GM813 Z80 CPU and 64k RAM

This was Gemini’s alternative (and more powerful) processor card. It could replace a NASCOM-1 or NASCOM-2 as the CPU on a NAS-BUS system, run as the main processor on a Gemini system or run as a standalone device. It was Gemini’s first card for their “Multibus” – what was to become better known as the 80-Bus. The GM813 continued in production for a while after the G811 was discontinued.

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. This PCB did not support a keyboard, being intended for use with the IVC card.

The processor was the tried and trusted Z80A running at 2 or 4MHz. A full 64k of dynamic RAM was installed.

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.

This particular example is missing a chip at the top left corner. This is the XR2211 tone decoder used for the cassette interface.

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.

Like the NASCOM-1 and NASCOM-2, both ports of the PIO were available, together with their handshaking lines, for the user. The GM813 even used the same 26-way connector/pinout as the NASCOM-2 (2 pins that are unused on the NASCOM-2 connector are wired as additional ground pins on the GM813). This made for a powerful system from a control point of view.

Note that this board did not incorporate any video or keyboard systems. This was not really a problem. It meant that a complete system could be built on just two PCBs and either of them upgraded later. A CP/M system only required the addition of a disc control board. 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 permanent or temporary console device by changing a link on the PCB. The unit was, however, intended to be used with the G812 IVC (Intelligent Video Controller) card.

Advertisement