Interfacing Signals - RCA COSMAC Operator's Manual

THE MODULE-ENABLE PHILOSOPHY AND TWO-LEVEL I/O (cont'd)
be used to activate the currently-enabled second-level I/O
device, with 60 reserved to select it. These latter devices
would be enabled by a second-level aecoder. For further
information, refer to p.53 of the COSMAC Microprocessor
Manual, where logic implementation diagrams are given.
(In the Microkit, one would use SI¢O-P as the "device select
clock" to enable the 8-bit I/O Device Select Register shown
there in Figure 14).
INTERFACING SIGNALS
User devices can be interfaced to the Microkit with signals
available at the I/O BUS (card position 21) and from the
I/O Decoder. There are seven sink signals which are sensed
by the CPU, namely, EFI-N, EF2-N, EF3-N, EF4-N, DMAIN-N,
DMA¢UT-N and INTERRUPT-N. These signals are pulled high
with 22Kn resistors on the CPU board, and are brought to
the I/O Bus. Control electronics transmitting 'these signals
should use a transmission gate which pulls the signals low
so that when no output is present the device appears as an
open circuit. Thus several devices may be wire-or'd to
these lines.
There are eight output data lines ¢TDO-N to
~TD7-N
which may
be connected to user devices. Data here is valid at TPB
of the I/O execution cycle. When an OUT N instruction
(machine code 6N, O<N<7) is executed, the I/O decoder sends
out a decoded signaT,-SI¢N-P, at TPB of the I/O execution
cycle. This
SI~~-P
signal may be used directly to strobe
a latch* which will hold the data appearing at ¢TDO-N to
~TD7-N.
There are eight input data lines, INDO-N to IND7-N, which
may be connected to user device data lines. The data is
written to memory during ~~R-PI of the I/O instruction
execution cycle. Eight decoded signals
I~8-P
to I¢F-P,
lasting one machine cycle, can be used to close transmission
gates connecting device data lines to INDO-N to IND7-N.
Certain other signals, which may be useful for added devices,
are also brought to the I/O Bus. 16-P indicates that some
I/O instruction is being executed. S2-P indicates that
the processor is responding to a D~~ request. S3-P in-
dicates that the processor is responding to an interrupt.
* TPB and the SI~-P signals are the shortest duration
pulses generated in the Microkit, each approximately
250 nano sec wide. This may be too short for certain
COSMOS latches at V
nn
=5V. As long as the leading edge
of TPB is used to strobe data, the strobe can be widened,
if necessary, to the leading edge of the following TPA.
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