FINS messages are composed by a header and a data portion. The header contains, among other information, data about source and destination. Data portion contains command codes and parameters for optional commands. Particularly interesting here are the source and destination parameters in the header, described next:

•DNA: Address of a destination network

•DA1: Number of a destination node

•DA2: Address of a destination module

•SNA: Address of a source network

•SA1: Number of a source node

•SA2: Address of a source module

This Driver always configures DA2 or SA2 parameters as 0 (zero). This means that communication must be performed between the host computer and the destination node of the CPU module. This Driver also configures SNA and SA1 parameters as 0 (zero), indicating that communication must be performed through the Host Link port of the interface device. The interface device, in this case PLC 1, reconfigures SNA and SA1 parameters as needed, when forwarding messages to other devices.