Colm, I had a customer ask this question recently. Can you help?
Yes, I get asked this question quite a bit. There are a number of factors which influence this.
Let’s consider a couple of cases
1. Equipment where each interface has a separate port (e.g. CAN, RS-485, RS-232/ USB)
a. The ports can be classified into two: field buses & diagnostic/ firmware upload ports.
b. If the field-buses can be connected at the same time (e.g. the equipment can be connected to a CANOpen network and Profibus network concurrently), then it is possible these could be on two different ground references/ lines/ buildings etc. In this case the ports should be isolated from each other.
c. If the field-buses can only be used in an either/ or situation, then they can share the same isolated ground.
d. It is better practice for the diagnostic/ firmware upload ports to have separate grounds than the fieldbus ports, to ensure that any events on the field bus side don’t damage the technician equipment for example (or worse! – the technician).
2. Equipment where multiple interfaces are switched onto one port/ socket
a. No more than one interface will be exposed to the external environment at any time => same ground can be used.
b. Best practice to have any debug/ technician ports on separate isolated interface.
3. Equipment where interface modules are swapped in and out depending on the customer configuration required
a. Each of these will have their own isolated ground as the modules are typically design for singular use.
Thanks Colm. The RS-232 appears as the physical layer transceiver for both fieldbusses and diagnostic ports (ie: one implementation of Modbus). What do you advise for this?
That's right Steve. Modbus can be implemented using RS-232 or RS-485 for asynchronous serial transmission (other options are fibre and radio). Of course there's also the TCP/IP implementation and Modbus+ for high speed links.
If RS-232 is used as a fieldbus and there are other isolated fiedbuses requied in the design, then we're back to whehter the ports can be used at the same time: in which case they need seperate isolated grounds; or if only one is used at a time, then it's OK to use the same plane.
The ground plane of the diagnostic port should not be on the same ground plane as the fieldbus side ground , if isolation is required in the design for the reasons stated above.
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