The 74HCxxx is the High speed CMOS version of 7400 series digital IC's
^ ^
They are totally CMOS devices, with the output drivers consisting of a
push-pull pair of complimentary MOSFET's. The logic output levels will
be very close to the Vcc input voltage.
In the 38-S ... the 74HC240 is operated off +8v and ground for the
voltage references, thus the logic outputs will be 0v (LO) and a bit
less than +8v for a HI level. Also, since the output stages are
complimentary MOSFET's, they can be placed in parallel, which they are
in the 38-S for the output PA.
The 74HCTxxx is High speed CMOS, TTL compatible versions.
^ ^ ^
TTL is powered off of +5v Vcc (+/- 0.2v), NOT +8v. For TTL compatability,
logic outputs must never exceed 5.0v. 5v is the HI for TTL. In the
74HCT's, there are clamping diodes across the output MOSFET's to
ensure the output never exceeds +5v.
In the 38S ... this presents two problems. First, driving the HCT240
off of +8v exceeds its TTL rating. Secondly, the clamping diodes across
the MOSFET drivers will limit the output drive to +5v AND these clamping
diodes interfere with the pure CMOS concept of placing the drivers in
parallel. When the output exceeds 5v, you'd have effectively 4 diodes
in parallel now conducting, placing nearly a short circuit across the
PA output times 4.
When I built the prototype, I socketized the HC240 just for the reason
of experimenting with different manufacturers of HC240's. I also plugged
in a 74HCT240 as an experiment, pretty sure it would destroy itself, but
to ensure it wouldn't destroy anything else (like the voltage regulator),
in case a HCT240 was used inadvertently. Well, the HCT240 creamed itself
very quickly after keydown. No smoke, fire or brimstone ... just
quietly rolled over and died after only a few heartbeats.
GL, Paul NA5N
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