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Welcome to the CBM-II B-Series Prototype Computer Page. The CBM-II line was Commodore's followup to the PET/CBM line. Commodore planned a range of computers with a brand new architecture with expanded memory and advanced features. To begin with, they designed the line around a new CPU called the 6509. This CPU was based on the 6502 but with additional memory banking features to support up to 1MB of RAM, plus the ability to run at 2MHz. The video was based on the same CRTC chip from the PET but using a larger 8x14 matrix to produce sharper video. The old PET beeper was replaced by the powerful SID chip, giving amazing 3-channel sounds with programmable waveforms. The system was designed to be highly expandable with the ability to add additional processors to run things like MS-DOS and CP/M. The keyboard was expanded to include programmable function keys. Also included was an industry-standard RS-232 connector, and even a reset button. All of this was housed in an attractive designed curved case with built-in swivel monitor and detached keyboard.
It all started with the CBM-II B-series prototype seen here. The prototype was hand assembled and housed in a standard PET-type case with an amber screen. This is an early prototype, given to software developers to get them familiar with the new architecture. The operating system (firmware) was not complete, and not all features were developed yet, but the prototype is fully functional. In fact, this machine may be the only surviving B Prototype in the world.
For additional information on the CBM-II line please visit my Commodore CBM-II Page.
The following are pictures of my prototype. Click on the picture for a larger version.
Okay, first off you'll see that the prototype is obviously in a standard PET enclosure. A lot of design went into the final plastic "SK" case, but those were yet to be designed at this point. The PET case is made of metal similar to very early PET machines, and unlike later PET/CBM's in the old square plastic cases (top part). You'll also see that this machine has an Amber monitor that doesn't quite fit the plastic bezel. Very early PETs came with a blue-ish-white monitor, and later models came with the typical green screen. I understand that early monitors were either 15 or 20 kHz scanning frequency, whereas the CBM-II's used a 17kHz rate to support the modified 8x14 pixel font. In any case, Amber monitors were never used in production PET/CBM/CBM-II's.
Moving on to the Keyboard, you will see it's black with white lettering, very similar to the older PET machines. If you look closely you'll notice that some keys have silkscreened letters and some are engraved. The "0" key is the same width as the letter keys, and not wide like the production keyboard. When I first received this machine I noticed that the keyboard didn't work properly. Some keys were mixed up. I had to swap some lines to get it working and to match the production key matrix.
On the back of the machine we find an IEEE Port, Audio Port, and RS-232 port. On the side is the power cable (permanently attached) and a reset button. The cartridge port is not accessable as it is recessed quite a ways. You'll also notice that that the RS-232 port uses a Male connector, rather than the female used in production units.
Opening up the machine we can see the motherboard. This is marked "8256010 REV A 2-8-82". Also there is a hand-written "34 OK" and a sticker marked "32". I'm guessing one of these is like a serial number or perhaps a developer number. Every chip is socketed. The 6509 CPU, 6581 SID, and 6526 CIA are in ceramic packages. The CPU is dated 2282 (22nd week of 1982), the SID is 1382, and the 6526 is 0882. The SID chip appears to be one of the earliest known revisions. On the front left of the motherboard is 256K of DRAM (32 x 4864). To the right of those are the Kernal and BASIC ROMs (well EPROMS on adapters). To the left again, we see the 68B45 CRTC controller (6545 compatible like the PET) and two 2114 RAMs totalling 1K for the SYSTEM BANK lower ram. Moving more we find the 6525 TPI chip and keyboard connector. The keyboard connector is not the usual inline pin header but a DB-25. It is also located in the proper position for the final casing, so Commodore must have already had a design for it at this point. Finally at the right edge we see the internal video connector.
Just behind the TPI chip we see the 6581 SID. Near the SID we find another connector for internal audio, and attached to it is a cable running to the back of the board and connecting to the RCA jack coming out the back. Also at the back is the reset connector with wire running to a switch mounted on the right-hand side of the case. Behind the SID we find a 2016 SRAM chip (2K) used for screen memory (80x25 characters) and the high-profile (HP) character set 2532 EPROM. Moving farther to the back of the board we find a 6551 ACIA chip, another 6525 TPI and a 6526 CIA.
Continuing along the back of the motherboard we find the power connector, the cassette port, and the IEEE port. To the right we see the cartridge port. Notice that both sides of the cartridge port have been cut. The CBM-II cartridge port is similar to the C64/Max Machine port but with reduced pins, so likely a standard connector was modified.
You may notice that there are no PLA chips. All decoding was done with standard TTL chips. PLA chips were just becoming available around this time. PLA's allow the designers to reduce chip count and modify behaviour without re-designing the circuitry. Also missing is some ram in the lower system bank. Production machines have 2K starting at $0000. This is a real problem as there is very little free RAM in the system bank for writing small machine language programs. Without it you are forced to write in one of the main ram banks and that means you need to deal with the stange memory banking and hoop jumping to call kernal routines.
Prototype ROMs coming soon
Page Created: Nov 7/2013. Last updated: Mar 11/2024, 9:00pm EST
Send comments or feedback to Steve Gray(sjgray@rogers.com)