Vector 4 - Page 99

EXHIBIT I-17 (A) PORT SIGNAL DESCRIPTIONS
PORT DECODER OPERATION
Port Decoder 0
The Port Decoder 0 selects ports between 00H and 0FH. The Port Decoders output
signal Selects a particular subsystem. Since some subsystems consist of single chips
(e.g. USART = Keyboard Interface Subsystem) this particular function actually acts
as a Chip Select signal.
When this occurs the LSB (Least Significant bit of 7-bit port address: ZAO0)
simultaneously selects the particular function at that chip. This arrangement allows
one chip select signal to operate as a port address gnd enable one of fwo ports
(See picture below).
The 8255 PPI has four ports. Therefore this chip uses fwo Port Decoder 0 Output
Signals (PO89, POAB) combined at an AND gate to control its active LOW Chip
Select pin.
Port Decoder 1
" This decoder selects ports between 10H and 1FH. This is accomplished using the
protocol given in the Port Decoder 0 description.
Port
.
Decoder
——
P001—p»1
CS
0
r‘A
[
ZA0
|
C/D
7
!
0
|ojojo]ololo]o]0]
USART
from
the
Port
00H
Keyboard
Interface
Subsystem
Port
L
|
Decoder
——P001—»
CS
)\
[
ZAOQ
P
C/D
7
"o
l0jlojojojolo]o]f 1]
Port 01H
I 5-8