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Unit Interface
The interface between the I/O board and the MPU board is described below.
I/O board
Interface circuit
Data is sent and received between this board and the MPU board by a synchronous
serial transmission method, through the differential input / output RS–422 interface
circuit, based on the 12.288MHz clock signal created by the system clock on the
MPU board.
Transmission format
Transmission bits are 2 start bits, 8 address bits, 2 dummy bits, 16 data bits, 1 parity
bit, 1 stop bit, totaling 30 bits. Read/write information is also included in the address
information.
Transmission procedures
When the MPU board sends the start bits as a result of action by the CPU, the I/O
board starts sending data to the MPU as soon as the start bits are detected.
Completion of transmission is determined by the MPU.
The DTACK signal is returned to the CPU by the serial control unit of the I/O board.
In the event of a transmission error, data is re–transmitted from the MPU, and this is
repeated until there are no errors. If DTACK is not returned after 256 clock signals
following CPU access, a bus error occurs.
6–12
Electrical and Wire Data
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