Date: Fri, 21 Jan 2005 21:43:19 -0500 From: Colin <[EMAIL PROTECTED]>
I assume all of this applies to an 8500, since they are the same family? Is the 8600/300 ROM SIMM hard to come by, because I'd like to put one into my 8500 if/when I decide to get a G3 upgrade. Or maybe Apple (or someone) made the ROM flash-upgradeable (like a PC's BIOS)?
I forgot to mention, it is possible to make an 8600/300 ROM DIMM out of a Power Computing ROM DIMM but it's probably too much trouble for most folks. I've seen a number of PCC machines where the seller or donator stripped out the memory and also took the ROM (darn-it). There should be some of the ROM DIMMs floating around in the hands of clueless people who don't realize what they removed. Of course, they probably think that they have a memory DIMM that doesn't work right or soemthing.
This is the ROM DIMM found in their Catalyst based machines such as the PowerCenter, PowerCurve, and PowerTower (not PowerTower Pro). The circuit board already has most of the stuff you need. The code on those boards is contained in four Apple ROM chips and it is the older x500 code ($77D.28F2). So the first step is to desolder the four ROM chips without harming the circuit board.
Then you get four Flash chips of 8 megabit capacity in a 512K X 16 organization. Most of these chips operate either as 1M X 8 or 512K X 16 depending on how one pin on the chip is biased (the pin is usually labeled Byte on the datasheet). The chips need to be in a 44 pin PSOP package (as opposed to a TSOP or DIP, e.g.) as that's what the discarded ROMs were in.
The Intel PA28F800 will work (PA means it's in the PSOP package). Also the AMD 29F800_-__S_ where _ is an unspecified (by me) character. Except for the speed, the _ are don't cares as far as what letter is there. The S means it's in the 44 pin SO (small outline package, basically the same as the PSOP). Or the Atmel AT49F8192-__R_. In this case the R indicates the 44 pin SOIC or SOP package. It would help if these companies could call the package by the same name...
Oh, or the Hyundai HY29F800_ G -___.
Anyway, the flash chips need to be 90 ns or faster. The 100 ns might work, but the 120 will not work consistently. So fill in the __ spaces in the chip numbers above with 90, 70 or 50.
Then you must get code for the $77D34F5 ROM somehow, which basically means desolder a set off of an 8600 Enhanced or 9600 Enhanced motherboard and read the contents using an EEPROM programming machine. Then write those contents into the blank Flash chips. There are four unique chips to copy.
Then solder the appropriate chip into the appropriate spot on the Power Computing ROM DIMM board.
But that's not all....
Some Flash chips have a Reset pin and they all have a Write Enable pin. You'll need to look at the datasheet for the particular Flash chip you use, but in most cases the Reset pin (if present) and the WE pin need to be tied to the 5V supply to inactivate them. So you run a little wire from the 5V pin on the chip over to the Reset and the WE pins.
But there's more...
The ROM chips on board the 8500 (or 7500 or 8600/9600 non-Enhanced) machine need to be disabled when the ROM DIMM is installed. Pin 116 of the ROM DIMM socket connects to the CE_ pins of the motherboard ROMs. So, if you connect pin 116 of the ROM DIMM to 5V, it will feed 5V through the DIMM socket to the motherboard ROM CE_ pins and effectively shut them off as long as the ROM DIMM is installed.
I think, as built, that pin 116 of the PCC ROM is tied to GND through a resistor, so you need to remove that resistor to open the GND circuit, and then you can use the pin 116 side pad where the resistor was to connect the wire from 5V.
Jeff Walther
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