CBMSTEVE.CA - Steve J. Gray's Homepage
CBMSTEVE.CA - Personal pages of Steve J. Gray
Changes Info Prototypes Remakes PET Projects CBM Projects Modding 3D Models KiCad Github Ohio Scientific Other My Software My Collections Facebooks

Proxa7000 Board - CBM8000 Emulator for B-Series

Introduction

The Proxa7000 board was made by the German company Ultra Electronic. It adds CBM8000-series compatibility to the CBM B-series High-profile machines. The board installs inside the B machine via the 6509 cpu socket. The 6509 is then installed onto the Proxa board. Connections are also made from the Proxa board to the Keyboard port, IEEE port, and User Port.

Theory of Operation

Looking at the board layout and parts list you will see that the board uses a 6512A CPU. This CPU is 6502 compatible but is able to be tri-stated so that it may co-exist with the 6509. We see two 6521A's and a 6522A which are interface adapter chips to handle I/O to the keyboard and IEEE. What you DON'T see on the board are - RAM, ROM or CRTC chips. These functions must be handled by the B-series itself. This means that the CBM8000 ROMs must be loaded from disk. I assume that those ROMs may be patched somehow to allow writing to the CRTC chip and to handle the B-series Keyboard, which is completely different than the 8000 keyboard. Or, another possibility is the emulated CBM writes to video RAM as usual and then an interrupt routine on the B side copies the video contents to the actual Bank15 video RAM.

The board also has a speaker, to make that distinctive chirping sound at startup, and two cassette ports. There are also some additional connections for future expansion.

According to Boris Kretzinger, who owns a machine with a working Proxa board, you could have up to 4 different CBM8000 machines at one time (not nesessarily running simultaneously). This would make sense for the 720 with 4 RAM banks. Thanks go to Boris for supplying pictures of the Proxa board installed into a 720 machine and the parts list.

Spec Sheet

Here is the Proxa 7000 spec sheet (german). Here is the english translation, courtesy of Richard Lagendijk.

Pictures

  • Unpopulated board - top
  • Unpopulated board - bottom
  • Board inside 720 (left side) with HRG board (right)
  • Board inside 720 - closer
  • Board inside 720 - angled showing Ultra Electronic label
  • Unpopulated board (Boris) - top

    Software

    The Proxa board is rare and the software that controls the board was missing for quite a while. Luckily, in late 2011 it was found! Richard Lagendijk found the disks and created disk images, which can be downloaded below. There are a handful of people hoping to get these boards running and now that the software is available they can! If you get your Proxa 7000 board running please contact me! I would be interested in hearing from you.

    The following are images of 8250-format disks. You will need to transfer these back to real disks or use one of the new SD card IEEE storage devices.

    Status

    In 2015 I received an HP motherboard with Proxa card pre-installed. In early 2016 I installed it into a case and finally got the proxa board running! I found the software confusing, and being in German didn't help. Below you can find some pictures of the board running in my CBM 128-80 high-profile machine. I don't think the board would work in an LP machine due to the different location of the cpu.




    As a side note: There is also a software PET emulator called 8432, which can emulate 4 8032 machines at once WITHOUT any hardware at all if run on a 256K B-series machine. It can even run up to 14 machines on a 1MB machine! This software is available in the CBUG library.

    Contact

    Last updated: Aug 25/2018, 9:40pm EST

    Send comments or feedback to Steve Gray(sjgray@rogers.com)