INMC 80 News

With all this done, you are now ready to offer up the Graphic board. You are supplied with a short strip of plastic U and two screws, with which you fix the U to the board. Under the U you can place two sticky pads, which are also supplied, and stick the graphics board into position on the N1. It is advisable to try this a few times to be sure you know where everything is to go – the pads don’t shift easily! At the same time as offering the unit up for final fixing, you also have to connect the 24 way umbilical with the graphics board lying beside the N1. Check that everything is working properly. You should be able to get the extended graphics set by holding down the graphics key and typing (assuming you have an N2 keyboard, or have modded the N1 one – otherwise use the Nas-Sys K options – Ed.). Then, all being well, fold the graphics board into position having removed the paper off the tabs.

The Graphics Rom supplied is the NASGRA, as used on the N2. It is 2716 compatible (single 5v rail), so the possibility of custom character sets is now with you. The whole unit, once you have found the track errors, works very well, albeit with a slight degradation of sharpness in both the standard character set and in the graphics set. This would look like a timing fault on the unit, but is not acceptable. Any suggestions on how to clear it up would be most welcome. It is to be regretted that the boards have the above described track errors on them,. and no mention of it in the accompanying documentation, as it is easily cured – either on the etched board by the constructor as described, or on the original negatives/​positives for print down (a knife and some opaque is very easy to use).

In case the track fault is peculiar to one series of boards, those I have seen with it were marked (on the component side) PF-020. One last thing, the tants go with their marked sides facing away from their legends.

For those of you who feel that Nascom’s Econo-Graphics kit at 30.00 is not cheap enough, try this instead. It was built for a total of 17.96, including all sockets, cable etc.

The circuit shown will fit comfortably on a 65mm by 80mm piece of Vero board. Solder the I.C. sockets first, breaking the Vero tracks where necessary to separate pins on the same track. Then wire point to point with thin solid core cable following the circuit diagram. Use the tantalum bead capacitor to decouple the 5V power supply to the Character Generator and the Graphics ROM and the ceramic disk to decouple the supply to IC15 and the buffer.

Use ribbon cable to attach the 24 pin and 16 pin DIL plugs to the Vero board. About 180mm should be enough. Pins 1 and 2 on the 74LS00 should be taken to IC17 on the Nascom board – pin 19. If the pin is bent out sideways, then a single soldercon socket and a small piece of heatshrink sleeving will make a socket to attach the wire. On the underneath of the board IC28 pin 18 should be joined to IC17 pin 18 to latch Bit 7 of the Video memory (used as Graphics select).

At this stage I took the opportunities to make the modification to IC18 detailed in previous issues (drive the clock divider chain and IC 17 from Pin 12 of IC 18 rather than Pin 5.)

The Graphics board is now ready for testing. Remove the Character Generator IC 16 and the shift register IC 15 from the Nascom board and plug the 24 pin plug in place of IC 16 and plug the 16 pin plug in place of IC 15. Insert IC 15 and the 74LS00 and the 81L895 into their places on the Graphics board. Switch on and check that nothing drastic happens.