Nuts and Volts - September 2015

difficult as it might first appear, but in order to understand why, we need to look at how the computer is designed, keeping in mind limitations of technology at the time.

After the 2001 model, Commodore started using the MOS6545 cathode ray tube controller (CRTC for short).

This chip is programmable and is responsible for producing the timing signals such as horizontal and vertical sync. The CRTC is fairly simple in that it accesses the video memory, but it does not actually produce the pixels we see on the screen. That job is left to additional circuitry.

The CPU, CRTC, and RAM are all clocked at 1 MHz, which was pretty typical back then. The CRTC reads one byte of video memory and latches it with a 74LS373 chip. That byte is sent to the character ROM along with lines from the CRTC, and then the appropriate character data is fetched and sent to a 74166 shift register clocked at 8 MHz, which puts out the actual pixels that we see. With monochrome video, our pixels are simply “on” or “off.” A 40-column by 25-line screen needs 1,000 bytes, or just under 1 KB of RAM.

These first CRTC-based PETs (with 40-column screens) were not really business machines. Businesses preferred 80 columns to do their word processing, spreadsheets, and databases more efficiently. To create a new 80- column PET it would need double the video memory. That memory would normally have to be twice as fast, and that would mean the CPU would also need to be twice as fast due to the interleaved access method that the video system used. All of these would increase the cost because memory was quite expensive in the late ‘70s and early ‘80s ... fast memory even more so.

To keep costs down, Commodore came up with a clever solution which was to have two separate banks of memory. The CRTC could then read and latch two bytes at a time, effectively making a 16-bit wide video buss. This meant they could still use the same speed memory and same speed CPU. The shift register is now clocked at 16 MHz and sends the two bytes out one after the other, effectively doubling the pixels without changing the memory or CPU speed.

With this, Commodore re-designed the PET motherboard for 80 columns and the 8000 series machines were born. A short time later, they re-designed the motherboard yet again, to support 40 or 80 columns simply by setting a few jumpers on the board (Figure 2). This allowed them to sell either a “home” or “business” computer using one motherboard. It is this version — called the Universal Dynamic PET motherboard — we will use for this project.

Commodore unknowingly provided us with almost everything we need to add color to the PET. We have the CRTC chip, 2K of memory split into two banks (even and odd), a character generator and shift register, onboard jumpers for memory configuration, plus the right signals to control a monochrome CRT.