PURPOSE: To improve the maintenance of a system by setting predetermined diagnostic information on an optional general line, and automatically diagnosing a scanner.

CONSTITUTION: When a signal distributor MSD for maintanance energizes a relay A, a scanner test unit part MTSCNTSTU for maintenance allows a relay AO to operate through a contact #0 of the relay A, and a scanner is held in readiness to send a signal "0" as an even bit. Then, line information on the equipment of the unit part MTSCNTSTU is sent out to a scanner to be diagnosed through X0∼X3, and Y0∼Y2. The scanner expands the signal received by a cable receiver CR at a decoder part DEC to read respective bits. They are read by a sense amplifier SA and sent back through a cable driver DC, and the sent-back information is checked to confirm the normalcy of the scanner.