Siemens cm1241 modbus rtu

[Rahul3005]

Dear Members ,

I am using a S7-1200 with a communication module CM-1241 , I want to read / write data from few MODBUS devices like PH meter , Density meter etc.

For every MODBUS devices , i have given a unique a node address , and same communication parameters like (baud , stop bit , parity).




I have did the connection as mentioned above in the photo , i have daisychained all modbus devices , I am getting + and -ve cable on my panel.
I have connected +ve to PIN 3 and -ve to PIN 8 and GND to PIN 5 (optional).

Apart from this connection , is there anything more which i need to connect , i heard we have to use terminal resistors in some cases , is using terminal resistors necessary ? also please can u please give me details of communication configuration which i have to do in s7-1200 PLC ?

Thanks and Regards,
Rahul P

 

[JesperMP]

Termination resistors are used in all RS485 networks. So yes.

Sample project S7-1200 with CM1241 to multiple Modbus slaves:
https://support.industry.siemens.com/cs/ww/en/view/47756141

 

[Rahul3005]

Is termination resistor compulsory ? for siemens cm1241 ?

 

[JesperMP]

For RS485 there is to the best of my knowledge always termination resistors.
You may omit the termination resistors, and it may work, especially at lower baudrates, but the resistance to noise is degraded.
Why even consider it ?

 

[Rahul3005]

Thanks for the reply , can u share me the photo of how to connect termination resistors and is my connection correct ?

 

[kalabdel]

Thanks for the reply , can u share me the photo of how to connect termination resistors and is my connection correct ?

I believe wiring information is in the S7-1200 manual

 

[kalabdel]

I believe wiring information is in the S7-1200 manual

The resistor values vary quite a bit depending on the manual/example you read. I use 12Ohm on V20 and they work fine but I've read 220Ohm and 330Ohm in two different Siemens manuals. It also states that those recommendations are for cable length longer than 50 meter, I terminate them anyway.



I would go with this link (also see attached image), it shows required resistors to bias and terminate D-Sub.




Read1 ( this is from my Mega, couldn't find the file on the net)

Read2