Direct Answer You do not NEED a ground for RS485 networks. Medium Answer The measurement is the voltage difference between a pair of wires. If the voltage difference is too large, or if one leg is out of range of the receiver's decoders, the device will not understand what information is being modulated on the line. In some rare cases, adding a common 0V connection (not technically a GND) between control panels can solve the problem, but IMO this approach should be approached with extreme caution. Long Answer All senders and receivers must modulate/demodulate data in/out of the serial line. Each devices' manufacturer has chosen some chips for this purpose, but they may not all have the same limitations for what voltage range is acceptable. Adding to this, not every power supply will generate equal voltages. In the following example, some device on the network has a power supply that's running above the ground state (36V & 12V compared to earth ground). Everything seems fine, there's 24V between +/- on the power supply, and the panel does its job without any complaints. Another device on that network has been rigorously designed and tested and adheres to every code and standard. It's power supply is spot on (24V & 0V compared to earth ground). This panel too does its job without any complaints. But both panels have very restrictive serial chips, and they both expect a very narrow range of voltages on the serial line. Their specifications call for a common mode voltage between -7 to +12V, with a minimum bias of ±1.5V. When these two devices attempt to talk to each other, the first device modulates high to +5V and low to -5V COMPARED TO ITS REFERENCE 0V. This is clearly within the common mode voltage, and with sufficient bias to adhere to the standard. The problem is that the second panel interprets these voltages TO ITS OWN REFERENCE 0V. Which means that +5V, which is actually +17V compared to earth ground, is WAY outside the common mode voltage allowed, and the serial chip throws errors. In this case, if the two power suppliers were bound together (same 0V reference, at whatever value above earth ground it falls to) then they would communicate without issue. Binding the two power supplies in this way can cause a number of issues that I'm not going to get into because the PTSD is too intense. I'll leave it to someone else to escort that cat out of the bag. Answer from itzsnitz on reddit.com