c) Configure the PSG directly as an I/O port. This is the approach I
have adopted. It is not too difficult to build, Inexpensive, plugs
staight into the NAS-BUS (absolutely no wires), very easy to progranme
and up to 124 (Heaven forbid) PSGs can be used with no modifications
to the NASCOM (except a IINK to the National Grid to drive them plus a
rather large mother-board).
The working design:
The circuit works by controlling the status of the BCl and BDIR inputs
on the PSG according to the following truth table –
|1||1||Latch resister address|
|1||0||Load register with data|
|0||1||Read resister contents|
IC 1 decodes the address bus to configure the PSG as ports 8 and 9
(address lines 8 to 15 are not used for port addressing on the NASCOM
2). Rearrange these inverters if alternative addresses are required
(but don’t use ports 0 to 7 as these are already dedicated on the main
If the predefined address is received then the output of IC 3 will go
low. Any I/O instruction will also push the IORQ line low (active).
These two signals are inverted (ICs 2a and 2b) and fed into the inputs
of ICs 4a and 4b. Address line A0 is then able, by using IC 2c, to
select either IC 4a or 4b, giving a low output. As the circuit stands,
addressing port 8 will give a low output for IC 4a, and port 9 a low
output for IC 4b.
If port 8 is addressed, then the inverted (IC 2d) output from IC 4a
raises the inputs of ICs 5e and 5d making their outputs low. These
signals are inverted (ICs 2e and 2f) and fed into the BC1 and BDIR
inputs of the PSG. As both will then be high, a resister latch
operation has been performed (see truth table at beginning), and the
contents of the data bus will select the appropriate resister. This
operation does not take into account the CPU RD and WR lines. The
programmer must ensure that “OUT” intructions only are used for port 8
(or else the CPU and the PSG will both read the data bus simultaneously
with no-one supplying the data). If required, the WR line could be
incorporated but it hardly seems worth the effort.
If port 9 is addressed the low output from IC 4b is NORed with the RD
and WR tines in ICs 5a and 5b respectively. For a read (“IN”or“INP”)
instruction, the output of IC 5a and hence the BC1 input, will so high
and the PSG will transfer the contents of the currently selected
register onto the data bus. For a write (“OUT”) instruction the same
will apply to IC 5b and the PSG BDIR input, allowing the PSG to load
the currently selected register from the data bus.
Lines BC2, A8 and A9 are additional PSG select signals but as their
status in this circuit needn’t alter, they are tied as show.
For 4 MHz operation, the clock is divided (IC 6) to provide the 2 Mhz
maximum the PSG can handle and is further divided (IC 7) and controlled
by the address enable output (IC 2d) and the I0RQ line (IC 2a) to give
a single positive-going pulse which is inverted (note open collector
inverter) to provide a WAIT state for the CPU, I have used 7472 JK
flip-flops here because I have a box full of them – a 7474 may be
preferable, the unused NAND gate (IC 4c) could be used to link the
address enable and the IORQ lines into the system to control the
pulses. If you are using the CPU at 2 MHz then forget about ICs 6 and 7
and the WAIT states – just feed the system clock into the PSG.