Micro­power

1. Solder wire 6 to pins 2 and 4 of IC 71.
2. Finally place the Snowdinger 2 board into IC 49’s socket using the long legs of the wire wrap socket

Check all your connections and when you are satisfied that they are correct, try your Nascom at 2 Mhz. Test the circuit using the Tabulate command, then, if all is well, try the system at 4 Mhz. If the circuit worked well at 2 Mhz but not at 4 tHhz there are 2 possibilities:

1. There is a wrong connection on the Snowdinger.
2. The VDU RAM is too slow.

Check all your work, if no faults are found, it is likely that you have slow RAM. On the circuit diagram there is an alternative connection for one of IC 1’s gates which is in the 4 Mhz signal path. Making this change will increase the access time at 4 Mhz but it will increase the risk of static hazards as mentioned previously. Alternatively you could get a faster 4118 RAM. This should be considered as a last resort as the first solution is simpler and cheaper.

Hopefully, by now your screen is clear at both 2 and 4 Mhz. Try playing Invaders or some other program that makes dynamic use of the screen, you’ll wonder why you put up with screen flash for so long.

NOTE – Sheet 1 in the Nascom 2 manual shows IC 7 as an inverter. This is an error as IC 7 is an 81LS97 which is a buffer. Nascoms clock is not the inverse of that supplied by the Snowdinger 2, but a buffered version. One small alteration that can be made to improve the shape of the clock is connecting a 220 ohm resistor from pin 5 of IC 11 to +5 volts (pin 14 of IC 11).

Figure 1