Scor­pio News

Over the past few years I have been using an 80-BUS system in which the MAP VFC card provides the video and floppy disk control normally provided by the Gemini IVC/SVC and FDC cards. The card was one of the original versions and was supplied as a kit which was well documented and presented, and worked satisfactorily after a simple setting up of the FDC with an oscilloscope. The VFC is supplied as a kit or ready-built, either as a video controller or floppy disk controller only, or with both facilities implemented; a keyboard interface and video switch are optional for the video-only version but are included in the complete VFC.

What It Consists Of

The VFC card can be used in 3 ways: as a combined video and floppy disk controller, as s video controller only or as a floppy disk controller (for 5.25″ or smaller driver only).

The video section provides a memory-mapped 80 column by 25 row display with a highly readable alphanumerical character set; there is a facility to switch an external video signal into the same monitor as the VFC, and the video output level can be regulated with an on-board potentiometer. A 7 or 8 bit ASCII keyboard can be accommodated by the VFC and there are both ‘normal’ and ‘inverted’ character sets, half of which are left for expansion (for example, as a game set) but still leaving 512 alphanumeric and other characters for immediate use.

The floppy disk controller will cope with any make of drive, apart from 8″. Some criticism has been levelled at MAP Systems for this, but few users will find it a limitation. Also included is 4k of RAM, 2K of which is used for the display and the rest by the operating software. It uses a total of 16 ports which are link-selectable, enabling the user to choose any group of adjacent ports from C0 to EF; the default port allocations are from £E0 to EF but alternatives are suggested.

The board contains a standard 6845 CRT controller and a 2797 FDC; unlike the Gemini IVC/​SVC, there is no on-board Z80 and the 2797 a more sophisticated version of the 1797 disk controller used on the Gemini GM829 FDC/​SASI board, and is from the same family a the 2793 disk controller used on the more recent Gemini GM849 FDC/​SCSI controller. ALL this is contained on a single 8″ square board which plugs directly into the usual 80-BUS connector. Some care is needed if the kit is constructed and an oscilloscope is needed to set up the 2797 – MAP will do this for you for a small fee, but the construction manual is very good and the average person would find little difficulty in producing a working version.

There is a significant design difference between the two types of video controller: the Gemini IVC and SVC have a Z80 incorporated on the board (Z80A in the case of the IVC and a Z80B which runs at 6MHz in the SVC). Effectively, these are single-board, highly specialized micro-computers, whereas the VFC has no microprocessor included and thus forgoes the benefits of parallel-processing. For many applications this will not be a great disadvantage since the VFC software is quite sophisticated and carries out the task of interfacing between the VFC and CPU very efficiently so that most users would not be aware of any difference in the performance of the VFC compared with the IVC. The SVC, running at 6MHz, with faster screen handling, would appear to be different, as Dave Hunt’s brief test (80-BUS News vol 3 issue 1) shows! The SVC has a lot more on-board memory as well and generally very impressive performance. It also uses a high quality version of the 6865 (its performance is degraded if the standard version is used).