Micro­power

Latter day ROMs work off 5 volts and take relatively little juice from bus lines, particularly as they have standby modes when not enabled. So, out with the hacksaw and dawn to work.

The first job is to remove the switches from the Monitor board and you will also need to remake the links to pin 20 (Output Enable) on the two sockets that are plugged through into the Nascom 1.

Next you need to cut the tracks to pin 19 and 21 of the other pair of sockets. These carry the +12 and −5 volts needed by 2708s but not by 2716s nor bigger ROMs.

The 2708 sockets provide all the data bus lines and address lines A0 ta A9. To accomodate a 2716, you need ta find A10 and connect it to pin 19. A 2732 needs A11 and that has to go to pin 18, and the BASIC ROM needs A12 on pin 21. Check with a data sheet, or any of the many articles on the subject more competent than this one, as to what to do with the pins that are now floating, but you can in practice pick up A10, A11 and A12 from anywhere on the Nascom 1. Suggested places in increasing order of ‘safety’ are the Z80 CPU Socket (pins 40, 1 and 2), the Nascom 1 bus/​43-way connector (pins 14, 13 and 9) and lastly the Nas/​Gemini/​80-bus connectors on the buffered. motherboard (pins 40,​41 and 42). These are also in increasing order of length, so a final decision must be made as a compromise between physical and electrical properties.

The last part of the modification depends on the RAM board that is fitted to the system and produces the correct enable lines for the ROMs. I use the Nascom RAM B card which has the Nascom 1 connection on the bank-select socket. Sixteen open-collector outputs are used to choose which part of each bank of RAM is mapped to which 4K memory area. The simplest solution and the one that I tried first, was to connect pin 20 of the BASIC ROM to the E and F outputs of the bank-select. No surgery is required since there is a convenient hole where the switch was taken out of the board.

So, the N1 select goes to block 0, the 16K of RAM goes from 1000H to 4FFFH and the BASIC ROM resides from E000H to FFFFH. Nas-sys in the 2708 sockets and switch on.

The first task was to test the RAM and that produced no problems at all. Nas-sys appeared to work through all its many features and, very happily, when the area of memory from E000H upwards is tabulated, we can see that BASIC is there. Give it a ‘J’ and it crashes.

Wait states…?? Yes, I have heard of them, but this is not meant to be a technical article. Is the ROM fast enough to work? Can the Z80 read it at 2MHz? It obviously is because it can be tabulated and you can even do a fast copy of its contents into a Ram area with the ‘C’ command but the ‘J’ still does not work.