Philips pm2521 - Page 53

55
ROMs
The two
ROMs
are each
addressed by the
eight
address
lines AO to
A7 and
three
of the
program
control
lines
P20,
P21, P22.
When
addressed,
the
ROM
responds by
sending the
instruction
or
data
in the
location
addressed
on
the eight
lines of the
databus.
This data
output
function is
controlled by
the PSEN
(program
store
enable)
signal (pin
9)
sent by
the ^xP.
This is
applied to
NAND-gate
D1
921,
wired as
an
inverter
and
gives a
logic
"1"
on
NAND-gate
inputs
D1921-12
and D1921-5.
Signal P23
is a
chip
select
input via
inverter
D1
921,
which is
used to
select
either D1909
or D1920
via
input
pin 20.
-lOV -5V
Fig.
39.
Memory