3 ms·

Noted that Ken had an issue when a 74S TTL part was replaced with a 74LS because the characteristics are different. Back in the day (mid 1980s), when I was goi

Noted that Ken had an issue when a 74S TTL part was replaced with a 74LS because the characteristics are different. Back in the day (mid 1980s), when I was going through an electronic engineering apprenticeship with a flight simulator company and many boards were end-to-end TTL, it was generally understood that power-hungry 74S series TTL were used for a reason (generally speed) and should not be substituted lest 'funny things' happen. Mind you, there was one time when debugging a glitch led to one chip being replaced with the same type from a different family - something like a 74LS replacing a 74F - and the propagation delay difference was enough to fix an edge-case timing issue. Due to project constraints, the root cause wasn't tracked down and so the board bill of materials made it clear that the 'odd one out' was correct and should not be changed. PS: Ken - I think I still have a quantity of 74S TTL 'pulls' from the day, so if all else fails sourcing a part, look me up!

WalterBright 9y ago
Back in the 70s when I designed TTL circuits, the rule was to do the math to work out all the worst case timings. The design was done with the expectation that any part could be replaced with a faster part, and the design would still work. (Because parts got faster over time, not slower, and the slower parts would become unavailable.)
Declanomous 9y ago
Wouldn't replacing the 74S86 with a 74VHC86 have worked? I'm a hobbyist, so my knowledge is pretty limited[1], but I was under the impression that the VHC series chips were drop-in replacements for the S series in both timing and logic levels. [1] Read as 'I know just enough to be dangerous, but not enough to understand why what I'm doing is dangerous.'
- CodeWriter23 9y ago
  The way to confirm your assumption is to look at the different typical timing waveform charts from the data sheet for each part. I once traced hundreds of thousands of dollars worth of bunk boards back to some RAM chips substituted on the basis of being a "drop in replacement" by the procurement department. The timing diagrams revealed the replacements possessed different timing characteristics. Enough so that the chips would occasionally leave the bus floating when the memory was read from the host side.
  - katastic 9y ago
    "It's not a hardware problem, it's a human problem!"

WalterBright 9y ago

Back in the 70s when I designed TTL circuits, the rule was to do the math to work out all the worst case timings. The design was done with the expectation that any part could be replaced with a faster part, and the design would still work. (Because parts got faster over time, not slower, and the slower parts would become unavailable.)

Declanomous 9y ago

Wouldn't replacing the 74S86 with a 74VHC86 have worked? I'm a hobbyist, so my knowledge is pretty limited[1], but I was under the impression that the VHC series chips were drop-in replacements for the S series in both timing and logic levels. [1] Read as 'I know just enough to be dangerous, but not enough to understand why what I'm doing is dangerous.'

CodeWriter23 9y ago
The way to confirm your assumption is to look at the different typical timing waveform charts from the data sheet for each part. I once traced hundreds of thousands of dollars worth of bunk boards back to some RAM chips substituted on the basis of being a "drop in replacement" by the procurement department. The timing diagrams revealed the replacements possessed different timing characteristics. Enough so that the chips would occasionally leave the bus floating when the memory was read from the host side.
- katastic 9y ago
  "It's not a hardware problem, it's a human problem!"