Start with topology and termination
Use one main trunk with short stubs. A star creates several reflection points and degrades quickly as baud rate or cable length rises.
Place termination only at the two trunk ends and match cable impedance. With power off, measuring the A-to-B resistance quickly reveals missing, duplicated, or miswired terminators.
- One terminator at each trunk end
- Keep stubs short
- Check whether vendors reverse A/B labels
Bias and common mode matter too
The idle bus needs a defined state. Calculate bias resistors together with termination so they create adequate differential voltage without overloading the driver.
Ground offset between distant nodes consumes common-mode margin. Long or industrial links may need isolated transceivers, surge protection, and a dedicated reference conductor.
Probe A-to-ground, B-to-ground, and A-minus-B. Looking at only one wire can hide a common-mode problem.
Check direction timing last
In half duplex, deasserting DE too early clips the final stop bit; too late can collide with the next node. Wait for the transmit shift register—not only the software buffer—to become empty.
If the waveform is clean, then inspect baud error, inter-frame timing, CRC, and retry behavior.
- Capture DE, TXD, and the differential bus
- Inspect the final stop bit
- Separate physical errors from protocol timeouts