The IWM chip in the Apple IIc is notoriously prone to failure, and both of the Apple IIc logic boards I own had defective IWM chips. This failure can sometimes be caused by faulty RAM, or external devices like Floppy Drives or Smartport devices.
The IWM chip comes in either DIP or PLCC form factors. DIP 28 chip model was consistently used across a range of models, including the Apple IIc, Macintosh 128K/512K Plus/SE. However, starting with the Mac SE/30, it was replaced by the SWIM chip, which supports 1.4MB floppies. The Mac SE FDHD model is an exception, this logic board is the same as the SE released in 1987, so it uses a DIP28 SWIM chip, as most other motherboards adopted the PLCC44 SWIM chip.
*There is a PLCC28 SWIM chip, but it is quite rare (used in models like the Mac IIx). Because of this, it is well known that the Mac II upgrade, which replaces the Mac IIx ROM and SWIM set, is documented as an Apple upgrade.
Using a PLCC44 SWIM Chip as a DIP28
Many logic boards from early 1990s Mac models with 68030 (or 68040) processors are now inoperable due to severe leakage from the infamous "Red Maxell Bomb batteries" made in Japan. However, these Mac models often contain PLCC44 SWIM chips that can be salvaged and repurposed for the SE FDHD conversion. This could be an enjoyable project for collectors with extensive Macintosh collections...
Designing... but Re-Designing the Prototype Board
Incidentally, I participate in a closed chat group called "Chips ’N Dip", where we exchange information about Mac chips. The group includes Kai Robinson, Alexandre Marcoux(aka. Alxlab), Drake, and Stephen Arsenault. Drake added me later for this group, and having discussions with such skilled hardware hackers is very rewarding!
After designing a prototype board and placing the PCB order, I remembered that I had previously designed a similar PCB for a Mac II SWIM adapter, but lost interest and never built it.
*This design doesn't look very good now hahaha...
Alex had also designed something similar. Upon revisiting our shared adapter designs, I found that alxlab had already created an adapter for using a PLCC44 SWIM in his SE FDHD! Unlike mine, his design was a piggyback type, which, despite its height, was compact and more refined than my prototype...lol The pin layout was logical, resembling common PLCC44 to DIP28 adapters.
This made me realize that my adapter needed more value-added features, so I redesigned it to be "low-profile" and match the height of the two ROMs and the IWM chip. I had to discard my initial prototype and create a new design. By moving the PLCC44 socket to the back layer with SMD mounting, I managed to achieve a flush fit.
*Both the prototype and the low-profile version worked without issues.
Interestingly, testing showed that a SWIM chip could work in a Mac Plus, wow! Provided there was no physical interference, though it was limited to 800KB floppies. With further ROM development, it might be possible to enable 1.4MB floppies on the Plus, though this would require the technical prowess of someone like Steve Chamberlin...a field for a select few genius hackers.
There was no battery in it so the Japanese OS was showing a Happy New Year splash screen.
Preparing the ROM for Mac SE FDHD
Kai Robinson sent me the FDHD ROM, which I split into two parts using SRecord. Alex kindly sent the already two split files, and I confirmed they were identical. Many thanks to them!
The wiring is messy for testing purposes...
I found that the M27C1001 EPROM, which has the same 128K capacity and pin compatibility, was almost a perfect match.
*There was already an article about it here, so I linked it.
I prepared two of these and programmed the 128K ROM file into Hi and Lo parts. Here's how I modified the pins, with a photo for reference:
- Lift pins 1, 2, 24, 30, 31, and 32 of the M27C1001 from the socket.
- Short pin 24 (now lifted) to GND using a bodge wire to pin 16 of 27C1001. *Ensure pin 16 of the M27C1001 inserts into pin 14 (GND) of the Mac SE ROM socket.
- Insert the lifted pin 2 (A16) into pin 22 of the Mac SE ROM socket(27C1001 pin 24 location) , avoiding contact with the lifted pin 24 of 27C1001.
- Bridge pins 32 and 31 with solder, and insert them into pin 28 (+5V, VCC) of the Mac SE ROM socket(27C1001 pin 30 location).
- Leave 27c1001 pin 1 and pin 30 lifted and unconnected (N/C).
After testing, to keep things neat, I ran the wires under the EPROM so they were not visible from the outside.
Finally, I confirmed successful booting from a 1.4MB floppy image! This project achieved all of its objectives...
Creating a Generic ROM Board
While the M27C1001 is sufficient, a more elegant project could be more useful. Some might dislike the abundance of bodge wires. Therefore, I designed a generic board using the SST39SF010 in TSOP form. I'll prototype this in the next phase. Stay tuned!
Where to buy it...
