Date: Wed, 1 Oct 2003 21:22:52 -0500 From: Derek Morton <[EMAIL PROTECTED]>
I am looking at making a memory card which allows the system to run full bore at zero wait states (RAM and ROM). Think of it as a cache card on steroids. There are a couple of problems, one of which is that the memory is not cheap and I have already dropped a bundle getting a stick of high speed SRAM (0 wait states up to 50 MHz and 8 MB worth if I remember correctly). This surely will not be sufficient for any real-world application, but it will show the maximum performance of the system.
That is a very cool idea. And a very expensive idea. I've been watching Ebay for over a year for some inexpensive SRAM and little has turned up. The prices from distributers are scary. Fortunately for me, I don't need anything as fast as you do.
Is the zero wait states based on the speed rating of the SRAM or are there other specifications that you must take into account? 50 MHz is a 20ns period, so are you looking at 20ns SRAM?
My current plan is to make the PDS card first (those connectors were IMPOSSIBLE to find
Did you find some? Or did you decide on a work around of some kind? I had a similar problem when I was looking for the CPU connector for the x500 PMacs. AMP has about 300 in stock, but their minimum order is 1008 at $9 each and all the distributers are out of stock. But for $9072 they'll run off another batch of 1008 for me...
and I will have the card (once programmed) replace the ROM on boot.
So you need to write some kind of utility to redirect the ROM reads to your SRAM card, plus add/modify the actual ROM code. That's the part that is a complete mystery to me. I wouldn't know where to begin, except at the beginning of several years of study of Mac programming.
make a ROM SIMM.
I agree that a ROM SIMM is much better than replacing the motherboard ROMs. It's just that getting the .050" board seems to be difficult these days. Will you be etching your own boards?
I have been doing electronic system design for years (it is my profession) and while the ROM mods do present a challenge, I feel confident it is quite doable.
I always believe these things are doable. The trick is having the skills to do it. It's very neat that you do.
As to my current board mods... I have replaced the SCSI and ethernet controllers. The new features are simply waiting for some code to release their potential (along with new crystals).
Did you put in a 53CF96 for the SCSI chip or something more advanced? The CF would give you the potential for Fast SCSI instead of just 5 MB/s and I think it's pin compatible with the existing 53C96. However, the part is very hard to find and rather expensive ($25/chip)--at least it was a couple of years ago. There was a lot of 10 or so of them on Ebay in the last year or so. Did you pick those up, if this is the chip you're using?
What did you do to the ethernet controller?
Thanks for the info on the ROM SIMM pin-out.
You're welcome. It was tiny compared to the other stuff you're doing. I made a small error. Replacing the ROM chips on the motherboard with 2 Mb chips wouldn't work, as they're 4 Mb chips. I was thinking in terms of pinout and you can't get a 4 Mb chip in a 40 pin package, so my brain settled on 2 Mb. I doubt this mistake would cause you any trouble even if it went unnoticed, becuase you won't be replacing the motherboard chips, but it's best if I mention it.
Jeff Walther
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