Sharp MZ-80B Owner's Manual page 130

121
7.0 APPLICATIONS
7.1 EXTENDING THE INTERRUPT DAISY CHAIN
Without any external logic, a maximum of four Z-80A-PIO devices may be daisy chained into a priority interrupt
structure . This limitation is required so that the interrupt enable status (IEO) ripples through the entire chain between
the beginning of M 1, and the beginning of IORQ during an interrupt acknowledge cycle. Since the interrupt enable
status cannot change during M 1, the vector address returned to the CPU is assured to be from the highest priority
device which requested an interrupt.
If more than four PIO devices must be accommodated, a "look-ahead" structure may be used as shown in Figure
7.0-1 . With this technique more than thirty PIO's may be chained together using standard TTL logic.
DATA BU.S
FIGURE 7.0-1
A METHOD OF EXTENDING THE INTERRUPT PRIORITY DAISY CHAIN
7.2 1/0 DEVICE INTERFACE
In this example, the Z-80A-PIO is connected to an I/0 terminal device which communicates over an 8 bit parallel
bidirectional data bus as illustrated in Figure 7.0-2. Mode 2 operation (bidirectional) is selected by sending the follow-
ing control word to Port A:
0 X X
1
Mode Control