Commodore PC20-III

Continuing the Commodore PC-compatibles line, after different variations of PC-10 and 20, Commodore engineers came with something smaller. PC-10-III and 20-III, released ca. 1988, were re-designed Commodore PCs with smaller casing, more compact mainboard and many peripherals built-in. CPU has been accelerated to maximum 9.54MHz, there was 640kB of RAM and most PC20 machines had some hard disk - starting from 20MB. Hard disks were connected using a new interface called XTA (XT Attachment) which was a predecessor of IDE.
It was also released under brand "Commodore Colt". Some modification was also known as HD40 or "Select Edition" - this one had 40MB hard disk.

Manufacturer Commodore

Origin USA
Year of unit 1988?
Year of introduction 1988
Class XT
CPU Intel 8088
Speed 4.77MHz
7.16MHz (Double Mode)
9.54MHz (Turbo mode)
RAM 640kB
ROM Commodore PC BIOS
Graphics CGA/Monochrome (Paradive PVC4)

This unit: ATI Graphics Solution SR

Sound PC Speaker
System expansion bus 8-bit ISA (3 slots)
Floppy/removable media drives 1x 360kB 5.25" floppy drive

Hard disk: 20MB WD XTA drive

Peripherals in collection:
 - Commodore 1404 monitor (monochrome, amber, manufactured by Hyundai)

Other boards:


ATI Graphics Solution SR graphics adapter
Non-standard expansions:  
Operating system(s): MS-DOS

My unit came to me in quite neglected condition - I had to replace RTC battery and one chip which was corroded by leak. What is unusual, it has an ATI Graphics Solution SR graphics adapter in ISA slot (manufactured ca. 1988). This is strange as PC20 already has a Paradise PVC4 CGA graphics chip. Both graphics adapters work perfectly when separated, so I think that someone had compatibility problems with Paradise. I left it there.
Another interesting part is a "PC10C OSC Tower" - a small PCB mounted instead of quartz generator. It seems that Commodore fixed the clock problems with triple-clock using this PCB.

Contents: Starting, usage Jumper settings Links


It's a PC clone. It starts, displays BIOS POST where it checks e.g. RAM, then it tries to boot from floppy. If this doesn't work, it tries to boot from HDD. During initialization it is only important to keep the disk in its factory low-level format, as using MFM low level format tools on XTA disks usually ends with damaged hard drive.

This machine does not have a "Turbo" button. If you use a normal XT keyboard, to set CPU frequency, you have to use SPEED.EXE (see files in files archive in Links section) program or keyboard shortcuts:
 - CTRL+ALT+S - Switches to "Standard" mode - 4.77MHz
 - CTRL+ALT+T - Switches to "Turbo" mode - 7.16MHz
 - CTRL+ALT+D - Switches to "Double" mode - 9.54MHz




Jumper settings

DIP switch bank i located on the rear of computer. It configures on-board graphics adapter. OFF (0) is when switch is up, ON (1) when down. So:
0110 - CGA, 40-column mode
0101 - CGA, 80-column mode
0000 - Monochrome




Links: - Interestong files in Bo Zimmerman's site. Service manuals, diagnostic manuals, disk images, ROM dumps... - Commodore 1404 service manual - Discovering Commodore Colt - Exploring PC10-III - similar unit