80-Bus News

Nascoms On The Rampage!

(or What to Do with the GM862 and a Nascom CPU)

by Warren Williams

Introduction

The Gemini GM862 is a 256K RAM board that supports Page-Mode addressing when used with Nascom 2 or Gemini GM811 CPU boards, memory mapped Extended Addressing when used with the Gemini GM813 CPU/RAM board, and Direct addressing within the 512K 80-BUS address range when used with the Gemini GM888 8088 CPU board. Although its advanced features are intended for use with current (and future) Gemini 8 bit and 16 bit products, owners of more venerable Nasbus/80-BUS machines can also put it to good use as a Virtual or RAM disk – given that they already have at least one Disk Drive in operation. The advent of 256K and 512K memory boards has led to used 64K GM802 boards being offered for sale at modest prices as professional users (and fanatical amateurs) upgrade their machines. This means that ‘normal’ Nascomaniacs can get to experiment with Paged RAM at relatively low cost. This article attempts to provide a few tips on installing the GM862 or GM802 as a Ramdisk on a Nascom 2 that are absent from, or perhaps beyond the scope of the original Instruction Manuals.

Having aquired a Gemini GM862 256K RAM board [Ed. – Mr. Williams won it in a contest, but perhaps is too modest to say so!] the task remained of implementing it on either of my trusty Nascom/Gemini computers. Each of these has a Nascom 2 CPU, GM812 IVC, GM809 FDC and (originally) a GM802 64K RAM board, plus interchangeable I/O, EPROM, and Programmer facilities. CP/M or Nas-Sys and various DOSs are available on both by means of switches. One machine is housed in a Kenilworth case while the other is built into a Vero card frame. Currently the 256K board is installed in the Kenilworth console, and both 64Ks are used in the Vero rack, providing respectively 180K and 60K of Ramdisk with 60K CP/M operating systems. However, to achieve this happy state a few problems had to be overcome, as shown below.

Documentation and Hardware

First a quick look at the GM862 and its documentation. Mine was an early production sample, and the original documentation consisted of photocopied A4 sheets without a circuit diagram; however an additional A5 booklet and A3 size circuit diagrams were supplied within about two weeks of my posting the request slip thoughtfully provided by Gemini. The Manual omits detailed discussion of how the circuits work but concentrates strongly on the setting of the two 8 bit DIL switches and drops carefully concealed hints on other matters. The information is accurate and some thought has been put into presenting the various complex modes and options available. The trick is to pick out ALL the parts vital to your own requirements and avoid bending your brain on the rest. Warning! There are (were?) vital omissions about Nascom usage.

As for the board itself; construction was well up to standard and all the RAM and a few other selected i.c.s are socketed. It has proved to work reliably on both a friend’s Gemini and my own two Nascoms. On the Gemini, the CM862 worked first time without a hitch in Extended Addressing mode when the appropriate onboard switches were set according to the detailed manual, and the software was configured to suit. Extended (memory mapped) addressing capability is not available on the Nascom 2 CPU board, and Page Mode operation