7 Jan
2005
7 Jan
'05
1:20 p.m.
On Fri, 7 Jan 2005 23:55:00 +0000 (GMT)
ard(a)p850ug1.demon.co.uk (Tony Duell) wrote:
Doesn't 4000-series CMOS logic operate pretty well at non-5 volt levels
if powered to such levels? I haven't used any in years, but all the
'building block' logic components are available, i.e. the 4049 and such.
A company I worked for made a muscle stim device out of a single 4000
series 'hex converter' using feedback to make multivibrators, crude
comparators, drivers, etc. out of the gates. It wasn't powered at 5
volts, the whole thing ran from a 9 volt battery. The logic thresholds
for 4000 CMOS are 1/3 and 2/3 of VCC, for whatever VCC you supply, if I
recall.
OK, this might sound like a stupid question, but being somewhat
ignorant of RTL logic in comparison to TTL and other designs, what,
if anything, can be substituted for RTL logic chips?
For example, if I wanted to build one of Don's devices and I wasn't
concerned with using "vintage" chips, could I do it "plug-and-play"
using current TTL?
It is difficult to mix RTL and TTL in the same circuit, the supply
voltage and logic levels are different.
RTL chips are slower, and have different switching characteristics to
TTL. If the devices are used as plain digital devices, and if the
timing delays are not critical, then you should be able to built one
of those circuits using entirley TTL devices. But if they use gates as
oscillators or monostables (read : If there are RC networks about),
then you will almost certainly need to make some changes.
The other thing to watch out for is that many older logic families
(certainly DTL, I suspect RTL too) had passive (resistive) pull-ups on
the outputs, and could be wire-ANDed without problems. TTL, of course
cannot, unless you use open-collector devices with an external pull-up
resistor.
The bottom line : You can almost certainly make a
functionally-identical device with TTL and the changes will not be
that major, but there could well be changes, It may not be just a
matter of replacing a NAND gate with a NAND gate, etc.
-tony