![]() ![]() ![]() ![]() 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. In this link it's suggested you could replace the 27XXXs with 28CXXX or 29CXXX with some simple modifications. If you don't have a UV eraser that means that you get one shot at programming them correctly. UV Erasable EPROMS can only be reprogrammed after having been exposed to strong UV (-C) light. If you buy replacement 27C512s, make sure you get the "windowed" type as they can be erased and reprogrammed. As the Mac-II ROM is 256KB in total, the ROMs should be 64KB each, indicating the EPROMs should be 27512s. ![]() 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. 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. In the Repair and Upgrade guide (page 197), the ROMs are refered to asĪs 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 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). ![]()
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