RS-422 Bus

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

[RS422
Description] [ICs
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EIA-422 Bus Description

EIA/TIA-422 Interface
Circuit

EIA/TIA-422 define a Balanced (differential) interface;
specifying a single, unidirectional driver with multiple receivers (up to
32). RS-422 will support Point-to-Point, 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 is 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 ground
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. 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}

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

Dallas Semiconductor Corp "Maxim"

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

Maxim {RS232-RS485-RS422 IC
Manufacturer}

National Semiconductor {Differential
Bus Transceiver-Receiver-Driver}

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

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

STMicroelectronics

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

Memory Chip IC Manufacturers
{This Site}
Processor Chip
Manufacturers {This Site}
Glue Logic IC
Manufacturers {This Site}

{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. 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].

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

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- 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 (This web Site)
RS-449,
37pin pinout (This web Site)
RS-449, 9pin
pinout (This web Site)

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

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

ARC Electronics {RS-232 to
RS-422/RS-232 to RS-485 Converters}

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.

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

Patton Electronics Co.

Telebyte {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.

{Back to
RS422 Interface Index}

Engineering Design Key words: EIA-485, TIA-485, EIA422, TIA422, RS485,
RS422, Balanced differential Cable Bus, Electrical Standard, Interface Standard,
Specification, Spec, Circuit layout, Electrical Interface, Semiconductor IC
manufacturers, Physical Interface, Engineering Description, electrical
characteristics, Device, Component Manufacturers

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Last Modified 1/27/07
Copyright © 1998 - 2007 All rights reserved
Leroy Davis