- All of the existing guides prescribe the destructive cutting of traces on the Z88 PCB, as if it's inevitable. I don't like doing this since then the mod cannot be reversed if it didn't work out. Perhaps people were a bit more cavalier about doing this when you could find a replacement machine for peanuts, but they're becoming quite expensive these days.
- As with the internal RAM replacement the choice of available parts to use for this mod in 2021 is very limited. In fact there is only a single currently manufactured 512K flash memory part in a PDIP-32 package - the SST39SF040. At the time of writing this can be acquired for around £10.
I had my chip programmed for me by someone with a programmer device, so unfortunately that task lies beyond the scope of this write-up.
My main source of wiring information was Dennis Berget's Z88 Hardware Modification guide, coupled with the SST39SF040 product datasheet. From these sources I could isolate the pin-out differences between the motherboard ROM socket and the 512K flash memory IC, indicated below with asterisks:
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Z88 PCB ROM 512K JEDEC Flash Memory
VCC |1 +--+ 32| VCC *A18 |1 +--+ 32| VCC
ROE |2 31| VCC *A16 |2 31|*WE
A15 |3 30| VCC A15 |3 30|*A17
A12 |4 29| A14 A12 |4 29| A14
A7 |5 28| A13 A7 |5 28| A13
A6 |6 27| A8 A6 |6 27| A8
A5 |7 26| A9 A5 |7 26| A9
A4 |8 25| A11 A4 |8 25| A11
A3 |9 24| A16 A3 |9 24|*ROE
A2 |10 23| A10 A2 |10 23| A10
A1 |11 22| CE A1 |11 22| CE
A0 |12 21| D7 A0 |12 21| D7
D0 |13 20| D6 D0 |13 20| D6
D1 |14 19| D5 D1 |14 19| D5
D2 |15 18| D4 D2 |15 18| D4
VSS |16 17| D3 VSS |16 17| D3
There are only five pins that need re-wiring which can be achieved by bending up those specific pins so they don't connect with their socket positions. Address lines A17 & A18 can easily be taken from the internal 512K SRAM modification. A16 and ROE positions are swapped. Dennis' guide mentions that WE (Write Enable) can be picked up from Slot 2 pin 22. This can be confirmed by consulting the slot pin-outs in the Z88 Developer Notes.
Wiring points - labelled in blue on the below photos
WE can be traced to a free pad very near the supercapacitor on the top side of the PCB.
ROE presents to a free pad near the right edge of the ROM socket.
A16 is not so clear-cut. It can be picked up from a free pad over to the right of the top side of the PCB, but in the end I decided to pick it up from the rear of the board, directly from pin 24 of the ROM socket to avoid needing to route a long wire around the SRAM chip.
You need to extract the solder from pin 24 of the ROM socket to properly insert and solder the wire quickly enough not to melt away its coating. Then it can be sent through to the top side through the via which is very close to that position. The risk here is that the kynar wire coating melts away very easily with the heat from the iron, so I played out a little extra length to make sure the coating was intact at the point it was threaded through the hole (see above photo). The wire must not short against the via.
To avoid lots of loose wires at the top of the PCB where the battery wiring is already in the way, I decided to thread the wires for A18 and A16 under the ROM socket between its legs. See below:
I clipped off the narrow parts of the bent up pins of the flash memory IC and tinned them up to make the wire soldering easier, as I had done with the legs of the SRAM IC. Some might argue that in clipping them I made it much harder to reprogram that IC if it was ever needed, but I think there is a very high chance of the legs breaking off after being bent even a couple of times, and the chip only cost about £10 including shipping. Once you have a working system, the ROM image on the flash chip can be updated using OZ's own update procedure, so I don't think this is much of a concern.