INMC 80 News


May–September 1981 · Issue 4

Page 55 of 71

Add ons

This is the first in a series of articles where we will be asking manufacturers of Nascom compatible “goodies” to outline their product range. In this issue we have details from Gemini Microcomputers Ltd.

Gemini is a very young company, started only some 7 or 8 months ago and yet they already have a very comprehensive range of products available, with several more exciting items about to be introduced. MD of Gemini is John Marshall, founder and ex-MD of Nascom, and Technical Manager is Paul Greenhalgh, an ex-Nascom Engineer. Perhaps this explains the company’s strong interest and capability in producing Nascom compatible product!

Gemini Microcomputers

Existing Products

EPROM/​ROM board

With an increasing range of firmware becoming available for the Nascom, users were rapidly running out of EPROM sockets and Nascom 1 owners were eager to find an easy way to add the BASIC ROM to their systems without recourse to surgery, so an EPROM/​ROM board seemed an obvious omission from the Nascom product range that had to be put right!

The Gemini G803 EPROM/​ROM board has 4 banks of 4 sockets for EPROMS. Each bank can take either multi-rail 2708s (1K x 8) or single rail 2716 (2K x 8) EPROMS, although each bank must contain only one type. Flexible decoding allows each bank to be located on any 4K address boundary. There is also a further 24 pin socket to take the MK36271 8K BASIC ROM. Any memory on the card produces a RAMDIS signal on the bus when it is addressed. This gives the EPROM or ROM priority over any RAM at the same address.

The card also contains a wait-state generator which, when enabled (a link option), is only activated whilst the card is being accessed. This means that a Nascom 2 owner can move all of his EPROMS and BASIC ROM onto the card and switch off the wait states on the N2. (I know many N2’s will run without waits anyway, but not according to the BASIC ROM spec!)

Finally (yes there is more) the card supports the Page Mode scheme. What’s that? Well, without going into great detail, (perhaps someone will write INMC80 an article?) it allows you to have more than 64K of memory connected to your system (up to 256K in fact) although only 64K can be accessed by the Z80 at any time – sorry, no 200K Startrek!

Disk System

After the EPROM board we started to think about disk systems, but we weren’t the only ones! One day Dave Hunt said “we’ve got a disk controller going, do you want to see it?” After seeing the card (of course we accepted his offer) we asked how it would be sold and were told “as a kit”. Having previously considered all the implications of a disk controller kit, drives, PSUs, interconnects, software etc. etc. we shuddered, and, as regular INMC80 readers already know, decided to produce a complete unit based on the Henelec controller card. A power supply and case were designed, a CP/M licence obtained from Digital Research, Pertec double sided drives selected and, with the addition of the Henelec controller card, the whole lot built and tested.

The Gemini G805 disk system connects to the Nascom PIO and is available with one or two drives and CP/M or D-DOS, a simple operating system. The popularity of the system has lead to Business & Leisure Microcomputers introducing DCS-DOS, a much more sophisticated version of D-DOS with file handling for BASIC, ZEAP, Nas-Pen and machine code files.

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