g813
This was Gemini’s second (and most powerful) processor card, replacing the GM811. It was intended to replace a NASCOM-1 or NASCOM-2 as the CPU on a NAS-BUS system and to be the main CPU on the 80-BUS. With the advances in technology since the NASCOM-1 days, Gemini had, effectively, put the CPU and RAM system together on the same PCB. This board did not have the expansion capability of a NASCOM-2 though.Across the top of the PCB you can see a preset for cassette “tuning”, 5-pin DIN cassette port (but no relay – although an open-collector transistor was fitted which could be used for cassette control), serial RS-232 port connector, reset button, HALT led and PIO connector.Just below the cassette preset there is an empty 16-pin DIL socket which is supposed to be fitted with a XR2211 PLL chip for the cassette interface.This PCB could not use the original “scanned” NASCOM-1 and NASCOM-2 keyboards. The Gemini keyboard(s) used the standard RS-232 interface.

The processor was the tried and trusted Z80A running at 2 or 4MHz. It was fitted with 64k of dynamic RAM.

The rather clever monitor fitted as standard was a 4k ROM called RP/M – Rom Program for Microcomputers (written by the writer of NAS-SYS). The version fitted to the GM813 was version 2.0 or later and is slightly different to the GM811 version. 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.

An alternative monitor (fitted when the GM813 was installed packaged as part of a disk system e.g. Gemini and Quantum computers) named SIMON or SImple MONitor (shown here in version 4.2). This acted as a bootstrap program to load CP/M (or similar) from a disk drive at start up. It was automatically paged out after use, leaving the maximum available RAM free.

Unlike the NASCOM-1, both ports of the PIO were available, together with their handshaking lines, for the user. This made for a powerful system from a control point of view.

Note that this board did not incorporate any video system. This was not really a problem as it meant that a complete system could be built on just two PCBs and either of them upgraded later. Indeed, the GM813 with RP/M 2.1 or later could be used without a video card – the serial port could be connected to a permenant or temporary console device by changing a link on the PCB. The unit was, however, intended to be used with the GM812 IVC (Intelligent Video Controller) card.
Unfortunately I don’t have any other documentation on this board apart from circuit diagrams. If anyone can help, please contact me. I am looking for the rest of the manual(s).

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