Signal board (SB): Only a few additional I/O points can be provided for the CPU. SB is installed on the front end of the CPU. The signal board is unique to Siemens S7-1200 PLC.
Signal module (SM): It can provide additional digital or analog I/O points, connected to the right side of the CPU.
Communication module (CM): Provide additional communication ports (RS232 or RS485) for the CPU. Connect to the left side of the CPU.