RS-485 Line Quality

How to check the RS485 line quality?

You can use the ZET 210 module (or other ADC/DAC module or a FFT spectrum analyzer) for physical check.

Instruction on Checking Quality of Cable Communication Line with RS-485 Interface

For diagnostics of the cable communication line with RS-485 interface, it is recommended to perform the following actions:

  1. Connect ZET 210 to computer port which will be used to control the cable communication line.

Note: ZETLAB software package must be installed on the computer.

Connect ZET 210 to RS485 interface converter in the cable line connection place (where the measuring modules are located). At the same time, the terminal 1 of the device is connected to D+ circuit, terminal 2 to D- circuit and terminal ┴ to GND circuit (for convenience of connection it is recommended to use adapter connection terminals).

  1. Activate interface converter and power supply on RS-485 line.
  2. Open ZETLAB software package.
  3. On ZETLAB panel in the “Service” field open the “Device manager” tab. Make sure that ZET 210 is displayed in list of devices.
  4. Disable channels except for “Sig_1_1” and “Sig_1_2” by activating channels indicators one by one (right click), each time selecting “disable”.
  5. Activate the “Sig_1_1” channel and in the “Features” tab in the “state” line check the “differential” (with ““). Press “Ok”.
  6. On ZETLAB panel in the “Display” field open the “Multichannel oscilloscope” tab.
  7. Select the “Sig_1_1” channel name (top right corner of the window)
  8. In the “Interval, s” field enter 1.
  9. Check the cable line with evaluating type of registered signal on the “Multichannel oscilloscope” window:

Figure 1 — no signal on the communication line (possible reason is that interface converter is not activated or faulty);

Figure 2 – asymmetric signal on the communication line (possible reason is “D+” or “D-” short circuit on the “GND” circuit);

Figure 3 – low level signal on the communication line, with signal range below 3 V (possible reason is low line resistance between “D+” and “D-” circuits or faulty drivers of the devices which form low level signals.

Note: to identify a specific device, activate its identifier on the “Device manager” tab and check its response in the “Multichannel oscilloscope” window.

Figure 4 — type of signal with normally operated RS485 interface.

Качество линии RS485

Figure 1 No signal

Качество линии RS485

Figure 2 Asymmetrical signal

Качество линии RS485

Figure 3 Low signal level

Качество линии RS485

Figure 4 Normal line state