zone
receiver. When using this interface, the maximum length of the communication line between
extreme devices can be up to 1200 m.
it is recommended to install end-of-line terminating resistors at points in the network
(terminators) to compensate for the characteristic impedance of the cable
to minimize the amplitude of the reflected signal.

The resistance of the terminating resistors depends on the line length and the number of devices. It should be between 100 and 620 OHM.

Both of these interfaces support synchronous transfer mode. Data
are sent in blocks (frames), the format of which is shown in Fig. 1.2. Transfer of each
frame starts with a start / bit signaling the receiver to start transmission
followed by data and parity bits. Ends sending a stop / bit, guaranteeing
pause between sending.
For the asynchronous mode, a number of standard exchange rates are adopted: 50, 75, 110, 150,
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 bps. Number of data bits
can be 5, 6, 7 or 8 (5 / and 6 / bit formats are not widely spread).
The number of stop / bit can be 1, 1.5 or 2 ("one and a half bits" means only
the duration of the stop interval).

Section.

RS-485 is used for data exchange between several devices over one two-wire communication line (twisted pair) in half-duplex mode. The transmission is performed simultaneously in one direction only. Reception is not possible. To receive data, it is required to switch the transceiver to receive mode.
In terms of electrical characteristics and principles of data transmission, RS-422 is fully compatible with RS-485, but is full duplex. In it, one twisted pair is constantly used for receiving, and the other for transmitting data.

Signal levels RS-485, RS-422
Data transmission goes along two lines, A and B, which are twisted pair (two twisted wires). The principle of differential transmission of one signal is used. The original signal is on the A wire, the antiphase signal on the B wire. When one wire has a logical 1, the other has a logical 0 and vice versa. This achieves a high immunity to common mode disturbances acting equally on both wires. Electromagnetic interference, passing through a section of the communication line, induces the same potential in each wire, while the informative potential difference remains unchanged.
The transmitter must be capable of 1.5 V signal level at maximum load (32 standard inputs and 2 terminal resistors) and no more than 6 V no load. On the receiver side, the minimum received signal level must be at least 200 mV.

Hardware implementation
RS-422 is a full duplex interface. Reception and transmission are carried out on two separate pairs of wires. Each pair of wires can only have one transmitter. Implemented in MAX488, MAX490 microcircuits.
RS-485 is a half-duplex interface. Reception and transmission go on one pair of wires with a time division. There can be many transmitters on the network, as they can be turned off in receive mode. Implemented in ST485, MAX485 microcircuits.

Distance and baud rate

Speed	Distance
62.5 kbps	1200 m
375 kbps	300 m
2.4 Mbps	100 m
10 Mbps	10 m

BORDER\u003e

The number of connected devices in the RS-485 line
The number of devices connected to one interface line depends on the input impedance of the receivers. According to the standard, the input impedance of the receiver must be greater than or equal to 12 kΩ. This resistance corresponds to a unit load (UL). One transmitter is designed to control 32 standard receivers. Receivers available with 1/2 UL, 1/4 UL, 1/8 UL load. When using such receivers, the total number of devices can be increased to 64, 128 or 256, respectively.

Agreement
The electrical signal reflects off the open ends of the transmission line. If the distance is large enough, the edge of the signal reflected at the end of the line and coming back can distort the current or next signal. In such cases, you need to somehow suppress the reflection effect.
At the far end of the line, between the twisted pair conductors, connect a resistor with a value equal to the line impedance. The electromagnetic wave reaching the "dead end" is absorbed by the resistor. Hence its name - a matching resistor or "terminator". The nominal resistance of the terminating resistor corresponds to the characteristic impedance of the cable and is usually 120 Ohm.

The resistor can be soldered to the pins of the cable connectors at the end devices. Sometimes resistors are mounted in the device itself and to connect the resistor you need to install a jumper (as in our products VTR-232/485, VTR-E / 485, USB-485M).

Protective offset
When the receiver is disconnected from the line, or when there are no active transmitters in the line, the level of the electrical signal on wires A and B can be arbitrary. To avoid sending erroneous signals to the UART receiver, it is necessary to set the pullup of input A to power, and B to ground.

Our products (VTR-232/485, VTR-E / 485, USB-485M ...) are equipped with 680 Ohm bias resistors.

When RS-485 is working for transmission, the output of the RO receiver is switched to the third state and the RX leg of the controller (UART receiver) "hangs in the air". As a result, any interference will be taken as an input signal during transmission on the UART receiver. To eliminate this situation, it is necessary to pull the RO receiver output to logic 1.

In our products (VTR-232/485, VTR-E / 485, USB-485M ...) there is a 10 kΩ receiver output pull-up resistor.

When the power is turned on or the equipment is rebooted by the "Reset" signal, the controller takes several milliseconds to initialize. It turns out that the power to the microcircuit of the RS-485/422 transceiver has already been supplied, but the enable inputs of the receiver / RE and transmitter DE are "hanging in the air". As a result, the transceiver can open for transmission due to interference and transmit garbage to the working line all the time the microcontroller is initialized. To eliminate this, it is necessary to pull up the transmitter switch to “ground” with a resistor. Thus, immediately after turning on the power, the transmitter is switched on for reception and does not litter the line.

In our products (VTR-232/485, VTR-E / 485, USB-485M ...) there is a 10 kOhm transmitter pull-up resistor.

Devices are often located at a great distance from each other, so galvanic isolation is usually required, the functions of which are to break the common "earth" circuit, protect the entire system from high-voltage transients, reduce noise and signal distortion, and increase the degree of electrical safety.

Technical characteristics of the RS-485 and RS-422 standards.

Parameter	RS-422	RS-485
Tx and Rx number allowed	1 Tx, 10 Rx	32 Tx, 32 Rx
Maximum cable length	1200 m	1200 m
Maximum baud rate	10 Mbps	10 Mbps
Voltage range "1" transmitter	+2 ... + 10 V	+1.5 ... + 6 V
Voltage range "0" transmitter	-2 ...- 10 V	-1.5 ...- 6 V
Maximum short-circuit current of the transmitter	150 mA	250 mA
Permissible load impedance of the transmitter	100 ohm	54 Ohm
Rx input sensitivity	± 200 mV	± 200 mV
Receiver input impedance	4 kΩ	12 kΩ
Rx input voltage range	± 7V	-7 ... + 12V
Logic-one level Rx	\u003e 200 mV	\u003e 200 mV
Rx logic zero level

BORDER\u003e

This article provides an introduction to RS-422 and RS-485 interfaces and explains why you might want to use them in your projects.

Related information

• Why and how to use differential signaling
• Interrupt friendly double buffering UART technology

Most of us are familiar with RS-232, a reliable but inconvenient standard that is forever linked to our memories of an increasingly obsolete serial port on a computer. You may be less familiar with RS-422 and RS-485, which are indeed (as the name suggests) related to RS-232.

However, make the mistake of assuming that these newer standards share the characteristics that make RS-232 so incompatible with modern electronic systems. RS-422 and RS-485 are major improvements in the RS-232 theme; both can be good choices for your next digital link.

First, RS-422 or RS-485

These two standards are usually grouped together because they have a lot in common. But they are certainly not identical, and RS-422 and RS-485 devices are not completely interchangeable. First, I will highlight the significant differences between the two standards. Then, in the rest of the article, we can simplify by referring to them as "RS-422/485".

Both standards (RS-422 and RS-485) allow multiple devices on the bus (i.e. you are not limited to one transmitter and one receiver). However, RS-422 can only be used for multi-site tires, i.e. a differential pair can have multiple receivers, but only one transmitter.

The maximum number of receivers on a two-wire RS-422 bus is 10 (well, sort of ... see the discussion below for “unit loads”).

On the other hand, with RS-485, you can have real multipoint a system where "point" instead of "subscriber" means that a single differential pair can support multiple transmitters as well as multiple receivers.

RS-485 also increases the bus capacity to 32 devices.

(Actually, this is not so simple - the standard specifies a maximum of 32 "unit loads", but you can connect many more than 32 devices using RS-485 chips, which represent only a small fraction of a unit load on the bus. It's a bit complicated, and frankly, this is where I start to lose interest ... but if you are more persistent than me, you can read the details.)

The fully equipped RS-485 bus is a high-performance interface. In addition to the benefits discussed later in this article, you can have many transceivers that use the same two wires, and any device on the bus can send data to any other device on the bus.

Another important point is that RS-485 is an important extension of RS-422. In other words, RS-485 adds and improves functionality, but does not conflict with anything in the RS-422 standard. Thus, an RS-485 device can be used on an RS-422 network, but RS-422 devices are not necessarily compatible with an existing RS-485 network.

The basics

RS-422/485 is a four- or two-wire, full-duplex or half-duplex, differential, medium-speed serial interface that supports multi-drop (RS-422) or multi-drop (RS-485) bus architectures. Here are some comments on these characteristics:

I like it

RS-422/485 features - long cable lengths, noise immunity, etc. - make it an excellent choice for industrial applications. However, part of my challenge in this article is to demonstrate that RS-422/485 is a good choice for many electronic and electromechanical systems, even if you don't need all the functionality it offers. My favorable view of RS-422/485 is based primarily on three considerations: ease of design, excellent support in IC datasheets and application notes, and noise immunity.

Keep it simple

Despite years of experience with various serial communication protocols, the UART is still my favorite. It is simple and reliable, requires minimal interconnections, and I would not be surprised to find that it is supported by every microcontroller on the market. It may be a little primitive, but you can always write firmware to implement any kind of data flow control, device identification, or error checking in your particular application.

Anyway, I want to say that I like to use the UART whenever I can and RS-422/485 is a great physical layer for UART communication.

Expert support

It's easy to include RS-422/485 in your project: almost all you need is a converter / transceiver chip, and there are many to choose from. These devices convert common logic signals to differential RS-422/485 signals and also handle the rest of the annoying details required to ensure compliance with the RS-422/485 standard. And if you're not sure how to accurately design your particular communications bus, you'll find plenty of guidance in the application notes and datasheets.