RTL Logic

[Scott Stevens]

On Fri, 7 Jan 2005 23:55:00 +0000 (GMT)
ard at p850ug1.demon.co.uk (Tony Duell) wrote:

> > 
> > 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

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.