Wire Break on RS422 Quadrature Input


#1

Problem
When wiring an RS422 quadrature encoder, RMCTools will not receive counts and the Wire Break status bit will be active. This bit can be seen by selecting the All tab of the Axis Status Registers and expanding the Feedback section.

Cause
The Wire Break status bit indicates the voltage between + and - input pins is less than about 0.4 volts.
Causes of this include wiring and power supply issues as well as damaged encoders.

Fix
Make sure the common (sometimes called 0 V) power pin on the encoder is connected to the common pin on the RMC input module.

While the encoder is completely stopped, measure the output of the encoder:

  • A+ to A-
  • A+ to Cmn
  • A- to Cmn
    Repeat for B and Z (opt)

Voltage levels should be:

  • Between A+ and A- should be more than 0.5V. Usually it will be around 2.5 to 4 V
  • Between A+ and A- or A- and Cmn should all be between 0 and 5V normally

If the voltage levels are outside these ranges and the wiring is correct, the next suspect is the driver chip in the encoder. Some driver chips are susceptible to damage if the pins are shorted out.