The MOS CIA Lives On, In 74HCT

It’s always pleasing to see a project we covered in its early stages reach maturity, so were very happy to bring you an update on [Daniel Molina]’s 74HCT6526. It’s a long-running effort to produce in 74 logic a faithful replica of the MOS Technologies CIA, the integrated I/O and timer chip found in so many of the 1980s Commodore machines. When we first covered it there was only one PCB, now the project has grown to a stack of three, with the remaining functions intended to fit on two more boards.

It was very common at the time for chips such as the CIA to integrate a set of common 8-bit peripherals onto one piece of silicon, both in general purpose with almost all functions of the original now implemented. hips and in more manufacturer specific parts such as this one. A project like this one is valuable because it provides a dive into the now less-common  world of interfacing directly to a microprocessor data and address line. It’s unlikely that many Commodore 64s will end up with this stack of boards inside them, but it’s not impossible the design may help a few old machines when put on an FPGA.

Meanwhile, remember it’s not the only custom 1980s home computer chip replaced with 74 logic.

11 thoughts on “The MOS CIA Lives On, In 74HCT

  1. I’m surprised that some chip manufacturer hasn’t started producing new chips that replicate and are pin compatible with these old MOS chips. There is clearly a market for them although it might be a small.

      1. There would probably be a slightly bigger market if FPGAs were not so convenient.

        Many retro enthusiasts, but such a plethora of ways to scratch the itch. If you could rope all the C64 enthusiasts into it, there would be plenty of demand for a run or two of such chips. After all, old systems need repair and it’s fun to build some replicas.

        The real trick would probably be to design an interesting and inexpensive product that uses the chip to do something entertaining, that way you can develop a secondary market at least for a little while.

    1. Western Design Center is still manufacturing the 65C02 as well as the 65C21 (PIA), 65C22 (VIA), and 65C51 (ACIA). None of them have the very cool RTC feature of the CIA, but you can do a lot with what they’re still making.

    2. PLA and SID replacement for C64 exists, CIA should be easily to do. And CIA blows fairly easily because CBM didn’t think to include any buffer from controller ports. An inconvenient short like from dodgy or poorly made controller or using Sega Genesis controller can blow one of the CIA.

      1. The only SID replacement for the C64 I could find was an FPGA emulated one. It did have nice features like being able to emulate both variants and emulate two of them at once to provide stereo sound, but not exactly the same as just getting a new SID chip.

  2. There was another project on here a couple years ago remaking the c64 cpu in 74 series logic. It was 5 years ago and the name of the post was “THIS 6502 MADE FROM 74-SERIES LOGIC CAN RUN AT 20 MHZ”

    Maybe someone else will fill in the gaps and one day someone could make a c64 in all discrete logic (except ram/rom)? That’d be a sight.

      1. I don’t see anything on that site that says they’re doing it open-source or for free – assuming it’s taken someone a lot of time & nonzero amount of money why shouldn’t they commercialise it if they want to?

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