Z80 Datasheet, PDF (81/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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BAI
Bus Acknowledge In (input, active Low). Signals that the system buses
have been released for DMA control.
BAO
Bus Acknowledge Out (output, active Low). In multiple-DMA configura-
tions, this pin signals that the CPU has relinquished control of the bus. BAI
and BAO form a daisy-chain for multiple DMA priority resolution over bus
control. Unlike the interrupt daisy-chain formed with the IEI and IEO lines,
this chain does not allow preemption of control by a high-priority DMA
when a lower-priority DMA is already bus master. The DMA that has the
bus is always allowed to finish, regardless of its priority in the chain.
BUSREQ
Bus Request (bidirectional, active Low, open-drain). As an output, this pin
sends requests for control of the system address bus, data bus, and control
bus to the CPU. As an input when multiple DMAs are strung together in a
priority daisy-chain through BAI and BAO, this pin senses when another
DMA has requested the buses and causes this DMA to delay its bus request
until the first DMA is finished. Because this bidirectional pin allows simul-
taneous bidirectional signals with no means of control, no buffers come
between this DMA and other DMAs. There can, however, be buffers
between it and the CPU because it is unidirectional into the CPU. A 1.8
Kohms pull-up resistor is typically connected to this pin.
CE/WAIT
Chip Enable and Wait (input, active Low). Normally, this functions only as
a CE line, but it can also be programmed to serve as a WAIT function. As a
CE line from the CPU, this pin becomes active when IORQ is active and
the I/O port address (up to 16 bits) on the system address bus is the DMA’s
address, thereby allowing control bytes to be written from the CPU to the
DMA. As a WAIT line from memory or I/O devices, after the DMA has
received a bus acknowledge (BUSACK) from the CPU, this pin causes wait
UM008101-0601
Direct Memory Access