This was a mono video control card with various operating modes and embellishments. It was an enhanced version of the earlier IVC card (G812) and was (very) upwardly compatible with it. It was actually Gemini’s first video card to be specifically designed for their own system. No attempt was made to remain compatible with the NASCOM video system.
Once again, the SVC monitor software was written by Dave Parkinson.
Available (software configurable) configurations were:
  • 4 pages of 25 lines x 40 characters with standard & alternate character sets
  • 2 pages of 25 lines x 80 characters with standard & alternate character sets
  • single 256×256 graphical display
On-board processing used a dedicated 6MHz Z80B processor, quite an upgrade over the IVC’s 4MHz device. Associated with this was an 8k operating system EPROM, 2k workspace RAM and 8k video space RAM. All 256 possible characters could be redefined in software. The video system used a HD46505 CRTC chip. Hardware features allowed for the board’s output to be blanked or inverted. Also blinking, half-tone background and half-intensity character attributes were available. It was really a single board computer in it’s own right.
Across the top of the PCB shown are:
PL6 – parallel Gemini keyboard
PL4 – 6-pin 240º DIN – serial ASCII Gemini keyboard
PL3 – 5-pin 180º DIN – Light pen socket
PL2 – 5-pin “domino” DIN – video output (includes separate horizontal & vertical sync signals – Gemini seemed to have a thing about using odd connectors for video!)
The card provided a type-ahead buffer and could also provide programmable function keys when a serial keyboard was used. The “beeper” which could be controlled in software. The light-pen returned character cell co-ordinates when triggered.
There is a set of holes on the board, marked PL5 (26-way). As far as I’m aware they were never used, but they carry expansion bus signals for a “piggy back” board. Originally this was intended to install a 8250 type UART and associated components so that the board could become a stand-alone serial intelligent terminal. There is no support for this in SVC.MON, at least up to version 4.01 (which is the last, as far as I’m aware).

Like the earlier G812, communication with the system processor was via three I/O ports. This meant that the 64k address space of the system processor was not taken up by video memory. The flipside of this was that some operations took longer – scrolling being noticeable due to screen updating not taking place during screen memory access by the CRTC. However, access to the screen memory, shared between the processor and the CRTC, was now managed in hardware rather than software, leading to a much faster display than the IVC had achieved.

Inbuilt 64-character keyboard buffering allowed for type-ahead so the system could keep up with fast typists!

There was 1kB spare RAM available in the workspace area and it was quite possible to write short programs to run in there – independent of the main processor. Just a bit of early multi-tasking there…


(Click these to get links to the full size drawings)




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