Mapping RS 422 Drivers and Termination by Signal Direction

A full-duplex RS 422 connection contains separate signal paths in opposite directions. Each path has its own driver and receiving end. Treating the entire connector as one bidirectional bus can lead to misplaced termination, crossed polarity, or an unsupported connection between multiple active drivers.

Draw each line independently

The TI overview of RS 422 and RS 485 configurations explains the single-driver RS 422 arrangement and places termination, when used, at the far end near the final receiver. That topology differs from a shared multipoint driver arrangement.

Create one drawing for each differential pair. Label the driver pins, connector contacts, cable conductors, receivers, and termination. Include any branch or intermediate receiver rather than hiding it inside a cable symbol.

Verify direction and polarity

Check the truth table at both ends instead of assuming that identical A and B labels always imply the intended polarity across different devices or documentation conventions.

Apply a known static state and a simple test pattern, then verify the receiver’s logical output. Repeat in the reverse communication direction, which uses a different pair and may have a different driver or load.

Audit installed termination

Locate termination physically and confirm its value against the selected cable and interface guidance. Check for termination already installed inside external equipment, adapters, or modules before adding another resistor on the PCB.

For example, two devices may each contain a configurable receiver termination. When connected, that can be correct for two oppositely directed pairs, but an additional connector adapter may place an unintended parallel load on one path.

Measure the complete route

Observe the signal at the final receiver under the intended cable length and rate. Include any intermediate receivers and their stubs. Check the common-mode voltage relative to each receiver’s local ground as well as the differential amplitude.

If several endpoints need to transmit onto one pair, reconsider the architecture rather than merely joining RS 422 outputs. The intended driver ownership must be supported by the chosen interface and control scheme.

Exercise power-up and power-down combinations to establish the behavior of inactive equipment. A system diagram that is correct only when every node is powered may be inadequate for the installation.

Save the directional drawings, polarity verification, termination inventory, and receiver-end captures. The final record should show exactly one intended driver for each RS 422 line and explain every load connected to it. A connector labeled full duplex does not by itself establish a correct electrical topology.