Right, suppose I got my super ttl (LS) logic design working perfectly ;-)
Now I want to offer some signals to the outside world (can be other ttl boards ) my thought was to past them trought something like 74LS244 line drivers.
My question is 244 really what I need to use, how resistant is a 244 to malfunction of the outside part, if a short happen on the outside will if feedback into my board and burn ttl gates.
Same question to input line from outside, will 244 protect my board for some accidents outside ?
I dont ned to have full proof buffer (opto seem to be to costly ... )
Does your "outside world" mean internal to the chassis, or external?
Internal might involve only being sure impedance matching (avoiding reflections screwing your edges on relatively long lines) is Ok, and crosstalk is not a problem.
External might warrant all of the above considerations as well as protective circuitry such as ESD, overvoltage, and short-circuit proofing. These can often be dealt with all in one blow with a decent output buffer design. In the extreme, isolation might be needed. I have not yet had to do that in my experiences, but if I was going to drive a single-ended signal to a location not in the same room, I might consider it.
My usual method is a pair of paralleled HC04 for medium speed, or pair of AC04 for high-speed, driving into a back terminating resistor matching the cable, followed by a pair of 1N4148 diodes to the rails, followed by a 0.200A 1206 SMD fuse. A shunt must appear across the rails as well such as a 6.2V TVS in parallel with a 5.6V zener (you could suffice with just the zener, but not be near-telco-grade).
This setup is short-proof, highly ESD robust, and safe from overvoltages up to the rating of the fuse.
If your "outside world" just means going from this board to that board on your workbench for hobby or proto purposes, then one generally starts by just connecting things directly and seeing if signal integrity is sufficient given the speeds and wiring lengths involved. At TTL/HC logic speeds, I have yet to encounter a problem with having
Interesting :-) I didn't yet do something with HC gates but it seems they offer alot of output (out and sink) current and in that case can replace my old 244.
I guess also it will be good enough for my design, its 2khz max king of digital signal.
Thank for the more short-proof way to do thing, I will take a deep look into that :-)
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