Commodore CBM-II B715 Page

Introduction

Welcome to the CBM-II B715 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.

After the CBM-II B-series prototype, which was housed in an old PET case, and before Commodore started selling the new systems, software developers were sent early pre-production machines to develop software on. This is one of those machines. The machine has a sticker on the back proclaiming it to be a "B 700" while the box itself calls it a "715". Internally it has 256K of ram, which would make it either a B700/256 or a CBM 720. However AFAIK both of those machines use a standard CBM-II "HP" (High Profile) motherboard like production machines. This machine actually has an "LP" or Low Profile motherboard. Although the LP and HP motherboards are functionally the same, they differ in mechanical dimension, parts, and port placement. Perhaps the "715" numbering was a way to distinguish LP vs HP. Before actually receiving the machine, another theory put forth was that perhaps it contained 192K RAM. Earlier announcements claimed x05 machines would have 64K, x10 machines 128K, and x20 machines 256K, thus it makes sense that x15 models could have 192K. Alas, we may never know what Commodore were actually thinking at the time.

Now, the problem with the LP motherboard, is that it doesn't actually fit into an HP case. That is, not unless you hack off bits of the board, which un-surprisingly, is exactly what Commodore did in this machine. Furthermore they also had to raise the motherboard up. No doubt some genius at Commodore remembered this, as this is basically what they did years later in order to stuff an 8032 motherboard into these same HP cases.

This B715 seems to have a standard LP motherboard, which means some additional hacks were needed to make everything work. First, the HP character generator ROM had to be installed. Second, there is no internal video connector on the board so a cable had to be soldered onto the bottom of the motherboad to connect to the internal monochrome monitor connector. Third, the metal backplate had to accomodate the LP port arrangement plus the additional motherboard offset. Fourth, the LP motherboard has an internal keyboard connector/header, so a cable was made to adapt this to the standard HP DB-25 keyboard connector. And lastly, since the LP motherboard is wider there is no room for the ROD that holds the case open for servicing.