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Going back to early 1975

Posted: 29 Jul 2018, 23:50
by marcelk
One of my side projects is to convert a board to original TTL series where possible, or at least to 74LS series with the oldest chips I can find. (The breadboard prototype was fully 74LS)

At a local electronics shop I recently found quite a few ICs that were manufactured more than 43 years ago. These are "new old stock": never used before and the pins still unbent. Some are indeed original TTL series as existed in the 1960s, not 74LS (Low-power Shottky) as introduced in 1971. They're plug-in compatible with the Gigatron design and the system works fine at full speed:

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Although it is cool, it also gets HOT! Original TTL consumes an order of magnitude more power. Due to this so far I only "upgraded" to original TTL the upper half the logic unit, the X register, the memory address unit and the program counter. Those 14 chips, together with a few 74LS replacements as well, already increase the current to 740 mA, or about a ten-fold from the kit version. Above that the multi-fuse and MCP start to do their thing. Remember that USB safety is the reason the kit comes with much more power-efficient 74HCT components in the first place.

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What happened in 1975?
  • The Altair 8800 computer was released
  • "Micro-soft" became a registered trademark
  • Stevie Nicks and Lindsey Buckingham joined Fleetwood Mac
  • The "Wheel of Fortune" gameshow premiered
  • Betamax and VHS launched
  • The original Jaws movie was released

Re: Going back to early 1975

Posted: 01 Aug 2018, 00:00
by at67
That is pretty cool, if you get a chance could you post a CRO screenshot of some zoomed in signals, original TTL vs HCT; would be great to see a comparison.

If you do find the time, maybe show some short and long PCB trace signals as well.

Re: Going back to early 1975

Posted: 12 Aug 2018, 16:06
by marcelk
Some annotated IR images. The original TTL reaches 60 degrees.

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The differences between original 7400-series, 74LS and 74HC/74HCT are clearly visible. The gold plated SRAM keeps it cool.

The attachment FLIR-2.jpg is no longer available

Re: Going back to early 1975

Posted: 13 Aug 2018, 19:16
by walter
You can play games while you are toasting bread! It's amazing!

Re: Going back to early 1975

Posted: 10 Feb 2019, 17:12
by marcelk
at67 wrote:
01 Aug 2018, 00:00
That is pretty cool, if you get a chance could you post a CRO screenshot of some zoomed in signals, original TTL vs HCT; would be great to see a comparison.
This was still on my todo. Reality is, I didn't have a 74HCT-based Gigatron any more. All my boards were populated with non-standard chips. Also, the recent developments at work have kept me busier than normal, until last week unfortunately.

But now I happen to own a 74HCT board again, so I can finally make the comparison. In the scope images below I probe U30, pin 10. This is the carry-in of the (high) X register U30, coming from (low) X register U29. On the hybrid board, these two chips are both 74163 TTL chips:

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In the kit edition, these are 74HCT161 chips:

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If you do find the time, maybe show some short and long PCB trace signals as well.
I couldn't find any long line between two true TTL chips on the hybrid board.