Macintosh II ROM Replacement

Question

I'd like to upgrade my Macintosh II to FDHD, I have a SWIM chip (to replace the WIM chip), but don't know how to burn the ROMS.

The upgrade process I'm trying to do is described in the book: "Macintosh If Repair and Upgrade Secrets"

Installing an FDHD/ROM Upgrade into a Macintosh II

The official Apple FDHD Macintosh II Upgrade Kit, pan number M0244, provides the five chips you need to get a 1.4 MB super drive working in an original Macintosh II. Table 8-2 provides a parts list with board references for various revisions of the Macintosh 11 logic board. Figure 8-3 shows the general area where the five chips are located.

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The current chip part numbers and board locations are

Location	Apple Part Number
U3	342-0108-B
U4	342-0107-B
U5	342-0106-B
U6	342-0105-B

I'm not sure what type of EEPROMS are compatible, and how the image needs to be split across the 4 chips. The ROM chips I'm attempting to replace are

I've tried looking around for previous attempts at this online, but haven't found any documentation about compatible EEPROMS or how to map the ROM file onto the 4 chips.

Answer 1

I'm not very familiar with Macintosh, so my answer is based on my general knowledge of the Motorola 68K architecture (like the 68020 that the Mac-II is based on).

ROM content

In the Repair and Upgrade guide (page 197), the ROMs are refered to as

• U3: Low ROM
• U4: Medium Low ROM
• U5: Medium High ROM
• U6: High ROM

As the Mac-II is a 32-bit "big-endian" machine this would mean U3 contains bits 0-7 of every 32-bit word, U4 contains bits 8-15, U5 contains bits 16-23 and U6 contains bits 24-31.

I'm not familiar with the way that Mac ROMs are distributed, but if you have a single ROM image file you should split it into 4 parts, with every first 8-bit byte going to the "low" ROM, every second byte going to the "medium low" ROM, every third byte going to the "medium high" ROM and every fourth byte going to the "high" ROM.

Original EPROMS

The first link in the comments together with the images of the motherboard indicate the original ROM chips are not EEPROMs but UV Erasable EPROMs of the 27XXX type. As the Mac-II ROM is 256KB in total, the ROMs should be 64KB each, indicating the EPROMs should be 27512s.

Replacement (E)EPROMs

If you buy replacement 27C512s, make sure you get the "windowed" type as they can be erased and reprogrammed. UV Erasable EPROMS can only be reprogrammed after having been exposed to strong UV (-C) light.

If you don't have a UV eraser that means that you get one shot at programming them correctly.

In this link it's suggested you could replace the 27XXXs with 28CXXX or 29CXXX with some simple modifications.

However, replacement with Flash or EEPROM might not be easy, as any suitable replacement Flash (29C512) or EEPROMs (28C512) I've found have 32 pins and are not a direct replacement for a 27512, which has 28 pins.

Replacement with electrically erasable 28/29 Flash/EEPROMs is technically certainly possible as most of the pins match with the "old" 27 type EEPROM, but you might have to build a separate custom PCB for this.