This is another part of my ongoing tester project; in addition to incorporating replacements for parts that have gone obsolete, I have been looking at streamlining the design where I can and removing parts specific to one tester mode I will not be using.
That brings me to the circuit section below. The left half is the original design after I removed one mode I don't need (pin 1 previously had a pull-down connection like pins 2 & 3)... the right side is what I THINK should work in its place, but was shot down in another forum (the respondent refusing to look any further than the ICs themselves rather than what they're wired to do), so am looking for a second opinion. The IC on the left is not cheap at $6+ a pop, the IC on the right I want to use is an idle section of an IC I already have on the board.
The 74LS348 is an 8-to-3 priority encoder, and the 74LS368 is an INVERTING buffer.
In both cases, the output to ED6 / ED7 are tri-state outputs controlled by the flag '80F9'.
Here's what I have worked out for the 74LS348 circuit, and the results I expect from the 74LS368 circuit:
Shunt on J5 (or J15) 3-2 = Pin 3 grounded = ED7 high, ED6 low, telling the fixture to expect an LMT-100 unit.
Shunt on J5 (or J15) 1-2 = Pin 2 grounded = ED7 low, ED6 high, telling the fixture to expect an LMT-2 unit.
I see you presented what you have but 'expect LMT-100' means nothing to us.
Please post your DESIGN CRITERIA which states
DESIRED TRUTH TABLE. (inputs xyz = outputs abc)
I'm not sure what you are asking? Are you asking us if an inverter will invert the signal or if pulling
an active low Enable input low will enable the outputs ? , or both ? What am I missing here.
It doesn't concern me that some other forum
shuns TTL. It's been around longer than they have
and will probably still be in use when they're gone.
What's changed is the technology use to implement the logic (like LS, HCT, etc.)
Can you rephrase your post as YES or NO questions ?
I apologize for my chronic inability to write clearly... 
J5 / J15 is simply a mode switch, configuring the unit to test two different equipment models, hence the 'LMT-2' and 'LMT-100' notations.
The truth table is attached to this post.
What I am trying to do is validate the behavior of the circuit on the left:
Do I understand correctly that when flag 80F9 is enabled, AND a jumper shunt is placed connecting J5-1 and J5-2, that ED7 will be low and ED6 will be high?
Do I understand correctly that also, when flag 80F9 is enabled, AND a jumper shunt is moved to connect J5-2 and J5-3, that ED7 will change to high and ED6 will change to low?
Now, the circuit on the right:
Do I understand that when flag 80F9 is enabled, and placing a shunt to connect J15-1 to J15-2, I will see ED7 go low and ED6 will go high?
Then, do I understand correctly that when flag 80F9 is enabled, and moving the shunt from J15-2 to J15-3 will cause ED7 to go high and ED6 to go low?
Lastly, am I correct in saying that both circuits have the same outputs, depending on the placement of the shunt on J5 or J15? My goal, if both circuits do the same thing, is to replace the circuit on the left with the one on the right.
YES
and
YES
and
YES
This is easily determined by reading the 74LS348 datasheet:
" http://www.skot9000.com/ttl/datasheets/348.pdf"
Case A:
Input 5 LOW
Input 6 HIGH
Outputs are LOW,HIGH, LOW (A0,A1,A2)
SINCE ED7 is connected to A0, it will be LOW
SINCE ED6 is connected to A1, it will be HIGH
CASE B:
Input 5 X-DON'T CARE
Input 6 LOW
Input 7 HIGH
Outputs are HIGH,LOW,LOW (A0,A1,A2)
SINCE ED7 Is connected to A0, it will be HIGH
SINCE ED6 is connected to A1, it will be LOW
NO DATASHEET IS REQUIRED FOR AN INVERTING BUFFER (74LS368) because the OUTPUT is simply the OPPOSITE of the input.
It can be clearly seen that for
CASE A
Input 1 is HIGH
Input 2 is LOW
so the opposite is 1 LOW, 2 HIGH
CASE B
Input 1 is LOW
Input 2 is HIGH
I think you can figure out what the opposite is.
YES.
Both circuits have the same outputs .
The only difference is that a Priority Encoder is a much more complex logic function than a simple inverter but this is not your concern.
