Jumper.

Shortly, with the advent of new hardware opportunities, ie: disks with CP/M, many Nascom 1 users will no longer want to RESET to 0000H, as unfortunately CP/M requires RAM down there, and system ROM somewhere towards the top of the memory map. The circuit used is lifted straight from the Nascom 2 circuit diagram, so I claim no originality – however, it works, so I pass it on to others.

The object of the exercise is to force the top four address lines (A12 to A15) to some value other than 0000H on ‘reset’. This is required for the first instruction cycle only, and this forced condition must be released before the next instruction cycle. What happens is that on ‘reset’, the top four address lines go to 0H (as usual), but because of the action of the ‘reset multiplexer’, the top four address lines are modified to the required address. This condition remains true, commencing at the first M1 cycle following the ‘reset’, and termiantes on the next following M1 cycle.

To ensure proper operation of the program thereafter, the first instruction encountered at the multiplexed address must be an absolute unconditional jump (C3XXXX) to the next instruction. This will force the next instruction address into the program counter, and from then on operation will continue normally, and the effect of the ‘reset multiplexer’ can be removed. For example, we wish to “reset jump” to F000H, and then continue the program from that address. On ‘reset’, an M1 cycle is put out by the Z80, and an op-code fetch occurs. The Z80 address bus is set to 0000H, but due to the action of the multiplexer, the op-code is fetched from address F000H. This op-code must be an immediate jump instruction, and for sake of argument, we will make this an absolute jump to F003H. The code will look like this:

F000  C3 03 F0    JP F003H
F003  XX          Next instruction

as the op-code will be translated by the Z80 as an absolute jump, the next two bytes will be fetched, advancing the program counter to 0003H, and the multiplexed address to F003H. Having fetched the byte at F002H, the address F003H will be placed in the program counter. The Z80 will then put out another M1 cycle to fetch the next op-code, this time from F003H. The falling edge of the M1 cycle will cancel the ‘reset multiplexer’ and the address bus will carry the next address put out by the Z80, which will be F003H as required.

The circuit is very simple and uses a 74LS257 (quad tri-state multiplexer) to gate the forced address onto the bus, or to gate the processor bus onto the bus depending on the condition of pin 1. The 74LS257 may be likened to a 4 pole 2 way switch. As we want to intercept the address lines prior to them getting to NASBUS, the buffer board is the logical place for it. Fortunately for us, Nascom left a vast open space on the buffer board, and this is ideal area for building a small ‘piggyback reset multiplexer’.

The four top address lines, M1 and RFSH all pass through IC3 on the buffer board on their way to NASBUS, so by removing this IC and moving it to the ‘reset multiplexer’ board, we have an ideal connector point for the ‘reset multiplexer’ board via a 16 way cable and header plug. Only one minor modification is required to the buffer board itself to get the RESET signal on to the ‘reset multiplexer’ board. Again fortunately IC3 has two gating inputs on pins 1 and 15. We only need one gating signal to the ‘reset multiplexer’ board, so one pin is spare. It is most convenient to use pin 1 for the RESET and pin 15 for gating. Thus we need to cut the track between pins 1 and 15 of IC3. As this is on the top side of the pcb, this can be a bit tricky. Remove IC3 and you will see two adjacent tracks (See alongside circuit diagram.)

Cut the track indicated and try not to cut both (as I did), and connect RESET from some suitable point on the buffer board to pin 1 of 1C3 via a flying lead. Use a piece of veroboard about 2″ square and construct the circuit shown on the circuit diagram, bolt (or use double sided sticky) the veroboard to the buffer board as close to IC3 socket as is convenient, having prepared a suitable length of 16 way ribbon cable with 2 16 way header plugs to connect from the multiplexer board to IC3 socket. A tip, my board didn’t work first time, so I found 12″ of ribbon cable useful, so that I could get at the ‘piggyback’ board to modify it. Having found and corrected the fault, the ribbon was shortened, and the board mounted permanently.