Fabbing 30 pin SIMMs

[Trash80toHP_Mini]

It might work, but it would be an even grosser hack than SIMMsavers were in the first place.

< . . . chuckles quietly [}] ]'> . . . >

. . . and it's still a stupid idea.)

[] ]'>

 

[trag]

(I never got the point of SIMMsavers. Every time I remember upgrading motherboards to one using a newer memory form factor the price of memory had dropped enough it made more financial sense to just buy more new memory than I had in the first place than try to save the old stuff. I can see some edge cases where if you'd *really* broken the bank maxing the memory on the last machine they might save you a few bucks, temporarily, but... factor in the headaches and it's still a stupid idea.)

The one place I've found such a thing useful is the SimmXpander in a Q605. This converts two 72 pin SIMMs to one 72 pin socket.

I've been able to get 260 MB of RAM in the Q605 using one expander and two 128 MB SIMMs. However, thinking about the addressing and architecture of the SIMMs, I can only think of one way it can possibly be working. The Q605 must have four independently operable RAS lines. I thought they were usually paired.

 

[Trash80toHP_Mini]

I've owned one SIMMsaver since back in the day for tinkering purposes, but I've actually never used it, IIRC.

On a less silly note: the DATA/Addressing lines aren't made all THAT much longer to make timing an issue for the SIMMspender hack.

In the straight thru iterations, it should be duck soup to keep all the traces/patches equal in length.

Since many of the signals are exposed on the top and bottom layers on the MoBo and on the SIMMs, why is shielding for the CAS/RAS lines above board so important? It's not an insurmountable obstacle to use tiny co-ax, by any means, but it's not like crosstalk or interference should be an issue with chunk combos being strobed sequentially.

Then again . . . I could easily be misunderstanding that last bit . . . that's usually the word on my assumptions. :-/

p.s. thanks for the clarification on the purpose of the really dumb boards in the dumb PCI Adapter pic, G. I was wondering about their function, addressing made a lot more sense to me, but the color coding of the wires said AC POWER COLOR CODING which is another issue in DC for PCI slot adapters. Looked like a stupid way of doing power, but it'd be waaayyyy better filtered coming from the other slots than from a Molex Y Cable.

 

[Gorgonops]

On a less silly note: the DATA/Addressing lines aren't made all THAT much longer to make timing an issue for the SIMMspender hack.

I don't know, you might be surprised. Once you get into the low-mid double-digit Mhz speed ranges is where you start seeing motherboard designers doing things like waving traces around to make sure that all the wires leading to a memory device are the same length. My guess is you'd *probably* get away with an inch or two of slop, but it's not kosher.

Mechanically the thing would be something of a nightmare in any case. You can't really make assumptions about how the SIMM slots are laid out on the host motherboard so the whole thing would have to be flexible. (30 pin SIMM sockets come in both vertical and slanted varieties, the socket-to-socket spacings vary, etc.)

Since many of the signals are exposed on the top and bottom layers on the MoBo and on the SIMMs, why is shielding for the CAS/RAS lines above board so important?

Someone might correct me, but I don't think you'd need the CAS/RAS lines from more than one socket, or at least the full set of them. You will need the 8 data lines, and you *might* need the strobes. However... taking a brief look at the pinouts of the two types of SIMMs and an operational description of how the two work, something tells me that there are gotchyas to this that might require putting some active components on the adapter for refresh to work correctly, at least if the 72 pin SIMMs used in such an adapter were made using components denser than those used on a 30 pin SIMM 1/4 of the size. IE, if a machine like a Quadra 950 supports 16 MB chips with only 2 [or 3, ignore the third] chips that's 8 chips for 64MB. If you were to install a 64MB SIMM using *less* than 8 chips in your SIMM-adapting nightmare adapter the logical layout of the chips might require a different refresh cycle than the memory controller in the host machine can supply. In particular I suspect this problem would crop up with 72 pin SIMMs that use 8 or 16 bit wide DRAMs.

Dual-rank 72 pin SIMMs would also be difficult to use without help. So an adapter like this would have significant operational gotchyas.

(There's an old thread on this board where the question is brought up why some machines will only work with the 8/9 pin versions of a given size SIMM, not the 2/3 chip version. The answer is that the wider DRAMs sometimes require more refresh cycles than the memory controller on a given machine can supply. Coincidentally this was brought to my attention recently when researching ways to upgrade an old TRS-80 Color Computer 3 from 128k to 512k. The obvious way, replacing the four 4464 DRAM chips with four 44256s, doesn't work because the denser chips require a 512 cycle refresh while the counter built into the GIME DRAM controller only counts to 256. So in order to get 512k you have to add a satellite board made up of 41256 chips that accept the 256 cycle refresh.)

 

[Trash80toHP_Mini]

Yup! Forgot about the data lines for the wider path, my bad! :beige:

It's lookin' like four small SIMM Cards connected to one loose, mother goose, card by flex-cables would be the only do-able way for this notion to work with timing integrity preserved in the majority of 30 pin macs.

Next!

p.s. hrmmm . . . where does short run flex cable design and prototyping stand?

 

[James1095]

I like the idea of putting newer chips on a 30 pin PCB a lot better than some contraption that plugs into the old SIMM sockets and provides "modern" 72 pin sockets. For one thing the PCB is gonna be a lot larger (= more expensive) and a lot more complex. It's pretty trivial to pop all the chips off a SIMM with a heat gun, hotplate or hot air reflow station and then reflow them onto new boards. I use a temp controlled hotplate I built but I've seen it done with various low tech solutions including a thrift store electric skillet. Anyway the end result has a tidy stock appearance that will fit easily in any machine, not a messy hack with a multitude of wires snaking around.

 

[Trash80toHP_Mini]

wohoo! p.2 continuity edit:

I like the idea of putting newer chips on a 30 pin PCB a lot better . . .

I like that approach a lot too . . . it's just not quite as much fun! )

It's also not for those who don't want to do that particular sort of hacking, but those who do, can sell SIMMs to those that don't.

The side discussion of such silly antics brings a lot of expertise out of the woodwork and puts that wisdom into .TXT in the forums. There's a lot of crossover application info for other hacks to be had this way, in several concurrent discussions ATM actually . . .

. . . like dougg3 and bbraun's exploration of the ROM SIMM wedge for more hacking opportunities. [] ]'>