Volume 1 · Number 1 · August 1981

Page 23 of 33

“Connect pad of ICxx/y to …” means: solder one end of wire to any suitable point on the P.C.B. track that pin y used to connect to, before it was lifted. Plated through holes are ideal for such connections.

“Connect IC xx/y to …” means: solder one end of wire to any suitable point on the P.C.B. track that pin y connects to.

Preliminary: Remove “snowplough” if fitted and restore board to its original state.

  1. Remove IC 31. This 7406 may be used, if desired, on the veroboard, or retained in case the 2102’s are too slow
  2. Lift IC 11 pin 5. Connect pin of IC 11/5 to IC 11/4
  3. Lift IC 45 pin 13
  4. Lift IC 36 pin 6
  5. Plug veroboard into IC 31, ensuring that the 7406 has the same orientation as originally. The major part of the board fits over the group of R’s and C’s adjacent to IC 31, and between the PIO and the character generator. Subsequent instructions assume that the switch is already wired in, and that wires using the colour code on the logic diagram have been soldered to the veroboard.
  6. Connect pin of IC 45/13 to WVSEL (black)
  7. Connect pin of IC 36/6 to VDUSEL (blue)
  8. Connect pad of IC 36/6 to RVSEL (white)
  9. Connect 1MHz pin adjacent to IC 19 to 1MHz (brown)
  10. Connect 2MHz pin adjacent to IC 19 to 2MHz (red)
  11. Connect 4MHz pin adjacent to IC 19 to 4MHz (yellow)
  12. Connect IC37/24 to WAIT (green)

NOTE – Those of you who are using Nascom buffer boards are currently driving their processors with a different oscillator to the on-board one driving the V.D.U, logic, this being Nascom’s solution to a bus skew problem. The modification presented here cannot tolerate independent oscillators since it uses a synchronisation technique, and continues to use the Nascom 1 on-board oscillator to drive the Z-80. The buffer oscillator must be disconnected in its entirety from both the Nascom 1 interface, pin 38, and the Nasbus, pin 5. The latter, if connected to pin 11 of the 7406 on this modification, produces the same solution to skew as the buffer implements – i.e., two gate delays. If you don’t like flying leads, and are prepared to do some P.C.B. track cutting, isolate the Nascom 1 interface pin 38 and use this as a method of linking the signal from the Nascom 1 to buffer and hence to Nasbus.

When you have carried out the above modifications, try the system at 2MHz. Any screen misbehaviour will probably show with “tabulate”, but experiment with several programs making dynamic use of the screen. If misbehaviour is seen, fit a 150 pf capacitor between pin 8 of the 7406 on the veroboard and ground. If the system still doesn’t work properly then your V.D.U. RAMs are too slow – either get some faster 2102’s or try an alternative suggested in the text. If all is well, and providing that you are quite certain that your system can run at 4MHz i.e., it did so before installing the modification – then carry out the same tests at the higher speed. Hopefully, you now have a clean screen without any disturbances.

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