Auto Graphics Selection On A Nascom

Most Nascom-2s are equipped with the graphics ROM, and many Nascom-1s have some form of graphics capability, either by means of the sadly-departed Econographics kit or a locally-produced or commercial system. Often there is need to switch between two sets of graphics if you use special characters – for example, to display the pieces for a chess program such as Sargon. This can be done by switching the CE lines of the chips on and off with mechanical keys, or, even worse, by using one line from the PIO as a latch to enable the required ROM; this ties up the PIO needlessly.

The simple circuit described here uses one of the two spare output lines – from port 0, the keyboard port. The spare lines are bits 2 and 5 of this port. The status of the port is reflected at £0C00. By modifying £0C00 to set the selected bit to 1 the corresponding line is set high without affecting the other lines, and it stays that way until set back to zero by a program command, or until the RESET button is pressed. if a program uses the special graphics ROM, you merely have to include the following machine code routine at the start of the programs :

and at the end of the programs:

Construction

Make up a “piggy-back” board, using a small piece of Veroboard or a small PCB, with one 24-pin wirewrap socket, one normal 24-pin socket, and one 14- pin socket. Cut pin 18 off the wirewrap socket, leaving about 1/4 inch for wiring. Connect pins 1 – 17 and 19 – 24 -from the wirewrap to the normal 24-pin socket. The 14 pin socket is wired as shown in figure 1, and the 74LS00 is plugged in. The standard graphics chip is placed in the normal socket, and the alternative ROM in the wirewrap socket. The board is then inserted into the socket vacated by the graphics chip on the main board, using the extended leads of the wirewrap socket as a plug. Connect a wire from the keyboard socket (pin 13 for bit 2 on a Nascom 1, pin 8 on a Nascom 2) to the input of the 7400 flip-flop as shown.

The circuit is shown for 2716-compatible chips, but the principle applies to almost any ROMs or EPROMs – just be sure that you wire the outputs from the flip-flops to the correct pin on the I.C.s you use. The 2716 chip can be ‘selected’ by voltages applied to pins 18 and 20. Pin 20 is the chip select line (CS), while pin 18 is Power down/​Program line. If EITHER line is taken to +5 volts the data lines of