Wednesday, May 31, 2023

Forty Years of Personal Computing - Tanner/Digital Research 64KB Memory Board

Digital Research Computers 64 KB SS-50 Board
I graduated from Georgia Tech in the fall of 1983 and got a full-time job. By 1984, 20 KB of memory didn't seem like enough for the MC6809E V1 board. My attempt to expand the  8 KB MP-8M to 16 KB didn't work. And I never built the dynamic RAM circuits I designed. I wanted more memory.

Digital Research Computers marketed an SS-50 card designed by Tanner Computers in the early 80s. It sported thirty-two sockets for 2 KB RAM or ROM chips. These 2716-compatible chips were quite popular at the time.

I bought a kit for about $225 with a full 64 KB of RAM in June 1984.

Assembling the kit was straightforward, along the lines of the SWTPc kits. The board worked right away, with no soldering issues -- largely due to the excellent solder mask on the board.

For SS-50 systems, this board has several flexible options. The first 48 KB presents as three 16 KB banks that are enabled individually. Each 2 KB segment in the top 16 KB is enabled individually, allowing one to navigate conflicts in the C, D, E and F blocks of memory. This allowed for I/O on the motherboard, or perhaps RAM or ROM on the CPU board.

The board supports extended addressing on the S0-S3 pins. When enabled, the entire board responds as one 64 KB block. Each socket can contain either RAM or ROM chips, selectable by the jumper next to each chip.

Initially, I used this board without extended addressing as a 56 KB board. I later enabled extended addressing to access the full 64 KB, after modifying the MP-B motherboard to decode the 20-bit address for the I/O slots. This allowed me to use that 8 KB of RAM for a virtual disk drive, briefly.

I discovered some  extended memory issues between BBUG/Flex09 and OS-9, so I disabled the MP-B decoding.

As pictured, the board has the E000 and E800 blocks disabled, with F000 enabled, and F800 disabled. This configuration was appropriate for the MC6809E V1 CPU board and MP-B motherboard without the 20-bit address decoding, although, technically, the E800 block could be enabled, and the F000 block would not be accessible after I modified the MC6809E V1 CPU board for a 4K ROM.