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
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.
46 September 2015
FIGURE 3. Early test with both monitors active.
FIGURE 2. Jumper section set for 40 columns.