During the past few months I have pieced together a IIfx without any RAM. I know trag has made some IIfx RAM in the past, but I thought it would be a fun personal project to make my own RAM SIMMs based on the info that trag provided in the linked post. This is really just a quick little personal project and I won't become a source for purchasing IIfx RAM.
Here's a quick summary of trag's R&D. Typically, IIfx RAM SIMMs use eight 1-bit RAM chips, because those are the only chips that have the separate data in and data out pins (D and Q) needed by the IIfx. The IIfx pinout also has a separate write control pin for each bit, so you can really tell that the whole pinout is designed around using eight x1 chips per SIMM. So...to make a 16 MB SIMM, you really want eight 16Mx1 chips. trag discovered that DEC 54-24123-AA 64MB 72-pin SIMMs readily available on eBay have 36 16Mx1 chips (presumably the extra four chips are for parity). Two of these SIMMs contain enough chips for creating nine 16 MB RAM SIMMs. trag also figured out how to use x4 chips by adding another chip, but I didn't want to bother with that.
I got a couple of the DEC SIMMs on eBay. They were covered in Mitsubishi M5M416100BJ chips, just what I needed. They use a standard pinout also shared by other suitable chips such as the KM41C16000BK. I removed all the chips with my IR preheater and hot air station. I'm not going to lie -- it was pretty tedious to desolder 72 chips, but it could have been a lot worse.
Because Seeed Studio does 4-layer boards now for a reasonable-ish price, I thought this would be a good project for my first one. The outer two layers are signal, the inner two layers are ground and power. Having four layers really simplifies routing, because you can get ground and power anywhere on the board by only adding a via. I put all eight chips on the same side of the board which eliminated the possibility of assembly by hand, but I think the design looks nice and clean! I printed it out to see if I got everything sized correctly:
Here's the finished PCB:
For the first few SIMMs, I was too lazy to set up my solder paste dispenser, so I cheated. I put solder onto all of the pads, added more flux, and stuck the chips on top of the soldered pads. The flux was sticky enough to hold them in place. Then, I baked the boards in my repurposed toaster oven and the chips dropped into place. That approach worked, but it was pretty ugly and a few of the chips misaligned themselves while in the toaster so I had to redo them. Then I also had to add the decoupling caps by hand afterward, which was boring. Anyway, they turned out OK:
For the rest of them, I used solder paste and baked them in the toaster oven. Those SIMMs all worked perfectly on the first try. Here's a shot before and after reflow in the oven:
Total price to build 9 SIMMs, not including shipping and my spent time: $84.40. Not bad! All 9 SIMMs work fine in the IIfx and pass all memory tests I throw at them.
Warning: To avoid degradation of signal quality, it is critical to adhere to the strict timing parameters of the IIfx and to use a good layout that takes high-speed circuits into account.
They didn't make a warning about this on any of the other models. Makes me feel like doing this as a 4-layer board was probably a good idea!
Another interesting (unrelated) tidbit from that article is that SIMMs which use 4 Mbit and larger chips in the II and IIx could potentially have random memory problems if they don't have a PAL with logic to prevent the chips from entering a test mode that JEDEC added to the spec for larger chips. So on those models, composite SIMMs are better, even though they're generally frowned upon. Weird...Well, I guess composite SIMMs are still probably a bad idea...you'd want SIMMs with the PAL on them.
Thanks! I just attached the original Eagle files and gerbers to the original post. Edit: To clarify, this is my own original design. I haven't even seen trag's schematics or layouts, aside from the pictures he posted on 68kmla.
Thanks! trag's design actually only has two layers and it still works, but I feel safer using a four-layer board for it. I made this with a freemium trial license for Eagle, BTW. I've also been replicating the design in KiCad and DipTrace as an experiment to decide what other PCB software is good.
Hey, thanks! Yeah, it's old stuff no longer in production. 5V DRAM chips seem to be pretty rare now, and any that might still be available are wider than 1-bit. Only the 1-bit varieties have the separate D and Q pins. The good news is lots of old RAM has those types of chips, you just need the patience to desolder them.
In order to fit all chips on the same side, the chips have to be spaced closely together. There's not enough room between each chip to fit a soldering iron. Maybe if you're super skilled and you have the right type of tip, but it would be pretty tough, especially on J-lead packages like these.
It's so easy to do the toaster oven stuff. I bought a random cheapo toaster oven, and funded this controller on Kickstarter. The only hard part is waiting for the controller to tell me to open up the oven, and avoiding breathing in the nasty fumes when I open it up. I do it outside because of the fumes. For my more complicated boards, I would never go back to hand soldering...reflow is so much easier!
Hmm, maybe I should check that out. Sorry for the topic hijack, but what do you use for solder paste? Do you keep it refrigerated? I've always been a little squeamish about keeping something like that night to my cheese and bread in the fridge.
Edit: Also, do you use a stencil? Or do you just run a line of solder paste straight across a whole row of pads?
No problem I feel the same way as you, I don't want the solder paste anywhere near my food. I just leave it out. Apparently it only has a shelf life of 6 months when it's not refrigerated, but mine has definitely lasted longer than that. I use Chip Quik SMD291AX10 paste. I think that the 6 month shelf life is just for professional environments. Maybe the flux doesn't work quite as well as it otherwise would after 6 months, but I did put a bit of extra flux on my PCBs before putting on the solder paste just in case.
For these RAM SIMMs, I'm not making a ton of them so I didn't want to pay for a stencil. I used a solder paste dispenser--details on my blog--and ran a line of solder paste across rows of pads. You can sort of see the lines of solder paste in my "before" picture in the oven above. It's really tough to shoot the paste out of the syringe through a needle by hand. Takes a lot more force than you would think. So I use the dispenser hooked to an air compressor.
I do use stencils (from Pololu) for my ROM SIMMs and programmer though...it's much easier and faster than the dispenser. I use a few junk PCBs to hold my PCB in place, hold the stencil in place with masking tape, and use an old Wal-Mart gift card to spread the paste over the stencil. Works like a charm! I could imagine having problems with it on really fine-pitch stuff (0.5 mm) though. I honestly sometimes get a couple of shorts on my AVRs too. Maybe it's because the paste is expired, not sure. It's not a huge deal to touch up afterward though.
Thats why We need a "micro" style caulking dispenser thingy. You know the large ones you stick the caulking tubes in and pump the handle? Need a micro-miniature one for those small syringes. Dont get me wrong, but I think having a box with an air compressor near by is a bit too convoluted for me.
Totally agreed. The air compressor is a pain to get set up. I wonder if such a product already exists? It would be pretty nice, especially if it's small enough. Holding the whole "caulking gun" would be kind of awkward when placing the paste if it was too big.
Hey, this is really great work, and timely! I just got a IIfx board and have a PSU coming in, and will need some RAM and don't want to pay out the nose for it. One note, though: there are those of us who also restore old VAXen and Alphas who don't particularly like the idea of chopping up DEC RAM to pull the chips.
I've put in a quote for a large-ish quantity of compatible chips with a surplus dealer and am interested to see what comes out; if lucky, we could possibly get these fabricated and assembled for around $5 each (in sufficient quantity, obviously), which would be excellent news for the hobbyist community.
Do you terribly mind if I make a shareable project of your files on Seeed? It would be useful for those who want to build their own to fabricate them. I tried putting them on OSHPark, but you don't have a separate board outline layer (which they require) and it didn't like the raw Eagle file for some reason. OSHPark would be better for smaller runs, but I like Seeed for big runs (125 boards for less than $200 is a pretty decent deal, and you can order assembly to go with it as well).
Anyway, again, great work! This is really helpful. There are a lot of similar RAM builds that could be done relatively easily with a decent supply of surplus RAM chips that would make other machines (SGIs, Suns, etc.) a lot easier to populate with memory.