Gemini started with the G811 processor card, intended to replace a NASCOM-1 or NASCOM-2 as the CPU on a NAS-BUS system should Nascom be forced out of business. They had made a decision to continue manufacturing for the bus, as there were already quite a few companies developing equipment for it, even without Nascom themselves. It was specifically targeted at upgrading existing NASCOM systems, which would almost certainly have expensive RAM boards already installed. It could also be the CPU card on a Gemini-only system, of course.
With the advances in technology since the NASCOM-1 days, Gemini had, effectively, put the CPU and ROM systems together on the same PCB. This board did not have the expansion capability of a NASCOM-2, though it included a Z80 SIO, a 8250 UART and a cassette interface. This PCB had a connector for the Gemini G613 parallel keyboard. Alternatively it could use a keyboard connected to an associated video card (e.g. the G812 IVC), which then gave keyboard buffering. The processor was the tried and trusted Z80A running at 2 or 4MHz.
There were 4 “bytewide” sockets for RAM, EPROM or ROM chips. The monitor fitted as standard in one of these was an early version (often 0.1) of a 4k ROM called RP/M – Rom Program for Microcomputers (written by the writer of NAS-SYS). This looked, to programs, like a CP/M operating system. It was possible to write, run and debug small CP/M applications without having a disk drive installed! These could then be re-loaded from tape when disks became available. Indeed, Microsoft’s MBASIC for CP/M would run if loaded from tape or EPROM.
If the remaining 3 bytewide sockets were fitted with type 6116 RAM chips (the only bytewide RAM chips at any reasonable cost at the time), then there was 6kB RAM available on board, not always enough so a separate RAM card was usually used.
Incidentally, the manual for this board is labelled GM811 on the cover, but refers to G811 in the text on the drawing and is labelled as such on the board itself. This is almost certainly because the manuals were standardised when Gemini began to produce boards for the “Multibus” rather than the NASBUS, so all the existing numbers took on the new format.
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