Nascom News­let­ter

second drawback would be in finding a particular entry – we could either read each record in turn starting at the beginning until we get to the right one, or more efficiently we could look at the middle record of the file, determine if this is before or after the required data item, and then look at the middle record of either the top or bottom half of the records, etc. There are time penalties in these approaches since disc access with floppy discs is relatively slow, although the latter, binary seach, method is quite efficient.

Next month we will discuss the alternative methods of controlling access to random files.

7. AVG Connection Standards

We have now defined some standards for connection of colour monitors to the AVC, and these ae used on those production machines which have the AVC fitted as standard.

These connections can also be used for monochrome monitors, although most users have preferred in the past to use the normal BNC connector as the output from the AVC. The advantage of the scheme described here is that the 48 x 16 display can be made permanaently available while using the AVC output for graphics. This can be very convenient if you want to run twin screens – one containing menus or other text. This technique is used in NAS-CAD. An alternative would of course be to use the TV output for the 48 x 16 screen. Although clarity of the picture is not as good on a TV display. A 25 way D-type plug is used for the colour monitor and 48 x 16 displays while the BNC connector carries the usual AVC card output (controlled by the MODE command to give either graphics or 48 x 16 display). Note that a plug is used on the rear panel of the computer (preferably in position CON 4) to reduce the danger of plugging a printer into the AVC and causing damage.

The connections between the 25-way D-type plug and the 16-way Scotchflex connector on the AVC are as follows: