| 摘要 |
PURPOSE:To easily rewrite a control program and to prevent the contents of the control program from being destroyed by inputting the AND of the output signal of a reset set flip-flop and a writing control signal sent from a central processing unit as the writing control signal of a memory. CONSTITUTION:A writing control circuit 6 is composed of an input/output port (I/O port) 7, a reset set flip/flop 8 and an AND gate 9. The flip-flop 8 is set when the output signal of a reset circuit 1 is turned on and it is reset when a CPU 2 sends a writing ending signal through the I/O port 7. The AND gate 9 makes the AND of a writing control signal WR sent from the CPU 2 and the output signal of the flip-flop 8 and the product is obtained as the writing control signal of a RAM 10. Thus, the control program is easily rewritten and even when the CPU executes a run-away, the contents is prevented from being destroyed.
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