Z80 Datasheet, PDF (216/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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When reading the PIO, the data returned to the CPU is composed of output
register data from port data lines assigned as outputs and input register data
from port data lines assigned as inputs. The input register contains data that
was present immediately prior to the falling edge of RD. See Figure 10.
An interrupt is generated if interrupts from the port are enabled and the
data on the port data lines satisfies the logical equation defined is not
generated until a change occurs in the status of the logical equation. A
Mode 3 interrupt is generated only when the result of a Mode 3 logical
operation changes from false to true. For example, Mode 3 logical equa-
tion is an OR function. An unmasked port data line becomes active and an
interrupt is requested. If a second unmasked port data line becomes active
concurrently with the first, a new interrupt is not requested because a
change in the result of the Mode 3 logical operation has not occurred.
If the result of a logical operation becomes true immediately prior to or
during M1 an interrupt is requested after the trailing edge of M1
Φ
Port
Data Bus
INT
IORQ
Data Match
Occurs Here
T₁ T₂ T_W* T₃
Data Word 1
Data Word 2
RD
D7–D0
*Timing Diagram Refers to Bit Mode Read
Data In
Figure 10. Control Mode (Mode 3) Timing
Data Word 1 Placed on Bus
UM008101-0601
Parallel Input/Output