Verifying RS-485 with an oscilloscope
Due to the frequency response of a multi-meter, RS-485 is very difficult to measure with this test equipment. To correctly identify whether RS-485 is present, use an oscilloscope or a service monitor to view the signal. The + and - Data should look similar to the Figure 5 below. The + and - Data lines are similar but just inverted from each other. The location of the signal compared to ground does not matter as RS-485 data is not based on differences between data and ground. Attenuated or distorted data indicates the master clock's remote port output is bad or a device on the RS-485 is loading down the line. No data present indicates the RS-485 cable is broken or the master clock RS-485 remote port output data is not present. Check the master clock RS-485 remote output to see if the data is present at this point. If it isn't , disconnect the bus and check the port again. If the data is present, a device is loading down the bus. Disconnect each device one at a time until the bad device is located. If the data isn't present, the master clock output is bad and the master clock needs to be returned for repair. Contact Tech Support for an RMA number.
FIGURE 1: RS-485 data viewed with an oscilloscope