Using a 2532 EPROM for the C64 character ROM (2332 masked ROM)

I’m a geek, you’re a geek, we’re all geeks.

And geeks likes to make things themselves.

So when a geek is building a commodore 64 from scratch, instead of sourcing new-old-stock parts, they like to build it geek-style

Today’s article will look at how to drop in replace the Commodore 64 character ROM with a 2532 EPROM.

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The longboard Commodore 64 has 3 ROM chips – the Kernal (yes, yes, Kernal – the Commodore Kernel is named Kernal); the Basics; and the characters ROM. While the Kernal and Basics are masked ROM based on 2364, the Characters ROM is based on a 2332 masked ROM, similar to what has been used on many ATARI carts in early 80s.

2332 Masked ROM like used for C64 Character ROM (longboards) or Atari cartridge

There are no modern replacement EPROM that works as a drop-in replacement; even the 2732 EPROM (same pin count) doesn’t have the same pinout. Mainly, the A11 data pin is not placed at the same location.

2332 masked ROM VS 2732 EPROM with highlight on differences.

But the 2532 does. Well, almost, It is pin-compatible for reading, with all data lines aligned correctly, as well as the power pins. The main difference is a VPP pin that is mainly used when programming, but with all data pins correctly aligned the chip can be used in-circuit for reading with no circuit alterations

2332 masked ROM VS 2532 EPROM with focus on the small differences

Problem is – it isn’t the easiest chip to program with the TL866 chip programmer. It’s voltage requirements and pinout are the issue, none of which can’t be solved.

The modern T48 (TL866 3rd generation chip programmer) has no issue delivering up to 25V to the chip (even powered by USB) but the pinout doesn’t permit that, so the 2532 is just not present in the list of compatible chips. Note: the TMS2532 requires a VPP of 21V – cranking 25V to it would damage it.

The solution is to build an adapter, and let the programmer think it is a 2732. Once the pins are properly addressed, it is logically similar: most of the pins are the same, except the D11, /PGM that goes to /Ep, and VPP that we connect to /G. This is to be able to program the chip, for in-circuit use, the VPP and /OE pins would be mapped differently.

2532 EPROM VS 2732 EPROM with the wiring required to build a programming adapter

There are 2532-2732 adapter boards available online, but most focus on using a 2732 in-place of a 2332 or 2532, linking the /OE pin to either GND or Vcc depending on usage. Some are configurable. All can be adapted. But geeks can also build their own.

geek-made 2532-2732 programming adapter for the TL866

Bill of material

  • prototyping perfboard
  • Socket
  • pin header
  • wire, solder

The circuit is rather simplistic, only following the pin mapping shown above. Ensuring the proper chip goes to socket, and the other chip to the pin headers, in order not to reverse the circuit.

Programming

Once the circuit has been assembled, the T48 chip programmer will see it just like a normal 2732 to read or program. It is a good idea to select the 2732 of the same company as the 2532 to get the voltages right.

C64 Character ROM 901225-01 burned on a TMS2532-35 and another (non-programmed) EPROM for reference

In theory, same chip could be used on stock Atari VCS cart PCB with the masked ROM removed, to make it a programmable one. but that’s a story for another day.

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by Categories: 100% Geek, C64ROM, Commodore 64, Computers and technology, Geekyness, Projects, RetrocomputingTags: , , , , , , , , , , 4 Comments