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Noob question about line drivers ....

?

:-)

Jan 1, 1970
0
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 ... )

:)
 
R

Rene Tschaggelar

Jan 1, 1970
0
:-) said:
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 ... )

Opto isn't by default shortcircuit proof either...
What is your outside world ? TTL ? CMOS ?
And what is the speed of the outside world ?
a Relay ?

Rene
 
C

Chris Carlen

Jan 1, 1970
0
:-) said:
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 ... )


1. 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.


2. 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 <12 inch
wires running from here to there, even with no consideration paid to
ground paths.



--
Good day!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
[email protected]
NOTE, delete texts: "RemoveThis" and
"BOGUS" from email address to reply.
 
?

:-)

Jan 1, 1970
0
Chris said:
1. 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.

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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