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. Without the software we may never know.

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

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