MOS 74LS logic chips

Commodore (in)famously owned its own chip manufacturer, MOS Technology, later known as Commodore Semiconductor Group. MOS was pivotal to helping Commodore keep prices low in the early 1980s, as it lowered Commodore’s overhead, and ensured a steady supply of chips. In 1983 and 1984, MOS produced 74LS logic chips. These chips are very failure prone. Here’s how to find MOS 74LS logic chips, and more importantly, what to replace them with.

Why MOS made 74LS logic chips

MOS 74LS logic
Say what you will about TI, but they were a lot better at making 74LS glue logic than Commodore was.

The 74LS logic chips are standard, off-the-shelf components. Almost all of the major chip manufacturers made them. MOS usually didn’t bother. These chips were common and inexpensive and didn’t have a huge profit margin. MOS only had two fabs. So usually there was something better for Commodore to do with that capacity.

But for a time in 1983 and 1984, Commodore had some supply issues and couldn’t get enough of the 74LS glue logic chips to keep up with demand for the Commodore 64. Commodore was selling about 3 million machines a year at that point, but just about everyone’s computers were also selling well at that time. So it makes sense that Commodore may have had trouble getting all the chips they needed.

These chips aren’t super difficult to make, so MOS made their own.

The problem with MOS 74LS logic chips

There are two problems with MOS 74LS logic chips. While Commodore had some brilliant designers, MOS didn’t always execute consistently. MOS wasn’t good at making memory chips, which was why Commodore started using other companies’ memory chips early on instead of making their own. But it turned out MOS wasn’t all that good at making 74LS logic chips either. The chips were fine at first, but their reliability fell off a cliff once the chips were a few years old.

When you open up a Commodore 64, 16, or Plus/4 and see MOS-branded 16-pin glue logic, there’s a very good chance those chips have gone bad. If they haven’t gone bad, it’s only a matter of time before they do. So when a Commodore from the 1983-1984 era is acting weird, there’s a good chance it’s some of that glue logic going bad.

The other problem with the MOS 74LS logic chips is Commodore didn’t use the same naming convention everyone else did. A 74LS257 is a 74LS257 is a 74LS257, whether it was made by TI, Motorola, Signetics, Goldstar, Hitachi, or anyone but MOS. MOS used its own 4-digit stock number, and it’s not clear from the MOS part number what the standard equivalent is.

The MOS Rosetta Stone

Here’s a list of MOS glue logic chips and their standard equivalents:

7708: 74LS257
7709: 74LS258
7711: 74LS139
7712: 74LS08
7713: 74LS04
7714: 74LS02

When you open up a machine and see a MOS 77xx part, it’s a good idea to remove it and replace it with a part from a more reputable manufacturer. The failure of a MOS 7708 or 7709 can appear to be a failed PLA or RAM chip. And certainly Commodore’s PLA is another infamously failure prone chip, and the Micron MT4264 RAM chips that frequently turn up in C-64s and other 8-bits of that era also tend to fail a lot. But as bad as those chips are, the MOS 77xx chips seem to be even worse.

When repairing old Commodore computers, replacing the MOS 77xx chips when you see them can save you a lot of headaches.

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