Do you have a network of devices, connected using MD485 interfaces, so spread out that extremely long cables are required? Are you worried that the distances, and resultant cable lengths, may be causing communication problems? In this brief article, we’ll look at four tips that can help ensure your MD485 network operates correctly.
MD485 networks use RS-485 communications hardware for which the total maximum length is quoted as “up to” 1, 200 meters (4, 000 feet), with potentially up to 256 devices on the network. As you might suspect, the words “up to” really do indicate that you may not achieve the limits quoted in practice—or at least without careful design.
So whilst setting up a small network with short cables is a simple task, as the network size grows, you can run into problems with data corruption that result in lots of retries and delays. Sometimes extending the network can “push it over the edge, ” and it will stop working altogether.
To help you set up a reliable network, here are some tips to follow.
#1 – Use a high-quality cable and a simple linear layout
Ensure that you are using a twisted pair, high-quality cable. Ideally, the cable should be shielded, with the shield connected to ground at one end. Campbell Scientific offers a suitable cable (CABLE2TP), but you can also use existing structured wiring such as CAT5/6 cable. Whichever type of cable you use, make sure that the A/B wires used for the data connection are from the same pair of conductors in the cable. Also, it is best to design a network with a linear daisy chain of devices rather than a star arrangement.
#2 – Check the resistive/reference ground connections
Make sure that all of the resistive/reference ground terminals between your MD485 interfaces are connected together using an extra conductor in the cable. Prior to doing this, you can check that the voltage difference between the grounds of the different MD485 interfaces are not too large. If the voltage differences are too large, this can prevent communication and could even be hazardous.
To check the voltage difference, disconnect the ground-referencing wire temporarily and then use a voltmeter to measure the voltage between the end of the wire (connected to the rest of the network) and the MD485 reference ground. If the potential is more than a few volts, review the grounding of the different systems. If the potentials cannot be brought closer, consider adding a third-party RS-485 isolating repeater.