A programmable logic controller computer system is provided which achieves great flexibility in communication among the components of the system. The computer system includes a host programmable logic controller coupled by a field bus to a programmable bus interface unit. The programmable bus interface unit is located at a field station which includes a main bus to which a micro field processor and a plurality of I/O modules are coupled. The micro field processor is capable of locally processing I/O data from the I/O modules without involving the host PLC. A hand-held programmer is coupled to the programmable bus interface unit to permit the user to program an I/O map into the bus interface unit. The I/O map specifies mapping among the I/O modules, the micro field processor and the host programmable logic controllers. In this manner, communication among the I/O modules, the micro field processor and the host programmable logic controller is flexibly specified and controlled. Moreover, field bus traffic from the I/O modules back to the host programmable logic controller is significantly reduced.