Bus Interface Description RS422

Posted on Feb 6, 2014

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 Enab

Bus Interface Description RS422
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le 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. The Pullup/Down...

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