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I have to communicate with a device. There are A,B,C Terminals.

A and B are standard. But What is The C Terminal? Is it ground?

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    \$\begingroup\$ It's the rocket launch signal. Seriously, do you think we are clairvoyants here? \$\endgroup\$ – Eugene Sh. Jul 6 '17 at 13:55
  • \$\begingroup\$ Which device? Name? Brand? Model? Try measuring resistance from C to device ground or chassis. \$\endgroup\$ – filo Jul 6 '17 at 13:55
  • \$\begingroup\$ The A and B terms were already enough of stupid signal names... so I'd ask whoever invented C what they were smoking. Instead, give the signals rational names. Tx+, Tx-, Rx+, Rx-, GND, Shield comes to mind. \$\endgroup\$ – Lundin Jul 6 '17 at 14:22
  • \$\begingroup\$ Modbus specifies a protocol. You didn't even mention what kind of physical connection you are using, but as Andy pointed out its probably RS485. \$\endgroup\$ – Rev1.0 Jul 6 '17 at 14:23
  • \$\begingroup\$ @Lundin: Actually more like D+,D- and GND would be appropriate for a 3 wire RS485. At least USB got it right. \$\endgroup\$ – Rev1.0 Jul 6 '17 at 14:26
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"C" is usually ground in RS485 configurations that need ground: -

enter image description here

And, given that MODBUS uses (or can use) RS485: -

enter image description here

I suspect "C" means ground for MODBUS too. I don't think the "C" designator is used in MODBUS TCP.

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In this context C means Common. It is to limit the common mode voltage if the interfaces are not grounded.

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    \$\begingroup\$ If I found a terminal connector on some PLC called nothing but "C", I'd be very careful about making such assumptions. \$\endgroup\$ – Lundin Jul 6 '17 at 14:24

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