Connecting Modbus Master to Siemens S7-300


Thread Starter


I am trying to figure out a method of connecting a Siemens S7-300 PLC to a Modbus Master device. I'm told that the PLC can communicate either Industrial Ethernet or Profibus-DP (among a few others). Is there some type of gateway device that would allow this type of connection?

Daniel Chartier

Hello, SLittle;
you can use a CP341 (S7-300 serial communications processor)with a loadable driver (this means Siemens will gladly sell you the driver) to become either a Modbus master or a Modbus slave.

I have set them both up with the S7-400 and have gotten good results. Just make sure cabling issues are looked into competently; programming examples available on the Siemens website are quite adequate for set-up and programming.

Hope this helps,
Daniel Chartier
Call your Siemens Rep. and ask them about the CP341, it is a module that goes into the S7300 rack for use as a modbus slave.
The standard solution for the connection of a Siemens S7 300 series PLC with a Modbus Master is through a point to point connection using Siemens communication processor CP 341 installed with a modbus driver which can be downloaded from Siemens's web site. The S7 300 PLC would be the slave.
Siemens has a communication card, I believe CP341, that can be configured as a Modbus master. You then interface with this card using Profibus DP. You will need to write a program using ladder and configure a data block to send and receive Modbus messages. I did a project few years back with a S7400 and GE Modbus Concentrator. It worked very well. Please contact me at [email protected] if you need more info.

Bob Peterson

There is a loadable FB for Modbus communication available that allows the normal serial ports on the pLc to communicate via Modbus. See your favorite Siemens distributor for details.