As the RS232 was not intended as a bus system when devised, a number of elements must be added to the data frame in order to enable several modules to communicate through a single serial interface.
The data frame is followed by two bytes (group/ID) indicating the module to be targeted or the module that sent the response. Only the first three bits of the first byte are used. The second byte constitutes a unique module ID. => up to 255 different modules can be addressed. The first three bits of the first byte are encoded as follows:
The other statuses are not used.
 | The method of uniquely identifying a module with 11 bits has been adopted for the CAN protocol. |
In order to ensure reliable data transfer with RS232, a checksum (CRC16 => 2 bytes) of all data including group/ID, D-Len and Cmd is attached at the end of the data frame. An algorithm for the calculation of a CRC 16 checksum is included in the appendix. As RS232 is not a real bus system but can be wired like a bus system, there is a risk of data collision if several modules are simultaneously sending data to the master. Such collisions can however be easily detected and the necessary measures can be taken in order to clearly identify the status of all modules. If a large number of modules are operated on one "branch", it might be necessary to disable impulse message .