ZiLOG Z80 Handbook page 28

AO
Al
A2
A3
A4
AS
A6
ADDRESS A7
BUS AS
A9
A 10
All
A12
A13
A14
A15
D4
D5
D6
D7
30 25 BUSRQ I BUS
31 Jt CONTROL
32 23 BUAK
33
34 19 WHO 1
35 21 RD
1
MEMORY
36 22 WR OPERATION
37 28 RFSH
38
39
40 20
FUR INPUT/OUTPUT
1 Z-80
2 MICROPROCESSOR
3 27 Ml
4
5 26 RES
MISCELLANEOUS
24 WAIET T
14
15 18 HALT J
12
8 _
7 17 NMI
1 INTERRUPT
9
10 16 IN-T
1
INPUTS
13
6
0
11 Fe -+5V
29 F+----GND
Fig. 3-1. Z-80 interface signals.
edge) signal BUSAK. Signal BUSRQ is an active low signal that is
generated by an external device to gain control of the CPU busses.
During the time the external device has control of the busses, it will
probably perform a direct-memory access (DMA) operation. DMA
permits an external device to go directly to memory and transfer
data between memory and the device. The CPU must be "locked
out" during a DMA operation to avoid the conflict of the CPU re-
questing memory service at the same time and from the same mem-
ory location as an external device. When the external device brings
down (logic 0) the B-L77TFQ-, Bus
Bus Request signal, the CPU responds
with acknowledge signal BUSAK, Bus Acknowledge. J AK is an
active low output that signifies that the address bus, data bus, and
CPU output-control signals are now in the high-impedance state
and can be controlled by an external device for DMA operations.
MEMORY SIGNALS
There are four signals associated with memory operation, MREQ,
RD, WR, and RFSH. The first, MREQ, Memory Request, is a tri-
state active low signal indicating that the address bus holds a valid
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