Author Topic: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?  (Read 2050 times)

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Offline tsman

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Re: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?
« Reply #25 on: September 10, 2018, 01:44:21 pm »
Earlier this year I repaired an Asteroids Deluxe board for somebody that had I think 5 bad 74153 TTL chips, and there was another that had obviously been replaced previously. I was surprised to see this happen but it must have been a bad batch. The failed ICs were all the same brand, the really odd thing is they all failed at the same time.
Probably bad encapsulation or pins that were prone to corrosion which spreads to the internal bond wires. Old Fujitsu TTL chips are notorious for breaking old arcade boards.
 

Offline glarsson

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Re: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?
« Reply #26 on: September 10, 2018, 05:09:12 pm »
No it's not. It's TTL with Schottky diode clamps. DTL is ancient technology prior to TTL.
This is 74S. 74LS went back to DTL style inputs for speed.
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_7/TTL/temp_LS_sm.jpg
 

Offline Gyro

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Re: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?
« Reply #27 on: September 10, 2018, 06:42:42 pm »
Well I'll be a ....., sorry for doubting you glarsson.

I've never noticed that before. Browsing through the TI TTL Data Book, volume 2, the internal schematics do indeed show diode logic feeding into the bases of Schottky clamped transistors, a quick search through seems to show this holding true across the range. I see that they stuck with the 'standard' architecture for the 74S parts.

I've attached a scan of the 74x03 parts - the first I could find with all four internal schematics on the same page (74, 74L, 74LS and 74S). I haven't located a 74ALS internal schematic yet.

EDIT: Checking Volume 3, they don't show internal schematics for ALS, shame.
« Last Edit: September 10, 2018, 07:03:59 pm by Gyro »
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline drussell

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Re: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?
« Reply #28 on: September 10, 2018, 07:22:54 pm »
Yes, fair enough, the LS style inputs are technically a form of DTL since they don't use a multi-emitter transistor directly but the rest of the circuit on most styles of gates resembles other, what we normally call TTL class devices, rather than the circuitry of the actual earlier generation DTL logic chip family gates.
 

Offline glarsson

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Re: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?
« Reply #29 on: September 10, 2018, 07:48:42 pm »
Well I'll be a ....., sorry for doubting you glarsson.

I've never noticed that before.
I noticed if 40 years ago... :-)
Haven't used TTL for a long time, but when I did use it we filled racks with TTL, ECL, DRAM and 5V@500A power supplies...
I never found out the inside of 74ALS and 74F.
 

Offline MK14

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Re: 1970s era CMOS TI 7400 chips - REPLACE or LEAVE?
« Reply #30 on: September 10, 2018, 08:11:20 pm »
I noticed if 40 years ago... :-)
Haven't used TTL for a long time, but when I did use it we filled racks with TTL, ECL, DRAM and 5V@500A power supplies...
I never found out the inside of 74ALS and 74F.

This seems to show the inside (schematic) of 74ALS:

http://www.nutsvolts.com/magazine/article/understanding_digital_logic_ics_part_2

« Last Edit: September 10, 2018, 08:14:51 pm by MK14 »
 
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