Hello

I see, that this probles is quite old, but I hope You have a solution. I have the exact same problem:

My CP 341-1CH02 with modbus dungle is configured, no error, no SF light, but the RXD light is not flashing, while my Modbus MASTER system is sending requests. I am using two wire system.

Please any help, who had same problem and have a solution. 

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Split from CP341 as modbus slave to DCS .

Hi, thanks for fast reply

I do not see any problems in buffer (just information about states changing). I am addind info from diagnostic buffer

Oh, have done that at the very begining.

I even used 2 chanel osciloscope to see the transmision of my modbus and the voltage levels of line A and B looks all right, but still no light on the modul.

Do I need to put any program block in my program, or is it enough to set parameters in HW configuration? If I need any extra block, please tell me about it.

Thank You in advance.

Hello Kvazzy;

Could we have more detail on the Modbus master you are connecting to, and the cable you are using (a sketch would be helpful)?

You will find pinouts for the RS 422 and RS 485 connections on the 15-pin Sub-D socket of the CP 341-1CH02 in the CP 341 Modbus slave manual:

https://support.industry.siemens.com/cs/document/1218007/simatic-s7-300-s7-400-loadable-driver-for-point-to-point-cps%3A-modbus-protocol-rtu-format-s7-is-slave?dti=0&lc=en-FR

Connector of the CP 314-1CH02: 15-pin Sub-D socket

Pinout for RS-485:

R(A)- : pin 4

R(A)+ : pin 11

Signal GND: pin 8

What pins are connected to these on the Master's side?

Hope this helps,

Daniel Chartier

Hi again

I have messured signal levels on A and B lines with osciloscope one more time and have got some more information (photo added)

The problem is that signal on line B (+) does not go so low and signal on line A (-) does not go so high. As I understand RS 485 communication, the signal is detected when the signal on line (-) is higher than signal on line (+).

My master system at the moment is converter USB - RS485 and some modbus simulation software on my PC. With any other modbus slaves my converter works just fine. The protective resistors on my converter on A and B lines is only 10 ohms and voltage drop on it is about 1 volt, it indicates that the curent in communication line is about 100 mA (it is maximum curent for RS485 line)

If I power off the CP341 modul, then the signal goes to normal levels. So, as I understand, the modul CP341 goes to the transmiting state and hangs up there. And again, my question is, what blocks I need to add to my program and what configuration to set to get my modbus slave working? 

Thanks

I am usin RS485 line (two wires), HW configuration set to half duplex (2 wires), ModBus cable is 6ES7 902-3AB00-0AA0, cuted in 2 parts, one side is connected to the CP341 (DB15 connector) and the other side is conected to my converter.