Industrial Serial Communication - How to Use S7-1200 Communication Module CB1241

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In Siemens S7-1200 serial communication solutions, in addition to the previously mentioned CM1241 module, another method is to use CB1241 for serial communication. Some users may find it challenging to distinguish between CM and CB, so let's briefly clarify.

CM is the abbreviation for Communication Module, while CB is the abbreviation for Communication Board. Communication modules are installed on the rail, whereas communication boards are plugged into the CPU's slot, differing in physical appearance. As mentioned earlier, we discussed that the CM1241 communication module has three types supporting RS232, RS422, and RS485 electrical interfaces. On the other hand, the communication board has only one type: CB1241-RS485, supporting only the RS485 electrical interface, with order number 6ES7 241-1CH30-1XB0.    













CB1241-RS485 doesn't utilize the standard 9-pin serial port; instead, it employs terminal blocks (labeled: X20). The following image illustrates the comparison between X20 and the 9-pin RS485 interface



TA and TB are not mentioned here, as they are not pins in RS485. The meanings of each terminal in X20 are as follows:

  • M: Shield grounding
  • TA: Connection for terminal resistance
  • TRA: A (transmit/receive)
  • TRB: B (transmit/receive)
  • TB: Connection for terminal resistance
  • RTS: Request to Send

CB1241 has internal terminal resistance, and you can achieve the ON and OFF states of terminal resistance through wiring. When it is necessary to activate the terminal resistance, connect TRA to TA and connect TRB to TB, as shown in the following diagram:







In the absence of a need for terminal resistance, simply leave TA and TB unconnected:



In the hardware configuration of CB1241-RS485:


the wiring diagram for CB1241-RS485 with the communication partner is as follows





















































































The hardware configuration for CB1241 is not found in the "Communication module" section but rather in the "Communication boards" section. Click on "Point-to-Point," and you will be able to locate CB1241 (RS485).ws:


Once the configuration is complete, you can set the communication parameters such as baud rate, data bits, stop bits, etc., within the port settings. There's no need to configure the electrical interface since CB1241-RS485 exclusively supports the RS485 interface.


CB1241-RS485 is a straightforward and cost-effective serial communication solution for S7-1200. It differs from CM1241 only in hardware, with programming being identical. For software programming guidance, please refer to the previous article: "How to Use S7-1200 CM1241 Module for Industrial Serial Communication."