TTL is hot

A project log for Gigatron TTL microcomputer

Just because! A home computer without microprocessor

Marcel van KervinckMarcel van Kervinck 08/12/2018 at 16:270 Comments

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, the kit edition is 74HCT.

At a local electronics shop I recently found quite a few ICs that were manufactured more than 43 years ago, in 1975. 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:

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.

The nice thing about the 7400-series is that improved series remained compatible in pin layout and voltage levels.

I have them all on a board now.  The difference in generations is clearly visible.  The old TTLs get up to 60 degree Celsius. The blue square isn't a cool oasis in the middle of a hot desert. It is a reflection on the gold plated ceramic lid of a vintage SRAM.