Commodore/CBM900/Technical Info

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This page contains random technical information discovered during my work with our CBM 900.

Power Supply

The powersupply is a Switchmode design, it delivers +5, +12 and -12V and two GND.

The sixth wire is a power-good signal which is not connected on the motherboard.

Floppy Drive

The four connectors for the two floppy drives are "crossed" on the motherboard.

See: [1]

Floppy #0 uses connectors CN202 and CN205 Floppy #1 uses connectors CN203 and CN206

The floppy drive should be attached as #1

The Internal fan draws power from the CN202 connector (see photo)

The extension-cable which attaches to CN203 should be oriented so the red strip does NOT mark pin 1.

The drive is a 96 TPI drive, otherwise identical to the 100TPI drive in a SFD-1001.

Using a drive from a SFD-1001 does work, but obviously the floppies are not compatible.

I have written a set of 100 TPI floppies for this eventuality.

Hard Drive

The harddisk is presumably a regula 20MB MFM disk, but substituting another disk has not worked, so it is probably formatted in some special way.

I have not discovered a way to format a harddisk, but the commands for floppy and harddisk controllers seem to be the same, so that (and the ROM-disassemblies) are a good starting point, should the disk need replacement.

Low-Res Video Card

Takes a Monochrome/Hercules Monitor

Character set is Codepage 437, ie the same as the original IBM PC:

Cbm900 char rom.svg

Hi-Res Video Card

I believe the output from this card is ECL monochrome signal.

Character set is pure ASCII:

Cbm900 char rom hires.svg

Boot Rom

If booting the floppy and hard disk fails, the boot rom will prompt to retry.

If you press '1' and '9' at this point, you get into the boot rom menus.

It is not obvious if there are other ways to get there.

The menus have options for selecting hard-disk type, formatting floppies and parking the harddisk in addition to boot commands and debug facilities.

EPROM Firmware

The motherboard boot rom, the floppy controller (6508) rom and the harddisk controller rom are all part of the github/bsdphk/PyRevEng project examples.