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How to test an Infinet bus using a volt meter.

How to test an Infinet bus using a volt meter.

Issue

What voltage should be seen across conductors on an Infinet bus?

Environment

  • Infinet
  • RS-485

Cause

Communication issues on RS-485 bus.

Resolution

 Measuring Ground Potential

Signal Potential refers to the horizontal plane along which signals are sent and received with respect to ground.  RS-485 requires that all devices communicating with each other be at a reasonable ground potential.

Measuring the ground potential of any Infinet controller

Use a voltmeter to measure between the earth ground connection (for power) and the Infinet shield connection. 

 Measuring the ground difference between the Infinet controller and the ground at the InfiLink or CX/NetController (shield grounding point).

  • Set the multi meter to measure AC voltage; measure and record the AC voltage (ACV) between + and -.
    • A reading higher than 2-3 volts indicates a grounding problem.
  • Set the multi meter to measure DC voltage; measure and record the DC voltage (DCV) between +and -.
    • A reading higher than 2 to 3V on either voltage indicates a grounding problem.
  • Calculate the combined voltage using the following formula.
    • (1.4 X ACV) + DCV = Combined Voltage.
  • If the combined voltage is greater than 7 a grounding problem exists
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