Going back to early 1975

Using, learning, programming and modding the Gigatron and anything related.
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marcelk
Posts: 98
Joined: 13 May 2018, 08:26

Going back to early 1975

Post by marcelk » 29 Jul 2018, 23:50

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
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at67
Posts: 48
Joined: 14 May 2018, 08:29

Re: Going back to early 1975

Post by at67 » 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.

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

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marcelk
Posts: 98
Joined: 13 May 2018, 08:26

Re: Going back to early 1975

Post by marcelk » 12 Aug 2018, 16:06

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.

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walter
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Joined: 13 May 2018, 08:00

Re: Going back to early 1975

Post by walter » 13 Aug 2018, 19:16

You can play games while you are toasting bread! It's amazing!

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