From: "waynegriffin" <[EMAIL PROTECTED]> Subject: Reading Macintosh ROM's Date: Thu, 30 Dec 2004 16:42:31 -0500 Message-ID: <[EMAIL PROTECTED]>
I have a nice Stag PP42 EPROM programmer here and I am familiar with its operation and how to handle electronics. Have any of you ever directly read the ROMs from older Macs straight out of the chip? I have seen ways that you can get them to dump their ROMs as files onto a floppy (basilisk). But I have never inquired about how to place the physical devices in something and read them.
The reason I am asking is because I have a Plus that hangs on a Happy Mac boot and I want to ask about checksum or crc from you guys. I will know if my ROMs are bad really quickly. Unfortunately, I do not have another Plus about to A - B swap them and find out.
Is it possible to burn eproms and then place them on the Macintosh motherboard instead of the custom fab chips?
Yes, with caveats. I have desoldered IIci ROMs, read their contents, and written them out to Flash chips in PLCC packaging in order to make a ROM module for the SE/30. Gamba did the module fabrication. It worked fine.
The caveats are that you need to make sure that the chips you are using have the same pinouts as the ROM chips. This is rarely a problem if the chips have the same capacity, as the data, address, power, ground, CE and OE pins are pretty standardized for a given capacity of storage chip.
However, the pin arrangement changes somewhere around 512Kbits. That is, chips smaller have one pin arrangement, and chips larger have a different pin arrangement.
Ideally, you would compare the pinout of the Mac ROMs to the pinout (as shown on the datasheet) of the EPROM you are proposing to use. The difficulty with that is getting the pinout of the Mac ROM. Some of it can be traced out with a continuity meter. For example, power should connect to 5 volt connectors on the MB and similarly with Ground. You can probably get the datasheet for the 68000 still and then check which pins on the ROM connect to which address and data pins on the 68000. If you use the datasheet for your EPROM, and the Mac ROM matches, then you won't even really have any hunting around to do. You'll just be confirming that the Mac ROM does match the EPROM pinout.
Also, if you use Flash, instead of an EPROM, you may need to make simple modifications such as tieing WE_ and Reset high by running a little wire from the power pin to those pins on the chip.
Jeff Walther
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