Does ISOLATED RS485 require a ground?

[SiliconWizard]

If the link is properly isolated, there is no common mode shared by both sides. Each side will have its own. Each side has to be ground referenced (you need a path to ground on the receiver's side), but with its own ground. Of course if there is a shared ground, then there is no isolation.

Still requires proper protection of course.

[Simon]

All transceivers I have looked at talk about common mode voltage and voltage levels between the but pins but not referenced to ground. If you have an isolated transceiver at one end surely it's ground reference effectively becomes the non isolated ground at the other end. It can take on it's reference because it is isolated, that's the point of the isolation. It's floating so when it get's referenced somewhere else it won't mind.

Is that the difference here? isolation at both ends and isolation at one end?

[T3sl4co1l]

Right, for the various caveats "what is ground, anyway?", as has been covered so far (i.e., at AC, for various styles of termination).

For example, the line itself may have capacitance to surroundings, or may be required to be shielded/grounded to facility or apparatus ground, and therefore pick up CM voltage with respect to one or both transceivers.  In that case, double isolation is preferred (or required even).  The same goes for the isolation barrier itself, which has to have some capacitance (no isolation is ideal).

Tim

[Simon]

OK so what you are saying is what I said earlier. The need for a ground is to over-ride any other accidental and inappropriate ground that would be formed without an explicit one. Here I am not talking about 1000's of volts, the two sides would actually have a common ground in a reasonable size cable but I do not wish to risk current spikes in the power draw upsetting the coms link.

The supply is around 24V so the ground cannot climb above 12V anyway. I will include plenty of bypass capacitance in the device and a choke so current spikes here are very unlikely so I am actually wondering if I want to worry about isolation. I have increased the supply voltage and reduced the current draw compared to where we started which also helps.