Every T100MD+ PLC has built-in TBASIC commands (such as NETCMD$, READMODBUS, WRITEMODBUS) for communicating with another PLC's or 3rd party MODBUS devices. This makes it possible for one or more T100MD+ to be used as slave I/O boards for a master T100MD+ PLC. Hence more analog or special I/Os than are available on a standard T100MD+ PLC are accesable for complex control tasks, or to provide remote communications to reduce the amount of wiring required.

Example, in the arrangement shown below the master PLC could have access to 24 analog I/Os, 6 PWMs, 6 stepper controllers and 6 encoder inputs! 

Each board can also have up to 2 expander boards connected to add more digital I/O.  Relay outputs an opto-coupled inputs are available on the EXP1616R Board.  Regular I/O expansion is available with the EXP4040 Board.

Typically the Master unit would be located near the driver and have an MD-HMI display attached, Slave 1 located in the cargo bay where your tanks are and slave 2 located at the rear of the coach for engine monitoring tasks etc.