80-Bus News


January–March 1982 · Volume 1 · Issue 1

Page 47 of 55

N1 Graphics

Still Cheaper Nascom 1 Graphics

by Theo Zelders, The Netherlands

The Nascom 1 character generator is in fact effectively a 2 kByte ROM. When it is replaced by a 2732 EPROM (and this is very easy, as you will see) there is room for another 2 kBytes to accept the information of the Graphics ROM. The 128 characters of the character generator are addressed with 7 bits only. The two lines necessary for input and output of the 8th bit (usually called bit 7) can be connected to the two NC pins of the original 6576 socket. Only one line on the Nascom 1 board has to be broken, and in case you do not like that, you can always bend pin 6 of IC 15 outside and leave the board as it is. This pin 6 of IC 15 has to be connected to pin 10 (NC) of the char. gen. socket. It is the output of the 8th bit. A few extra connections have to be made: Pin 12 of IC 20 (8th data bit) to pin 18 of IC 17 (74LS273 byte latch); pin 19 of IC 17 (output of 8th bit) to pin 14 of the char. gen. socket. That’s all for the board.

Now you have to make an adapter socket for the 2732 EPROM. The pin layout’s of the two IC’s are totally different. The easiest way to solve this is to mount two 24 pin wire wrap sockets alongside each other in such a way that they fit best to the Nascom. Later on you will cut the pins of the 2732-accepting socket so you can push the other socket (with the long pins) into the socket of the char. generator, and you will find the other socket floating somewhere over your Nascom board.

Now interconnect the two sockets according to the following scheme:

long pin
short pin

That is all, apart from programming the 2732. How do you get the bytes for this EPROM? The easiest way is by copying it from someone who did the job earlier. But some one must be the first. The most difficult way is programming it by hand by coding each byte in turn from the Nascom-documentation and the characters published in an earlier Newsletter. This is easy but time consuming. I did it earlier, at least partially, and it turned out to be fun. The characters are built from 16 consecutive bytes in the EPROM. Each byte defines a line of eight dots. A bit is set when the corresponding dot is white; zero in case the dot has to be black. Highest order bit corresponds with leftmost dot. Each character is addressed with it’s own number from 0 to 255.

There is one other way to at least get the original contents of the character generator IC. Before you cut the pins of the 2732 socket, push it into a 2732 socket somewhere on your system; connect the two extra supply potentials (+12 and −5) to the appropriate pins of the 6576 socket and read the characters into computer-memory or onto tape. Also a 2716 socket will do. (By the way, I converted the 4 2708 sockets on my memory board into 2732 sockets and now I can choose by switch which of the EPROMmed utilities in the 4 2732’s I will use by simply pulling the chip-enable down when I need one).

I wish you success, and fun, in programming chess and other symbols into your Nascom.

Page 47 of 55