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Duane Benson
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Re: SN7400N in stock at Digi-Key
Duane Benson   6/27/2014 2:44:27 AM
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I still have a few 7400 series chips that I bought in the late 70's or early 80's in my parts bins

John Birkner
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Re: The Soul of A New Machine
John Birkner   6/27/2014 1:57:31 AM
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I remember that it was Data General using PALs in the MV8000 to replace TTL.

Kevin Neilson
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Re: The Soul of A New Machine
Kevin Neilson   6/27/2014 1:37:31 AM
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"Ah, the memories," says Doug.  Pun intended?

Kevin Neilson
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Re: Unequal output source/sink
Kevin Neilson   6/27/2014 1:36:24 AM
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A lot of engineers still use active-low signals in HDL, which drives me nuts.  It just adds confusion, and isn't really helpful unless you accidently time-warp back to 1982.  Then when they OR them together they'll DeMorganize which makes everything more confusing:

if ~(rst_a & rst_b & rst_c) ...

Homer J Simpson
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Re: Unequal output source/sink
Homer J Simpson   6/27/2014 1:27:04 AM
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Those would be Schottky diodes, not zeners. Probably the Low-power Schottky 74LS series TTL - my personal favorite because that's what I grew up with. Oh boy, where did the last 30 years go?!

David Ashton
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Re: SN7400N in stock at Digi-Key
David Ashton   6/26/2014 9:56:37 PM
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You still see 7406s - hex inverters - used as drivers.  Open collector with Vce = 30V and Isink = 40 MA.  But most people now would use the ULN2003 or similar.  Even better specs.

traneus
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SN7400N in stock at Digi-Key
traneus   6/26/2014 9:26:17 PM
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Digi-Key does stock Texas Instruments SN7400N quad NAND gates (p/n 296-14641-5-ND, $1.64 for one piece) and a few other parts in the original series.

DougInRB
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Re: The Soul of A New Machine
DougInRB   6/26/2014 4:23:37 PM
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Ahhh the memories....  That was required reading in one of my EE classes at Cal Poly (mid '80s)  I had never dreamed there would be so much intrigue and espionage in the design of a computer.  I recall that GI was one of the first to utilize PALs.

 

TanjB
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Re: Unequal output source/sink
TanjB   6/26/2014 11:59:24 AM
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I believe the origin of the asymmetry traces back to relay logic.  Relays were either on or off (subtly different from high or low) and that conditioned the way of thinking about logic.  It suited bipolar transistors very well, especially since they could only make NPN on the chip in the early days (to do both would add process steps).  Well, they could have made a chip with all PNP but that performed worse then as they still do today, so TTL was all NPN and engineers could happily continue in the mindset of a "1" being switch on, pull down.  There was not much concern about capacitance so pull-up current could be low.  External wires themselves became extra logic (a bus line served as a "wired NAND" when it had a pull-up resistor).

CMOS was exotic technology requiring the chip to go through extra process steps.  The first forms which came out in the early 70s were aimed at low power and almost seemed like magic (they ran on less than a micro-amp!).  But even these, initially, had the same asymmetric ability to sink 20mA (a standard which went back to teletypes and relays) with very little pull-up (which was fortunate since the PMOS transistors, like PNP, were lower performance than NMOS).  It was not until the late 80s when CMOS was pervasive and wired-NAND style busses finally died in pursuit of higher frequency / lower power operation that CMOS outputs became routinely symmetric and the idea of pull down faded entirely.

I'm pretty sure that version with zener diodes must have been a very late iteration of TTL.  I recall zeners being a novelty in the early 70s as discretes, and it probably added complexity for them to learn to make them on the same chip as the transistors.

zeeglen
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Re: Unequal output source/sink
zeeglen   6/26/2014 11:51:05 AM
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FINALLY an explanation of why active-low switches and LEDs were preferred.

Can still remember my first time with CMOS and how it was so nice to have the choice of LED drive.

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