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4.1
ADDRESS DECODER
North Star HSIO-4
The Address Decoder converts I/O address and .
timing signals from the processor into a set
of signals for each of the four I/O ports.
The HSIO-4 board is selected by address bits
A4 - A7. These address bits are compared with
the address switches. If a match occurs, the
board is selected. Address bits AO - A3 select
functions within the board. Address bits A2
and A3 select one of the four I/O ports as
follows:
Al
A2
0
0
Selects Port A
Selects Port B
0
1
1
0
Selects Port C
Selects Port D
1
1
Address bits AO and A1 select the operation to
be performed by the selected port as follows:
M
A1
0
0
Write to Baud Rate Select Register
0
1
Write to Interrupt Mask Register
1
0
Write or read USART data
1
1
Write or read
57
USART control or status
User/Technical Manual
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