112
lEI Interrupt Enable In (input, active high)
This signal
is
used
to
form a priority interrupt daisy chain when more than one interrupt driven device
is
being used. A high level
on
this pin indicates
that
no other devices
of
higher priority are being serviced by a
CPU interrupt service routine.
lEO Interrupt Enable
Out
(output,
active high)
The
lEO signal
is
the other signal required to form a daisy chain priority scheme.
It
is
high only
if
lEI
is
high
and the
CPU
is
not
servicing an interrupt from this PIO. Thus this signal blocks lower priority devices from
interrupting while a higher priority device
is
being serviced
by
its CPU interrupt service routine.
INT
Interrupt
Request
(output,
open drain, active low)
When
INTis
active the Z-80A-PIO
is
requesting an interrupt from the Z-80A-CPU.
A
0
- A
7
Port
A Bus (bidirectional, tristate)
This 8 bit bus
is
used
to
transfer data and/or status or control information between Port A
of
the Z-80A-PIO
and a peripheral device. A
0
is
the least significant bit
of
the Port A data bus.
A STB Port A
Stroh~
Pulse from Peripheral Device (input, active low)
1)
Output
mode: The positive edge
of
this strobe
is
issued by the peripheral
to
acknowledge the receipt
of
data made available by the PIO.
2)
Input
mode: The strobe
is
issued
by
the peripheral
to
load data from the peripheral into the Port A
input register. Data
is
loaded into the PIO when this signal
is
active.
3) Bidirectional mode: When this signal
is
active, data from the
Port
A
output
register
is
gated
onto
Port
A bidirectional data bus. The positive edge
of
the strobe acknowledges the receipt
of
the
data.
4) Control mode: The strobe
is
inhibited internally.
A RDY Register A Ready
(output,
active high)
The meaning
of
this signal depends on the mode
of
operation selected for
Port
A
as
follows:
1)
Output
mode: This signal goes active
to
indicate
that
the Port A
output
register has been loaded and
the peripheral data bus
is
stable and ready for transfer to the peripheral device.
2)
Input
mode: This signal
is
active when the
Port
A input register
is
empty and
is
ready
to
accept data
from the peripheral device.
3) Bidirectional mode: This signal
is
active when data
is
available in the Port A
output
register for trans-
fer to the peripheral device . In this mode data
is
not
placed
on
the Port A data bus unless A STB
is
active.
4) Control mode: This signal
is
disabled and forced to a low state.
B
0
- B
7
Port B Bus (bidirectional, tristate)
This 8 bit bus
is
used to transfer data and/or status or control information between
Port
B
of
the PIO and a
peripheral device. The Port B data bus
is
capable
of
supplying 1.5ma@ l.SV to drive Darlington transistors.
B
0
is
the least significant bit
of
the bus.
B STB Port B Strobe Pulse from Peripheral Device
(input
, active low)
The meaning
of
this signal
is
similar to
that
of
A STB with the following exception:
In the Port A bidirectional mode this signal strobes data from the peripheral device into the Port A
input register.
B RDY Register B Ready
(output,
active high)
The meaning
of
this signal
is
similar
to
that
of
A Ready with the following exception:
In the
Port
A bidirectional mode this signal
is
high when the Port A input register
is
empty
and ready
to accept data from the peripheral device.
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