| Electronics > Projects, Designs, and Technical Stuff |
| Experimenting with TTL Cpu, 74LS chips, old vs New? Retro style switches? |
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| tggzzz:
Signal trace inductance is less of a problem than ground lead inductance. "Ground bounce" can be a real problem, since the low level voltage margin is only 0.4V |
| David Hess:
--- Quote from: rwgast_lowlevellogicdesin on June 04, 2020, 12:29:31 am ---So would I gain anything from leaving all the 70s 74LS chips in the drawerer and ordering all the parts in HC/HCT format? --- End quote --- Compared to LS, HC/HCT would draw less power for close to the same performance. AHC/AHCT and AC/ACT would also give a performance improvement over LS. Of course the same applies to the improved TTL families like ALS, AS, and FAST. FAST is my favorite of the TTL families but unfortunately it has a limited selection of functions. Unless you need more performance, then HCT would be my first choice and it has full compatibility with TTL signal levels so you can include some LS parts if needed and replace them later or not at all. --- Quote from: tggzzz on June 04, 2020, 09:21:45 pm ---Signal trace inductance is less of a problem than ground lead inductance. "Ground bounce" can be a real problem, since the low level voltage margin is only 0.4V --- End quote --- AS and FAST never reached their intended performance because of lead inductance in DIP packages. If anything, the fastest CMOS parts were even worse because of the difference in how CMOS and bipolar transistors switch. |
| tggzzz:
--- Quote from: David Hess on June 04, 2020, 09:27:44 pm --- --- Quote from: tggzzz on June 04, 2020, 09:21:45 pm ---Signal trace inductance is less of a problem than ground lead inductance. "Ground bounce" can be a real problem, since the low level voltage margin is only 0.4V --- End quote --- AS and FAST never reached their intended performance because of lead inductance in DIP packages. If anything, the fastest CMOS parts were even worse because of the difference in how CMOS and bipolar transistors switch. --- End quote --- That tallies with my recollection, but one of the claimed advantages of FAST was that controlled edge rates minimised the problem. Nonetheless, excellent PSU planes were required - and that is difficult with a homebrew construction. Putting the powe pins on the opposite corners always was a pessimal choice. |
| rwgast_lowlevellogicdesin:
Just a curious question, when using the FAST or the A families cant you still reap there speed benefits whiles using appropriately sized resistors to slow the rising edge slow enough in order to avoid issues? People do this to control the rise time of a FETs gate all the time, and aren't the newer chip family's based on FETs is that part of what makes there layout issues so much more crucial? Seems like if you were going to use them without trying to slow there edges the key is low impeadence ground, and smashing there SMT versions as close together as possible to avoid trace length. |
| CatalinaWOW:
I used several computers of the blinken light era. Switches used for register entry and memory loads varied all over the map so you can use whatever you like and be "period correct". The ones I liked best were on the HP 1000 series of of computers. Lighted push buttons. Press them and they latched down and turned on. Press again and they popped up and turned off. I am sure they weren't and aren't cheap. |
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