RS-422 / RS-485 Industrial Standards

Different plugs and female jacks with serial RS-422 / RS-485 interfaces

The serial interfaces RS-422 and RS-485

The industry standards TIA/EIA RS-422 (1994) and TIA/EIA RS-485 (1998) were developed by the Telecommunications Industry Association and Industries Alliance during the 90s. Both standards define a technology for data transmission largely free of interferences that has not existed in this form up to this time. The standards only determine hardware basics resp. their electrical features (bit transfer layer), but the software protocol for data transfer keeps independent. In the following years this led to further developments for many modern interfaces such as USB, Profibus, CAN Bus, Ethernet etc.

These industry standards enable complex networks or point-to-point and point-to-multipoint connections. In general, these networks are called field bus systems. Actuators, sensors, machines, gateways etc. are interconnected in multipoint bus networks (half duplex RS-485 or full duplex RS-422) and are usually organized with a server or so-called bus master PLC as control unit.

Electromagnetic compliance of RS-422 and RS-485 interfaces

An important feature of the RS-422 and RS-485 interfaces is an improved electromagnetic compliance (EMC), which makes the devices much less susceptible for interferences. The EMC results from requirements of the two specifications of TIA/EIA RS-422 (1994) and TIA/EIA RS-485 (1998). These have significant advantages in comparison to other transfer methods:

Symmetcric / differential signal transmission method

In the following, examplified by RS-485 specification, the symmetric resp. differential transfer method will be described. RS-485 uses the halfduplex transmission method and can transmit and receive data time-shifted in both directions. For signal or data transmission, one conductor pair per direction is always required.

The figure below shows exemplarily the data signals 101010 at a RS-485 wire pair. The red line TX+ sends a logical 1 with 5 V output voltage (nominal voltage) and a logical 0 with 0 V (not inverted). At the same time, but with inverted polarity, the blue line TX- sends the signal with 5 V logical 0 and with 0 V logical 1. The green arrows show the voltage difference resulting on the wire pair RX and TX. The receiver generates from this voltage difference the data bits for 0 and 1.

Schematical view of the power and voltage course | * 2.5 V resp. -2.5V apply to common mode voltage

The example shows the power and voltage with following defined voltage levels:

  • A = Nominal voltage (e.g. 5 V)
  • B = Minimal output voltage at the transmitter (1.5 V)
  • C = Minimal input voltage at the receiver (200mV)

The connectors on the devices are usually designated T/R+ and T/R-, where R stands for Receive and T for Transmit. The plus resp. minus signs mean that they are using a not inverted (+) resp. inverted (-) signal.

Sub-D 9 pin socket and labeled pin assignment on the terminal block Item 62406

Attention: The designations A-, A+ and B-, B+ or D-, D+ are also possible. But there is no binding norm for designations. For that reason, maybe connectors at devices and plugs are compatible, but labeled in different ways. Most important is, plus is plus and minus is minus, D+ and B+ for example have the same function.

Protection against interference (common mode and push-pull interferences)

Protection against common mode interferences
The frequently occuring electromagnetical interferences (EMI) like voltage impulses or voltage fluctuations generate the so called common mode interferences. These external low-frequency interferences also effect the wire pair. But, thanks to the inverse phased and symmetric signal transmissions, the voltage difference between the wire pairs can be maintained. Further improvements can be made with well shielded cables. These prevent the emission of high frequential fields.

Common mode interferences by electromagnetical fields can be avoided by the inversely phased transmission

Protection against push-pull interferences
Push-pull interferences are inductive couplings between the wires in a cable. The so called cross-talk can almost completely avoided by twisted wire pairs.

The induced currents and resulting voltages of two wires cancel each other out when twisted

Impedance or line matching

By impedance or line matching, signal cancellation and signal reflection are avoided. For this, termination resistors at the end points of the lines are used. So all signals of a transmitter can be absorbed at the end of a line by a resistor without reflections.

  • RS-485 requires a termination resistor on both end points of a line
  • RS-422 RS-422 requires a termination resistor only at the end of the line
A BIAS resistor at one end of the line, a terminating resistor at the other end

Different methods are used for the line adjustment. The terminating resistors must correspond to the wave resistance of the implemented lines. Regularly, 120 Ohm termination resistors or BIAS resistors are used.

Application fields

The more current TIA/EIA RS-485 standard is used more often than the RS-422 standard. Examples are Industry 4.0, facility automation or Smart Home devices (like HomeMatic, Modbus, SmartBus G4, Link Network Protocol), machines and automation control systems (BACNet, CC-Link, InterBus-S, Profibus-DP, DMX), etc. In this way, the requirements for modern infrastructure can continue to be met.

Product examples RS-422 / RS-485


Adapter USB 2.0 > 1 x Serial RS-422/485 + Terminal block

This adapter enables the connection of a serial device, e.g. printer, modem, IoT hardware etc., via a USB Type-A port.

Connectors
      Adapter:
      1 x USB 2.0 Type-A male >
      1 x Serial RS-422/485 Sub-D 9 pin male
      Terminal block:
      1 x Serial RS-422/485 Sub-D 9 pin female >
      1 x 6 pin terminal block
Serial connector with nuts
Chipset: FTDI

Item 62406

Converter USB 2.0 to Serial RS-422/485 with 3 kV isolation

This USB 2.0 to serial RS-422/485 converter provides an RS-422/485 connection for measuring instruments, printers, IoT devices, Modbus etc. Galvanic isolation via optical isolators prevents data transmission errors due to uneven earthing potentials, ground loops or interference from voltage peaks. The converter is suitable for DIN rail mounting.

Connectors:
      Host: 1 x Mini USB 2.0 B female
      Device: 1 x RS-422/485 terminal block 5 pin
3 kV optical isolation
Dimension (LxBxH): ca. 101 x 42 x 22 mm

Item 62501

Adapter USB 2.0 Type-A male to 1 x Serial RS-422/485 DB9

This USB 2.0 Type-A to serial adapter provides an RS-485 port for measuring devices, POS systems, printers, modems, IoT devices, etc.

Connectors:
      1 x USB 2.0 Type-A plug >
      1 x serial RS-422/485 DB9 male with screws
Data transfer rate up to 921.6 Kbps
Signals: T/R+, T/R-, RXD+, RXD-, GND

Item 66286

Converter 1 x Serial RS-232 DB9 to 1 x Serial RS-485 • 15 kV ESD protection

This RS-232 to RS-485 converter is suitable for all serial devices, e.g. on machine controls, fieldbus systems, measuring instruments, computers, printers etc. The level converter can be used as a coupling directly between two DB9 connections without an additional power supply.

Connectors:
      1 x Serial RS-232 DB9 female with screws
      1 x Serial RS-485 DB9 male with nuts
      1 x RS-485 Terminal block 4 pin
Auto data flow control

Item 63969

Adapter USB 2.0 to Serial RS-485 • 15 kV ESD protection • compact serial connector housing

This USB 2.0 Type-A to serial adapter provides an RS-485 port for measuring devices, POS systems, printers, modems, IoT devices etc. The user-friendly RS-485 connector is characterized by its small dimensions (39 x 33 x 14.7 mm) and thus offers a comfortable handling at device connections, cable glands, patch panels, etc.

Connectors
      1 x USB 2.0 Type-A male >
      1 x Serial RS-485 DB9 male with screws
Data transfer rate up to 921.6 Kbps
Signals: T/R (A+), T/R (B-) und GND

Item 66283

Extender RS-232 DB9 female • RJ45 female to RS-232 DB9 male • RJ45 female ESD protection • 1200 m range

This RS-232 Extender with two converters and RJ45 connectors can bridge long distances up to 1200 meters between RS-232 devices bidirectionally and OS independently. The differential transmission technology prevents data errors that can occur caused by external interference or different ground potentials. Additional power supplies on the converters ensure the quality of the data signals. The RS-232 Extender connects scanners, measurement instruments, machines, computers, IOT devices etc.

Connectors
      1 x serial RS-232 DB9 female with screws >
      1 x RJ45 jack
      1 x serial RS-232 DB9 female with nuts >
      1 x RJ45 jack
±15 kV ESD protection for all serial signals
Signals: RXD, TXD, CTS, RTS

Item 63934
Application example with extender 63934

Adapter USB 2.0 to 4 x Serial RS-422/485 • ESD protection

With this adapter up to four serial devices can be connected via one USB Type-A port.

Connectors
      1 x USB 2.0 Type-B female >
      4 x Serial RS-422/485 DB9 male
      1 x 5 V DC jack
Automatical recognition and switching between RS-422 and RS-485
±15 kV ESD protection on all signal pins

Item 87587

Also available with
USB 2.0 to 2 x Serial RS-422/485Item 87586
USB 2.0 to 1 x Serial RS-422/485Item 87585

PCI Express Card zu 2 x Serial RS-422/485 • ESD protection

This PCI Express card extends the PC by two external serial RS-422/485 ports. For example scanners, protocol converters, POS systems, infrared -, IoT - devices etc. and different modes can be used. With the included low profile bracket the card can also be installed in a Mini-PC.

Connectors
      internal: 1 x PCI Express x1, V1.1
      external: 1 x DB44 female
      Connection cable: DB44 male > 2 x Serial RS-422/485 DB9 male
RS-422 or RS-485 setting via jumper
±15 kV ESD protection on all signal pins

Item 89320

PCI Express Card > 2 x Serial RS-422/485 • ESD protection + surge protection

This PCI Express card is suitable for industrial field bus systems, machine controls, converters, measurement data acquisition etc. Due to an extensive overvoltage protection and the extended operating temperature range, this card is also highly suitable for use in demanding environments. With the supplied low profile bracket the card can be installed in a Mini PC.

Connectors
      internal: 1 x PCI Express x1, V2.0
      external: 1 x DB25 female
      Connection cable: 1 x DB25 male > 2 x Serial RS-422/485 DB9 male
±15 kV ESD protection on all signal pins
2 x Terminal block with surge protection, 600 W peak pulse

Art. 65841
All Delock RS-422/RS-485 products
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