An expandable, modular microcomputer system for an off-road vehicle having a controller which is operable to generate a plurality of independent pulse-width modulated control signals and a plurality of independent on/off control signals to a corresponding plurality of metal oxide silicone field effect transistors (MOSFETS). An RS-485 bi-directional serial communications interface enables the system to communicate with at least one other ingredient, external, controller. Thus, the control system can be modularly expanded to handle the needs of increasingly complex powershift transmissions. A watchdog circuit monitors operation of the controller and is supplied with a regulated voltage independent of the regulated voltage used to power the microcomputer. The watchdog can provide a reset signal to the microcomputer when spurious operation of the microcomputer is detected, as well as initiate operation of a back-up voltage supply circuit. The back-up voltage supply circuit is responsive to at least one analog input and operable to pulse-width modulate at least one of the MOSFETs to thereby control a clutch of a powershift transmission while operating in a "limp-home" mode. The system is particularly well-suited for controlling transmissions of heavy-duty off-road vehicles such as earth moving and other like vehicles, but can be readily adapted for use with transmissions of vehicles to be used to widely varying applications.