| 摘要 |
PURPOSE:To prevent a user from rewriting a prescribed control program of a microprocessor by providing a control program allowed for rewrite by the user and a control program not allowed and providing a means inhibiting the rewrite of the unpermitted control program. CONSTITUTION:A master set 1 is provided with a random access memory (RAM)7 stored with a control program allowed for the user for rewrite and a RAM 8 stored with a control program to be rewritten by a manufacturer (person in charge of installation work). The user can rewrite a program in the RAM 7 via a key telephone set 2, a connector 3, a switch SW1 and a CPU 6 but cannot rewrite the program of the RAM 8 because of the presence of an AND gate 9. Thus, when the key telephone set 2 is connected to a connector 3, the program of the RAM 8 cannot be rewritten because of the presence of the AND gate 9. Thus, so long as the user does not understand it, the program of the RAM 8 is not rewritten via the key telephone set 2.
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