Date: Mon, 29 Mar 2004 05:10:01 -0500 From: "Byron Q. Desnoyers Winmill" <[EMAIL PROTECTED]>
On Mon, Mar 29, 2004 at 07:11:41PM +1000, Charles M Gascoigne wrote:I have a general question/topic for discussion - how long can compact macs last, and what can be done to maximise their lives?
- ROM. This is probably folklore, but I've heard that the data in ROM (PROM, EPROM, EEPROM?). This form of bit rot is supposed to happen on the order of 20 years. You can backup your ROM on certain models, but you'll need a special gizmo to do so.
I'm not certain, but I think that true mask ROMs will last as long as any other fixed content chip. The problem (I think) comes from programmable devices. I have also heard the ~20 years lifetime. In some cases, the "ROM" in our machines may actually be POEPROM (Program Once Electrically Programmable Read Only Memory). In other cases they may be actual EPROM, EEPROMs or Flash (probably not Flash in a Compact). These types of chips may also have a limited content life.
The good news is that there's probably a PC shop near you with an EEPROM programmer who can copy your ROMs to a few sets of EEPROMs for a modest price. Most non-volatile chips of a given size and organization have a pinout similar enough that they are compatible for reading. I hope this would fall under the allowable back-up copy of copyright law, but I'd advise not mentioning it to Apple.
So a 128K X 8 ROM in a 28 pin DIP has the same pinout as most 128K X 8 EPROMs and EEPROMs in a 28 pin DIP so far as reading the chips goes, which is all that goes on while it's in a Mac anyway.
The bad news is that machines like the SE/30 have their ROMs soldered on a SIMM module so the individual chips are not easily movable to an EEPROM programmer, and Apple used chip packages (size, shape, pinout) in those ROM modules which are nearly impossible to find now days. One can substitute cheap and common PLCC-32 chips but the ROM module (the circuit board) is not configured (pads misplaced) to take PLCC-32 chips.
Gamba and I experimented with manufacturing our own ROM module configured to accept the common PLCC-32 chips and discovered an irritating fact. SIMMs from that era are only .050" thick. Standard circuit boards these days are .063" thick. So it is very difficult to make a compatible SIMM module for the old machines. Forget using some of the specials that circuit board companies offer and standard PCB stock will be too thick.
This means that manufacturing regular memory SIMMs will also be either difficult or (and?) expensive. That is less of a concern, but it might be nice to churn out a few 16 MB SIMMs for the IIfx, or my pet desire is for some standard 30 pin SIMMs manufactured with SRAM chips for low power use in the Outbound laptop.
Okay, all that blabbering about ROM and I haven't gotten to my real point. I've been told that the ~20 year life also applies to programmable logic chips. This is a much greater concern to me. Most machines have at least a few PLDs, PLAs, PALs or GALs (they're all pretty much the same thing) on board. It's usually next to impossible to extract the programmed content of these chips and if they start going there won't be any way to copy them.
Does anyone understand why, ten years or more after the EOL for a product, companies aren't willing to release these kinds of details to the hobbyist community? It's not like they're ever going to use that particular PAL design again.
Jeff Walther
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