Solder wire 6 to pins 2 and 4 of IC 71.
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:
There is a wrong connection on the Snowdinger.
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
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).