RCA COSMAC Operator's Manual page 61

DMA INPUT
Assuming only one device needs the DMA Input port, the
following arrangement is possible:
JL
BYTE FROM
DEVICE
CK FROM
EVICE
52-PI
•
MICROKIT
_ DMAIN-N
R
C
CLO
Q
~
D
~
INTERFACE
D
TRANS-
IV
oo
INDO-N TO IND7- N
MISSION
GATES(8)
-
(8)
....
(8)
Fig.16: DMA Input Example
(See Figure 15 (p.ss) of the Microprocessor Manual for a
corresponding circuit.) If this were the only uevice
attached to the IND bus, then the eight transmission
gates shown can be eliminated. Note that this arrangement
will work as shown even if a DMA Output port is also im-
plemented, because the Bus Separator on the I/O side
prevents the IND bus from connecting to the two-way system
data Bus, even though an S2-Pl in response to an Output
request enables the Input gates shown above. Expansion
of DMA Input to serve more devices will require aduitional
transmission gates for data, added selection logic (in
conjunction with S2) to enable the proper data transmission
gates, and a technique to logically OR the uevice clocks.
Note that the DMAIN latch may be shared among multiple
devices.
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