Provided is a CPU unit comprising: a first system bus I/O circuit part (11) which carries out signal input/output; an expanded function control circuit part (12) which carries out communication with an expanded function circuit part (22) which a target unit (3) comprises; and a switching part (13) which performs switching such that the first system bus I/O circuit part (11) or the expanded function control circuit part (12) is connected to a first signal line (B1) which is included in a system bus (B). When carrying out the communication with the expanded function circuit part (22) which the target unit (3) comprises the expanded function control circuit part (12) causes the switching part (13) to connect the expanded function control circuit part (12) with the first signal line (B1). In a configuration in which the expanded function part shares a common signal line with another configuration element it is possible for the expanded function part and the other configuration element to communicate without signals being mixed up even if the signals are changed by external environmental effects.
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