nascom.infoThe first Gemini disk system, the G805, was designed to give a ready-built alternative to the Henelec kits. It used the same drive control card as the Henelec system (not suprising as the same people designed it) but now housed in the drive box with a Gemini-built power supply. The complete system was designed to match the Kenilworth case, which had become extremely popular. The drive control software was re-written to handle double-density drives.
GEMINI G815
Gemini didn't sit back once the G805 started selling. Lucas Logic still hadn't got the official Nascom disk system off the ground (although it had been promised for some time) when Gemini produced their first bus-specific disk drive system. The drive box contained one (G815/1) or two (G815/2) drives. The controller, G809, connected directly to the 80-BUS (or NAS-BUS) rather than to the PIO. It used a very new disk controller (WD1797 and its support chips) with built-in double-density support. This format was becoming the latest standard. It could also handle 48 or 96 tpi drives and 8in drives (but the special 50-way connector was an optional extra).
The normal formatted capacity was 350kB per drive.
To go with your new hardware, Gemini would quite happily sell you part number G513, the CP/M 2.2 operating system for NASCOM-based systems.
An important point was that, when using CP/M, the G815 system could read disks produced by the Henelec system because the Gemini CP/M was "auto density seeking". When it was given a disk which failed to read in drive B it would retry in single density mode before rejecting it as unreadable.
POLYDOS 2 was also available for this system. This was an enhancement to the original POLYDOS offered for the HENELEC disk system.
The Gemini system was booted using the Reset Jump facility to jump to a 1k EPROM called SIMON at address F000H. SIMON (SIMple MONitor) would detect the Gemini IVC card and use it if possible, look for a drive containing a CP/M operating system and boot from it. If this failed it would act as a monitor to allow low level system debugging - a very useful little program indeed!
The BIOS for Gemini's version of CP/M 2.2 was written by Dave Parkinson.
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A further enhanced card, the GM829, allowed up to four mixed 5.25" and 8" drives. These could be single or double sided, single or double density formats. Switching between single and double density and between 5.25" and 8" was under software control. It used the WD1797 disc control chip. The board also included a SASI connector suitable for Gemini's own GM835 "Winchester" drive. Recommended floppy drives were listed as:
- Shugart SA200
- 5.25" single-sided 48tpi 40 track
- Pertec FD250
- 5.25" single-sided 48tpi 35 track
- Teac FD-55E/55F
- 5.25" single/double sided 96tpi 80 track
- Micropolis 1015F5/F6
- 5.25" single/double sided 96tpi 80 track
- These were the principal drives supported.
- Pertec FD650
- 8" double sided
- Shugart SA800/850
- 8" single/double sided
- Tandon TM848-1/-2
- 8" single/double sided
:-)
CP/M 2.2 was available configured for the Pertec FD250 and Micropolis 1015 drives for:
- Nascom-1 or 2 with Nascom screen or Gemini GM812 IVC board
- Gemini Multiboard system (GM811 CPU or GM813 CPU/RAM)
Polydos was available for the Pertec FD250 and Micropolis 1015 drives.
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The final version of Gemini's disk control board was the G849. This could handle up to eight floppy drives and also incorporated a SASI (SCSI) port for connection of hard disk drives.