Micropower

2)	/XROME	When address E000-FFFF is active low and BS2 (Bank Select) is low (indicating any bank except 4 is selected) then gate 2 ensures that /XROME goes active low. It is looped along the /OE pin 20s of the 8 extra EPROMS. Whether one of these is actually enabled depends on the output of IC 46.
3)	/XROMC	When address C000-E000 is active low and BS2 is low (any bank selected except 4) then gate 4 ensures that /XROMC goes active low and is looped along link blocks 1 to 8 (NOT 9) pins 10, to connect to the /OE pin 20s of the original EPROMS in the eight Nascom sockets. Again, which one is actually selected depends on the output of IC 46.
4)	/BROM	When Port 3, Bank 4 is selected, BS2 goes high and gate 2 being an ‘OR’ gate /XROME must be deselected. Meanwhile, /BS2 is low and gate 1 is an ‘OR’ so, as long as address is E000+, /BROM is active and BASIC occupies bank 4 E000-FFFF.
5)	BS1 & BS0	These send binary signals 0, 1, 2 or 3 to IC 46 to control the bank select. The output is steady depending on the last inputs to D0, D1 and D2 of the 3 flip-flops contained an the 74LS175.

The reset line connected to pin 1 of the 74LS175 ensures that bank 0 is always selected on power-up or reset. Pin 9 may be regarded as a chip select pin, enabled by pin 9 of the IN/OUT PROM IC 26.

Construction takes one evening and is easy but painstaking. Wire wrap sockets are soldered carefully to the 8 EPROMs that are to be selected at C000-DFFF. These go into the eight Nascom sockets. The 8 EPROMS to be selected at E000-FFFF have pin 20 carefully bent out to take a seperate /OE signal. They then piggy-back into the 8 new sockets. Wire wrap sockets are not essential but give some cooling space between the mated EPROMS. I lost no EPROMs doing this.

The control board uses a wire wrap socket to plug into LKS1 in place of the header plug. It therefore holds 4 sockets, 2 fourteen pin and 2 sixteen pin sockets. It has 10 leads to the board. Pins 2 and 13 of IC 46 have to be bent out of their socket before connection. Do not forget the 5 volt and Ground.

I cannot claim to have tested the board fully. I cannot afford enough EPROMS. I have piggy-backed six 2732s and moved them around and can find no faults. Indeed, the board appears to work with no wait states at 4 Meg. I drew a wiring diagram but it was such a mess I thought you would da better without it. I have checked the circuit diagram very carefully to ensure it contains all necessary information. I can only marvel at the flexibility of there fantastic littie computers.

Micro­power

Volume 2 · Number 4 · September 1982