EIA/TIA-422 Interface Circuit
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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. |
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.
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}
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}
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} |
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)
| 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.
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}
Lascomm {Fiber Converter EIA232 to RS485 Converters - RS232 to V.35/X21}
Telebyte {many Converter types}
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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