Discs and Ports on a Nascom 1

by C. Godfrey

When installing the Nascom F.D.C. card or any assembly using an off-board P.I.O. on a Nascom 1, you will find that the on-board P.I.O. will no longer function. This is because the decoding for selecting ports 0 to 3 (keyboard and UART) from ports 4 to 7 (P.I.O.) relied on the I/O select link being wired to ‘INT’ which is actually a2.

Once the I/O select is moved to ‘EXT’, which is actually NasIO, the address line a2 is disconnected and the port decoding is incomplete, causing any reference to ports 4 to 7 to be directed to ports 0 to 3 respectively.

If the P.I.O. was in use prior to the addition of discs then the only advice that Nascom can offer is that you buy a P.I.O. board, not a very economic method of sorting out this simple problem. There follows a method that will solve the port decoding problem for approx. 20p plus labour.

Parts Required.

1 x 74LS02 Quad NOR Gate
3 x lengths of wire 30cm long

Documentation.

Nascom 1 circuit sheet 1

Method.

1. Locate IC 46 (74LS00)
2. Remove, bend out pin 2, replace.
3. Take 74LS02, bend sideways all pins except pins 1,7,14.
4. Solder 74LS02 on top of 74LS00 using pins 7 and 14.
5. Bend pin 1 of 74LS02 to join pin 2 of 74LS00.
6. Solder these two pins.
7. Remove link on I/O select completely.
8. Connections to 74LS02 are as follows:
   pin 10 to pin 11
   pin 3 to pin 4
   pin 2 to pin 8 to I/O select ‘EXT’
   pin 5 to pin 9 to I/O select ‘INT’
   pin 13 to I/O select ‘COM’
   Ground from pin 7 to pin 6 and pin 12

Once the wiring changes have been made, all that needs to be done is to reassemble and test.

Testing.

Using the NAS-SYS commands ‘O’ and ‘Q”, type the following:

1. O 6 0F   Initialise port A as an output.
2. O 7 0F   Initialise port B as an output.
3. O 4 55   Send 55H to port A.