You're getting there Lardo
But some more to do...
In total there are six track cuts to do, and six wires required (not including the link wire for LK2).
Pins to isolate: 1, 23 and 26.
On the bottom (solder) side:
Cut the track to pin 1. As shown in your picture.
Cut the track to pin 23. As shown in your picture.
On the component (top) side:
Cut the track to pin 1. The track going towards the Z80.
Cut the track to pin 23. The track going towards the Z80.
Cut the track to pin 26 either side of the pin (two track cuts). One cut is shown in your picture. The track going to the edge of the board via LK1 from pin 26 also needs cutting.
Because we need to maintain the rest of the circuitry, we now need to link up the isolated tracks beyond the cuts, but keeping the relevant SRAM chip pins isolated.
The former pin 1 cut tracks need linking to maintain /RFSH to edge-connector. See the green dotted line on RetroTechie's picture of the component (top) side.
The former pin 23 cut tracks need linking to maintain /WR to pin 27 and to the edge-connector. See the green dotted line on RetroTechie's picture of the component (top) side.
The former pin 26 cut tracks need linking to maintain the +5V rail to the remainder of the board. Connect a wire on the bottom (solder) side from pin 18 of the now unused pad for IC4A (2114 / 21L14 SRAM) and connect it to the +5V pad for LK1 (edge-connector side).
Address lines can go to pins 1, 23 and 26 in any order. The SRAM does not care which address line connects to which address pin. And the Z80 CPU does not care either.
So I suggest wiring as follows:
A13 to SRAM chip pin 1.
A12 to pin 26.
A11 to pin 23.
IC4 SRAM pin out now:Code: Select all
Old signal 24p 28p new signal
/RFSH - 1 * A13
+5V - 2 +5V
(1) 3 A1
(2) 4 A9
(3) 5 A8
(4) 6 A7
(5) 7 A6
(6) 8 A4
(7) 9 A2
(8) 10 A0
(9) 11 D0'
(10) 12 D1'
(11) 13 D5'
(12) 14 GND (0V)
(13) 15 D6'
(14) 16 D2'
(15) 17 D7'
(16) 18 D3'
(17) 19 D4'
(18) 20 /RAMCS (via R2)
+5V via LK1 (19) 21 * A10 via LK2
(20) 22 /RAMCS (via R2)
/WR (21) 23 * A11
(22) 24 A3
(23) 25 A5
+5V (24) 26 * A12
- 27 /WR
- 28 +5V
* indicates changes. Pin numbers in brackets refer to a 24-pin IC in that position, as shown in ZX81 schematic.
Mark