RS-422 Linear Magnetic Encoder shows ERROR LED after power-on (Mesa 7i76 + 7i85)

I recently bought some Chinese linear magnetic encoders with RS-422 output and connected them to my Mesa 7i76 and 7i85 boards. Everything mostly works, but I’m having a strange issue with the encoder status LEDs.

Each encoder has LEDs:
Blue = IDLE
Red = ERROR

Here’s what happens:
When I power on the machine, the ERROR (red) LED turns on immediately.
If I unplug and replug the encoder connector, sometimes it switches to IDLE (blue) — but not always.
If I disconnect and reconnect one of the signal phases (A or , I can get the IDLE state.
If I power the encoder alone (without connecting signal lines), it always shows IDLE.
It looks like the signal lines or differential inputs might cause some initialization conflict when LinuxCNC / Mesa powers up.

Has anyone experienced something similar with RS-422 encoders on Mesa boards?
Any advice or tests to help diagnose this would be appreciated.

Maybe the scales cannot drive the RS-422 termination on the 7I76/7I85
or perhaps the 5V is marginal (check at encoder end of cable)
 

You are right.

Switched the 7I85 channel to TTL mode, disconnected A−/B−/Z−, and kept +5, A+, B+, Z+, GND only.
The encoders now start in IDLE every time and looks work.

These Chinese “RS-422” linear scales most likely don’t have a true RS-422 line driver and cannot drive the 120 Ω termination on the Mesa inputs.

As a side note — it seems these low-cost Chinese sensors are often mislabeled:
one of my units was even shipped with 1 µm resolution instead of 5 µm as specified.
So if anyone else is buying similar “RS-422” magnetic scales, be cautious — the specs are not always accurate.