is channel ground separated on NI 9871

[Steffen01]

hello,

on the NI 9871 are 4 R485 channels. Does each one has its own ground or are they connected? Reason I am asking, if I am connecting 2 modbus cables, the NI 9871 would be the centre / star point. Then it would be better to connect both modbus grounds to the same earth.

[oscarfonseca]

Steffen01,

The four ports have a common ground, which goes to the external power supply you connect to the module.

Regards,

[Steffen01]

Hi Oscar,

thanks. I measured to double check.

Now just have to think about proper grounding. One modbus cable controls a VSD drive and is earthed at that end. Another modbus cable is connected to a PLC. So now that PLC earth goes through the NI9871 to the pump VSD.

So I better ground things on the NI9871, because this would be the centre?

Or maybe better ground the modbus cable. Then cconnect this through a 1 ohm resistor to the NI9871? Soon is maintenance month, I better check, where the shield of that modbus cable is connected to. Separate shield from ground and connect to earth on the VSD drive. Ground power supply on the cRIO side.

[oscarfonseca]

I usually refer to this document when in doubt on how to properly ground a signal: Field Wiring and Noise Considerations for Analog Signals .

I also recommend ensuring the signal is within the module's specification before powering up for the first time. Maybe you can try connecting one and see its behavior and then add the rest.

All the best,

[Steffen01]

I made a new shielded cable and connected the shied to earth on the sensor side. Now the modbus signal drops out constantly. When I connected the shield to earth on the cRIO end, it dropped out just a little bit. But without shield its stable.Now thats scary. We run VSDs, dirty earth I guess. Maybe an earth stake helps