Screen Reverse

A. Marshall

This circuit, for both Nascom 1 and 2, gives port control over Background Colour (0), Reverse Alphanumerics (1), Screen Blanking (2) and Graphics Switching (3). The latter may be required when graphic codes have been used in machine code programs assuming that no graphics are available. (eg. the Othello program in an early edition of INMC )

The circuit in figure 1 shows how this is achieved using only 3 TTL chips, and the truth table of figure 2 shows how the Exclusive-OR gate can be used as an inverter. If A is low then C=B but if A is high then C=/B.

In the following description of the construction, the ICs referred to within the square brackets, [], are on the Nascom 1 board and those in round brackets, (), are on the Nascom 2. Note that I have a Nascom 1 and that the Nascom 2 connections are taken from the circuit diagram, but I believe them to be correct.

The prototype was constructed on Veroboard and the layout is shown in figure 3. The power supplies and port connections were taken by a 6-way ribbon cable to a 16 pin DIL header plug which was plugged into Skt a, Port 4. As the prototype was added to an Econographics board, the Serial In connection was made to the bent up pin of IC 5/6 and the Serial Out connection plugged into the socket. [On a Nascom without the Graphics, IC 15/9 provides the Serial In in the same way.] (Nascom 2 IC 65/9)

The switch or link on the Econographics board between socket 1/20 and IC 6/12 is replaced by the A7 wire connected to the socket side and the Graphics Enable connection made to the IC 6 side, [A7 to IC 17/19, GE not connected. Als o connect IC 17/18 to IC 20/12] (A7 to IC 67/19 side of LSW 2/9 and GE to IC 66/18 side). The Graphics clock is connected to the wire link to the side of IC 4/11 [IC 17/11] (IC 67/11). With the Econographics board fitted, the Veroboard can be mounted on to it, but if not then a wire wrap socket could be used to plug into the PIO socket instead of using ribbon cable and a DIL header.

After connecting the board to your Nascom, the display will be black on white as, all the inputs are held high by the pull-up resistors but as yet, the Port will not be controlling. To initiate the Port, type O 6 F and the screen will go blank as the output of the Port is zero. Type O 4 F and the display will return but now under Port control. Ta change the display under program control, send the appropriate value to the Port.