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 first 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 on 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 (shield) 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.
