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Field bus (Infinet or MSTP) will shut down as soon as device is connected

Issue

Infinet/MSTP was running fine until a device was added or replaced, after wiring in the new device it brings down the Infinet/MSTP field bus when powered up, disconnecting the new device from the bus makes the bus resume operation.

When the new device is connected the green (RD) LED goes solid instead of blinking and all communications to all devices on the bus stops, in some cases the master controller will reset.

Product Line

Andover Continuum, EcoStruxure Building Operation

Environment

  • Infinet devices
  • i2s
  • MSTP devices
  • b3s

Cause

Power lines reversed.

Continuum field devices (i1s, i2s and b3s) have their RS-485 communication lines connected to ground via an isolator as seen in the diagram below, if the LIVE and GND lines are inverted then LIVE is connected to the 485 communication lines instead of GND.

This will make the field bus electrically unable to communicate and in some scenarios can even cause hardware damage.

Resolution

When powering 24VAC  devices off the same transformer make sure to continue the wiring chain LIVE to LIVE and GND to GND.

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