Electronics > Projects, Designs, and Technical Stuff
Experimenting with TTL Cpu, 74LS chips, old vs New? Retro style switches?
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rwgast_lowlevellogicdesin:
I am interested in building Ben Eaters 8 bit TTL breadboard computer, I have never actually used 74 series logic for anything, with the functionality of all the chips out there these days and rarity of parallel buses these days, i have just never had a need. I have always been in to computer design and over the years I have wanted to build an old school style computer with a blinken lights and switch panel, along with a punch card reader and serial terminal. I decided to go TTL instead of z80 or 65c02, is because for some reason I am a real bottom up type of learner. A lot of this project is also about writing a ground up software stack too.

I plan to start out just following ben's design (and reading Petzolds "Code:Hidden language of computer hardare and software") and soldering through hole on veriboard, instead highly capacitive breadboard design. But once i know what im doing i would like to take it in the direction of the Magic-1, and make it do something useful while learning more. At that point I would probably want to use a proper PCB and surface mount 74AHC components to get a better clock speed.

Right now im on digikey making an order for parts and I have realized I have about 90% of the parts on the projects BOM. A while back I got a box full of 74 and 4000 series logic chips, But being fairly uneducated about the 74LS series im wondering if there speed/rise time difference between 74LS from the 70s & 80s compared to the newer TI ones sold on digikey today? If there is a difference what effect would having chips with different rise/propagation times have? I'm thinking I should just order all new chips but if it is just a waste on 30 bucks that wont give any better speeds/noise/power consumption then I would probably be wise to just use what I have sitting around in case I need it one day. Lastly what are the names of the type of switches used on PDP an IMASI8080 front panels? If its possible I would like to pick up the "duck bill" style switches on digi key as well.
T3sl4co1l:
Y'mean comparing new 74LS?  Wouldn't think there's a difference...

74HC and HCT are compatible, HCT being fully compatible and HC being HC-output to TTL-input compatible (and TTL-output to HC-input compatible with a pull-up resistor, if you don't mind it being slow; but HCT is better to just drop in).  HC and HCT have identical outputs.

Also 74F, 74AC(T) and 74ABT are faster, if you start running out of propagation delays in critical paths.  Mind the signal quality, and provide good grounding and bypassing!

Tim
jfiresto:
I always feel some cognitive dissonance when people refer to 7[A]HC[T]... parts as TTL, but I have been a CMOS fan for the last 45 years. Could that misnaming be an issue?
greenpossum:

--- Quote from: jfiresto on June 03, 2020, 03:35:50 am ---I always feel some cognitive dissonance when people refer to 7[A]HC[T]... parts as TTL, but I have been a CMOS fan for the last 45 years. Could that misnaming be an issue?

--- End quote ---

Well strictly speaking CMOS also uses transistors, they don't have to be BJT. And TTL is used to distinguish from RTL, DTL, ECL (which really would have to be renamed if ever there could be a CMOS equivalent), etc. But I understand your feeling of dissonance.
David Hess:

--- Quote from: rwgast_lowlevellogicdesin on June 02, 2020, 07:40:28 pm ---But being fairly uneducated about the 74LS series im wondering if there speed/rise time difference between 74LS from the 70s & 80s compared to the newer TI ones sold on digikey today?
--- End quote ---

There is no difference; new low power schottky has the same performance as old low power schottky.


--- Quote ---If there is a difference what effect would having chips with different rise/propagation times have? I'm thinking I should just order all new chips but if it is just a waste on 30 bucks that wont give any better speeds/noise/power consumption then I would probably be wise to just use what I have sitting around in case I need it one day.
--- End quote ---

Newer logic families like ALS, AS, or FAST will provide an improvement in speed or power consumption but the selection of logic functions is more limited than with LS.  The same applies when moving from HCT to AHCT or ACT except static CMOS power consumption is practically zero.


--- Quote ---Lastly what are the names of the type of switches used on PDP an IMASI8080 front panels? If its possible I would like to pick up the "duck bill" style switches on digi key as well.
--- End quote ---

They are just another form of toggle switch but they will not be cheap.  They were not cheap in the past either.


--- Quote from: greenpossum on June 03, 2020, 03:51:21 am ---
--- Quote from: jfiresto on June 03, 2020, 03:35:50 am ---I always feel some cognitive dissonance when people refer to 7[A]HC[T]... parts as TTL, but I have been a CMOS fan for the last 45 years. Could that misnaming be an issue?
--- End quote ---

Well strictly speaking CMOS also uses transistors, they don't have to be BJT. And TTL is used to distinguish from RTL, DTL, ECL (which really would have to be renamed if ever there could be a CMOS equivalent), etc. But I understand your feeling of dissonance.
--- End quote ---

It does not help that the later generations of TTL use DTL input structures in place of the multiple emitter inputs usually associated with TTL.  And then some replace the input diodes with PNP emitter followers.

CMOS ECL should be SCL (source coupled logic) and that term is sometimes used but more commonly it is referred to as CML (current mode logic).
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