RS-422 Bus

TIA/EIA-422 Electrical Characteristics of Balanced Voltage Digital Interface Circuits

[RS422 Description] [RS422 ICs ] [Standard Organizations]
[RS449 Cabling] [RS422 Pull-Up/Pull-Down Resistors] [OEM RS422 Converter Devices ]

EIA-422 Bus Description

RS422 Bus Circuit implementation
EIA/TIA-422 Interface Circuit

EIA/TIA-422 defines a Balanced (differential) interface; specifying a single, unidirectional driver with multiple receivers (up to 32). RS-422 will support Point-to-Point, and Multi-Drop circuits, but not Multi-Point [EIA485]. EIA-485 devices may be used in 422 circuits, but EIA-422 may not be used in 485 circuits (because of the lack of an Enable line). EIA422 is the differential "brother" to EIA423. One application note indicated that the combination of cable length (in meters) and data signaling rate (in bps) for RS422 should not exceed 108. The example provided a maximum speed of 200kbps with a maximum cable length of 500 meter [500 x 200,000]. An EIA-422 driver can support up to 10 loads, at 4K W per load.

Circuit grounding is not shown in either of the RS422 or RS485 circuits. However, the circuits should be grounded. The drivers and receivers should be connected directly to circuit ground. The circuit ground should be connected to chassis ground via a 100 ohm resistor. If the far-end circuit resides in another chassis, then the two circuit grounds should be connected together via a 100 ohm resistor at each end.

For additional ESD protection TransZorb [back-to-back] diodes may be added between the input/output lines and ground. This link provides part numbers and pin-outs for a DIP Package TVS Array. Steering Diodes [1N6101] may also be added to the input lines to protect the RS422 receivers. One diode between the I/O and Vcc, and another between the I/O and ground. Three different diode configurations and part numbers are shown on the DIP Package Diode Array page.
Cabling is not specified in RS422 or RS485, but a rule of thumb is listed above for 24 AWG twisted pair copper wire with 16pF/ft. Refer to 24AWG Attenuation vs Frequency Chart.

Cabling standards are listed lower down the page. The common name for this interface is RS422.

{RS422 Bus Index}

EIA-422/485 Standard Organizations

TIA/EIA-422: Electrical Characteristics of Balanced Voltage Digital Interface Circuits
TIA/EIA-485: Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems

Electronic Industries Alliance, EIA was disbanded & ceased operations in 2011.
Telecommunications Industry Association {TIA}, []

V.11: Electrical characteristics for balanced double-current interchange circuits operating at data signalling rates up to 10 Mbit/s

ITU, International Telecommunication Union,

FED STD 1030A is identical to TIA/EIA-422-B. It is intended for United States, non-military government use.
FED STD 1020A is identical to TIA/EIA-423-B. It is intended for the United States, non-military government use.

{RS422 Bus Index}

EIA-422/485 Bus Interface IC Manufacturers

Voltage Switching Levels
RS422 Voltage Transition Levels

IC Interface Vendors:
Dallas Semiconductor Corp see Maxim

Exar {RS-422 Differential Driver/Receiver IC Manufacturer}

Linear Technology {EIA422-EIA485 Transceiver-Driver-Receiver ICs}

Maxim {RS232-RS485-RS422 IC Manufacturer}

National Semiconductor {RS422 Differential Bus Transceiver-Receiver-Driver}

ON Semiconductor {EIA485 Transceiver-EIA562-EIA422/EIA423 IC Manufacturer}

STMicroelectronics {EIA422 components}

Texas Instruments 'TI' {RS-232/RS-422/RS-423/RS-422 Diff Driver/Receiver ICs}

Memory Chip IC Manufacturers
Processor Chip Manufacturers
Glue Logic IC Manufacturers

{EIA-422 Bus Index}

EIA422 Differential Bus Pull-up/Pull-Down Resistors

The Pullup/Down Resistors (Idle-line failsafe) are used to keep the +/- inputs of the Receiver(s) inputs at a minimum of 200mV differential. The resistors are normally required once the system has reached it's quiescent state, when no drivers are driving the bus. Only one end of the bus requires the resistor network.

The resistor values are dictated by four factors: The input impedance of the receivers (between 5K and 90K ~ check the data sheet), the number of loads, the termination resistance 60 ohms (120 ohms per side), and the required (turn-on) current of the receivers.
Many (on-line) App-notes use 12K ohms as an input impedance, 60 ohms (as a termination), 3.5 mA as the minimum 'load' current, and 10 loads.

Using these values {Check the data sheets for the real values!}:
..... Number of loads (10) x 12KW ~ 1200W {That's ten 12Kohm resistors in parallel, 12K//10 loads}
......Line Termination ~ 60 ohms {120 ohms at each side of the link, in parallel, 120//120}
......Taking 1200 in parallel with 60 ohms gives 57 ohms
......Turn-On current ~ 3.5 mA {check the data sheet for the device used}

The required load is 5volts / 3.5mA, which results in a load impedance of (around) 1428 ohms. Subtract 57W which is already present leaves; 1371W.
Placing half on the +5 volts line and the other on the ground line, gives 675W per resistor / per supply.
The nearest 5% standard value is 680W placed between both the 'A' line and +V connection and the 'B' line and the GND connection will produce < 3.5 mA (turn-on current}

{EIA422 Bus Index}

EIA422/423 Cabling

RS422 and RS485 do not define cabling or connector information. RS449 and RS530 [listed below] are two common cabling specifications which are used with the RS422/485 electrical standards. Use 24 AWG twisted pair copper wire with 16pF/ft for cables. Maximum cable length is determined by the combination of cable length (in meters) and data signaling rate (in bps). RS422 should not exceed 108 for length x bps. The example provided a maximum speed of 200kbps with a maximum cable length of 500 meter [500 x 200,000]. Charts of Cable Data Rate vs Cable Length.

EIA/TIA-449; General Purpose 37-Position Interface for Data terminal Equipment and Data Circuit-Terminating Equipment Employing Serial binary Data Interchange

TIA-449 is a serial mechanical interface standard for transmission of balanced and unbalanced signals between a variety of higher-end computer, media, and multimedia peripherals. EIA-449 allows a maximum data rate of 10 Mbit/s and uses a 37-pin or 9-pin connector.

EIA/TIA-530; High Speed 25-Position Interface for Data Terminal Equipment and Data Circuit-Terminating Equipment, Including Alternative 26-Position Connector

EIA-530 is a replacement for EIA-449 that uses a DB-25 (EIA-232-D) connector instead of a 37-pin connector, while keeping the most important electrical signals intact. EIA-530 is to be used in conjunction with EIA-422-A.
RS-530, 25pin pinout
RS-449, 37pin pinout
RS-449, 9pin pinout

V.35/RS449 Max length
Data Rate Cable Length
2 Mb/sec 15.24 Meters
1 Mb/sec 30.48 Meters
512 Kb/sec 60.96 Meters
256 Kb/sec 121.92 Meters
128 Kb/sec 243.84 Meters
56 K 487.68 Meters
1.2 Kb/sec 914.40 Meters

RS422 pinout is not defined in the standard, any pin out table found for RS422 is non-standard.

{RS422 Bus Index}

EIA-422 OEM Interface Converters

I/O Converter Vendors;
Acces I/O Products Inc. {RS422 to RS232 Converter Devices}

B&B electronics {422 Converter/Optically Isolated 485 Converter/232 to 422 Converters}

EQUIP'TRANS {V11/V24/20mA to RS-485 Converter Devices}

KK Systems Ltd. {RS232-RS422/RS485 Isolated Interface Converter}

Lascomm {Fiber Converter EIA232 to RS485 Converters - RS232 to V.35/X21}

Patton Electronics Co. {RS-232 to RS-422/449 (V.36) Interface Converter}

Telebyte {RS422 and many Converter types}

{RS422 Bus Index}

Normally EIA422 and 423 systems may not be connected together. EIA-449 cabling of RS422 sends and receives data as differential pairs and control signal as single-ended, but for 423 cabling it sends and receives single-ended data and control signals. Receiving the single ended signal in 423 is accomplished by grounding the 'B' side of the differential receiver at the connector.

So if the system follows the EIA-449 (cabling) specification than one (B) side of the differential receiver of the 423 side will be grounded at the connector forcing the differential driver on the 422 side to drive ground. The EIA422 side also uses a termination resistor between the ends of its differential receiver, providing a serious 120 ohm short to ground for the 423 driver.

If the systems were to be connected together (with out regard for RS499) than the system would revert to EIA423 (single-ended) distance and data rate ~ Only because the driver on the RS423 side is single ended, while the receiver on the RS422 side would receive the single-ended 423 signal and ground on its differential pair. How ever because 423 and 422 use the same receiver chip; going from 422 to 423 provides a differential path.

EIA422/423 Interconnection
RS422 to RS423 Conversion

Design Note; Signal inversion is easily accomplished by transposing the EIA423 lines [signal and ground] at the RS422 receiver.

RS422 to RS232 Circuit Translation implementation
RS422 to RS232 Conversion

{RS422 Interface Index}

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Modified: 2/26/12
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