IO Module Bus Voltages

Issue

What are the expected voltages on the RS485 IO Module bus?

Product Line

Andover Continuum

Environment

Netcontroller IO Modules

IOU Modules

Cause

Require typical values for troubleshooting 1st steps.

Resolution

Although the IO Module bus uses RS485 comms similar to Infinet or BACnet MSTP, the typical bus voltages when measured with a voltmeter are usually lower.

Exact voltages will vary from system to system and depend in the amount of bus activity at the time, but the following will allow general troubleshooting:

Typical DC voltage measured across the bus + to - = 0.7 to 0.8VDC but working systems may read as low as 0.4VDC

Typical DC voltage measured Screen (shield) to + = 0.6 VDC

Typical DC voltage measured Screen (shiled) to - = 1.3VDC

Note: If using Free Topology (FTT-10A) controller and modules, these use transformer bus coupling and will measure 0VDC across the bus.