RS-485: how to check a bus step by step
Short answer: most intermittent RS-485 faults come from the physical layer, not from the code. Check the bus in this order: topology, termination, bias, common reference, cable and, last, the serial settings. Most of these checks need only a multimeter and the cable drawing.
Download the checklist (PDF, 1 page)
Descarga la lista de comprobación (PDF, 1 página)
These checks work for any PLC, drive, meter or gateway with an RS-485 port. At the end you will find the page for your PLC range.
The idea in one minute
RS-485 sends data as the voltage difference between two wires, A and B. Noise that reaches both wires in the same way cancels out. This is why it works over long distances in noisy plants, as long as the bus is built as one line from node to node, with a 120 Ω resistor at each end.
Six checks, in order
| Check | What to do | Good result |
|---|---|---|
| 1. Topology | Draw how the cable is really installed | One line from node to node, short stubs, no star |
| 2. Termination | Power off and measure the resistance between A and B | About 60 Ω |
| 3. Bias | Power on, stop all traffic and measure the DC voltage between + and - | At least 200 mV, stable |
| 4. Common reference | Disconnect the bus at one node and measure between the signal grounds of two distant nodes | Not several volts, and no jumps when machines start |
| 5. Cable and shield | Check the cable type and its route | 120 Ω twisted pair, shield to earth at one end, away from VFD outputs |
| 6. Serial settings | Compare the settings of every node | Same baud rate, data bits, parity and stop bits, and no duplicate addresses |
What the readings tell you
- About 120 Ω: one termination is missing, or you are measuring only part of the bus.
- About 40 Ω or less: three or more terminations are enabled. Check the DIP switches of each device.
- Very high or open circuit: no termination, or a broken cable.
- Near 0 V with no traffic: the bus has no bias. Enable it at one point only, unless all receivers are true fail-safe.
If the bus has never worked
- Swap A and B at one device. Manufacturers do not use these letters in the same way, so follow the + and - marks.
- Compare the baud rate, parity and address on both sides.
- Check that the port is enabled and in the right mode. This depends on your PLC range.
Commenting is not enabled on this course.