Z80 Datasheet, PDF (151/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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CPU has relinquished the bus. Therefore, if this DMA is bus master, it
samples the WAIT signal for these requests. A simple 2-input multiplexer
steers the CE/WAIT signals as depicted in Figure 50. Using BUSACK
assumes that there is only one DMA. In systems with three or more
possible bus masters, BAl active and BAO inactive identify the master.
Simultaneous Transfers
The highest-speed DMA transfer method is the simultaneous transfer
method, or flyby method. This requires some external hardware to
generate simultaneous read- and write-control signals to the source and
destination ports.
Because the address bus is used for memory address, only transfers
between I/O and memory can be accomplished directly when the I/O port
selection is performed by hard wiring. The DMA is put into search mode,
and a circuit, such as that illustrated in Figure 51, generates separate simul-
taneous read- and write-control signals, which may be ORed into the read-
and write-control paths at memory and I/O. Figure 52 depicts such an
arrangement. This arrangement allows both the CPU and DMA access to
the I/O peripheral. (If the peripheral communicates only through DMA, it
only needs to use the IORD and IOWR signals.)
Pay careful attention to access, setup, and hold times in this mode. Because
the DMA is programmed to do searching, the MWR and IOWR signals are
derived from the DMA RD signal and mimic its timing. This does not cause
a problem for write operations, which are trailing edge activated. To make
MWR appear more like a CPU or DMA write cycle signal, the circuit of
Figure 50 may be used to delay the leading edge of MWR until after the
falling edge in T2. The programmable variable timing features of the DMA
may be useful, too.
UM008101-0601
Direct Memory Access