PROGRAMMABLE CONTROLLER AND ITS MAINTENANCE METHOD

PROBLEM TO BE SOLVED: To attain the effective maintenance of a programmable controller(PLC) to perform the maintenance job of the PLC via a communication port which is prepared for an optional CPU module. SOLUTION: A communication sub-processor D2 controls a communication port P1 independently of a main processor D1. A shared memory C1 transfers data between both processors D1 and D2. A bus I/F circuit B1 is connected to a bus A1 which secures connection between the CPU modules 5 or communication modules. An optional module 5 can access the memory C1 owing to presence of the memory C1 and the circuit B1. Furthermore, it is possible to perform the maintenance job of a PLC via a communication port of an optional module 5 by making another module 5 to process the memory C1 as a kind of communication module.