G812 IVC

g812
The EPROM with the damaged label, to the left, is IVC-MON 1.0. This is the IVC  monitor software (written by Dave Parkinson, who also wrote the Revas disassembler for the Nascom and the monitor for the later SVC card).
This was a very clever card at the time. The IVC (Intelligent Video Controller) had only a mono display, but the resolution was variable. It made an excellent display card as it could more or less emulate the standard Nascom display and also display graphics. The cost was also reasonable (as these things went at the time!) so it proved to be very popular. It was well named, as it included its own Z80A processor, 2kB RAM, CRTC chip and operating system in EPROM. This made it independent of the main Z80. Only 1kB of the RAM was actually used, the remainder could run user programs.
Being designed to be NASBUS-compatible, it would work with either existing Nascom or the new Gemini systems that were just starting to appear (with the new G811 processor card).

An interesting addition was a parallel keyboard input for use with Gemini parallel keyboards. (Gemini GM821 Cherry 59-key or GM827 Rotec 87-key). It was thus possible to build quite a nice system using just this card with a G811 CPU/Bytewide ROM/RAM card or G813 CPU/64k RAM card!

  • Dedicated 4MHz Z80A processor
  • 6845 graphics control processor
  • 80 characters 25 line mono main display
  • Second width-programmable mono display (supplied as 48×25)
  • Inverse display possible
  • 128 Programmable character generator
  • 128 Fixed characters in ROM
  • 160×75 “pixel” graphics
  • Keyboard input (Gemini keyboards only)
  • Combined video output (not via an RF modulator)

On switch-on the character generator ROM was copied into the programmable character generator RAM but with foreground & background reversed, so graphic characters would be the normal character set but black on white. All the graphic characters could be changed at any time in software.

This card was designed to run without using the main computer’s memory so that the full 64k of address space could be used for CP/M. A system of commands was sent to the board via three of the Z80 I/O ports. Because of this, existing software using the NASCOM memory-mapped display would not work. Another problem with this form of communication was that of rather slow scrolling of the display, since that was generally handled by the main CPU card, although a 64 character buffer on the IVC helped. Printing text to the screen was at about 156k baud, so was easily fast enough for most purposes.

An unusual choice of connector was used for the video output. If you
look at the top of the card in the picture you can see a standard 1/4 inch “jack” plug – not normally associated with video signals!

GM812 IVC Hardware Manual

GM812 IVC Software Manual

Nascom Pages