Eberline ESP-1 - Page 48

4-6
Port
PO,
Al,
is
the
address-data
bus.
It
is
bidirectional
(input
and
output).
Port
P2,
A1,
ошрш
the
upper
eight
bits
of
the
address.
On
a
typical
program
step:
(1)
The
address
(program
counter)
is
output
at
PO
(low
byte)
and
P2
(high
byte).
(2)
ALE
(address
latch
enable)
is
asserted
and
latches
the
address
low
byte
in
АЗ.
(3)
PSEN
(program
store
enable)
is
asserted,
enabling
the
addressed
byte
to
be
input
to
the
computer
via
PO,
(4) The program step is executed.
The
program
is
stored
in
A2.
On
later
versions
of
the
ESP-1,
the
program
is
contained
within
the
microcomputer
chip,
which
eliminates
A2
and
A3.
ESPI.MAN/February
1992