Siemens ET200SP + Modbus RTU, termination resistors?

[boneless]

Hi all,


Happy belated turkey day!


I have an ET200SP PLC with serial unit. We read out date from two energy meters using modbus RTU. We run a cable from the RTU unit terminals, from there a cable goes to each of the meters. Technically, we have a star network. We use no resistors currently.



We are experiencing drops to one of the devices. And I think we may need termination resistors here.



My question, where to put these? At the two meters, AND the PLC end?


Thanks guys!

 

[JesperMP]

RS232, RS422 or RS485 ?
Distance ?
Baudrate ?
And star does sound problematic in any case.

 

[boneless]

Hi Jesper,


RS485, baudrate 19200. Distance not sure, but I imagine 10-20 meters for each device.



Do you recon we should put the two meters in series, or rewire our cabinet such that both cables terminate directly to the CP?


Thanks!

 

[JesperMP]

https://en.wikipedia.org/wiki/RS-485

I would daisychain the connections.
And I would add termination resistors at the ends of the daisychain.
If this is half-duplex I would use Profibus cables and connectors.

 

[boneless]

Thanks Jesper, I will check it out.


Can't use Profibus plugs unfortunately, all devices are just screw terminals.

 

[danw]

Do you reckon we should put the two meters in series?

Yes.