Configuring The Communication Module - Siemens SIMATIC S7 System Manual

You must connect the communication interface of the CM 1241 RS232 module to the RS232
interface of the PC, which is normally COM1. Because both of these ports are Data Terminal
Equipment (DTE), you must switch the receive and transmit pins (2 and 3) when connecting
the two ports, which you can accomplish by either of the following methods:
● Use a NULL modem adapter to swap pins 2 and 3 together with a standard RS232 cable.
● Use a NULL modem cable, which already has pins 2 and 3 swapped. You can usually
12.3.5.1

Configuring the communication module

You can configure the CM 1241 from the Device configuration in STEP 7 or with user
program instructions. This example uses the Device configuration method.
● Port configuration: Click the communication port of the CM module from the Device
● Transmit message configuration: Accept the default for transmit message configuration.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
identify a NULL modem cable as one with two female 9-pin D connector ends.
configuration, and configure the port as shown:
Note
The configuration settings for "Operating mode" and "Receive line initial state" are only
applicable for the CM 1241 (RS422/RS485) module. The other CM 1241 modules do not
have these port configuration settings.
No break is to be sent at message start.
105BCommunication processor
12.3 Point-to-Point (PtP) communication
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