RTL Logic

[Tony Duell]

> 
> 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