Sega Teradrive Simms RAM upgrade help needed..

I've been doing some research and work with the Sega Teradrive PC for quite a while now (big thanks to Shane Mcretro, Nemesis and anyone else involved).

I was recently asked about possible Sega Teradrive CPU & memory upgrades . I learned via another member that apparently the Teradrive 30 pin simm slots are missing the "high address lines" connections, therefore it can't access anything larger than 1MB sticks per slot.

Supposedly there is/was a Japanese site were someone was successfully able to hand wire the Teradrive missing connections to the slots, and access 4MB per slot making it a total of 8.5MB

Does anyone happens to have any information about this? or has the knowledge as to which of the 30 pins may be missing or correspond to the "high address" lines?

Any help is much appreciated.

Here's some of the work I've done among other things:

Recapped the whole thing from top to bottom (found various caps around the main motherboard that had leaked)
Removed the factory 286 10Mhz CPU and OSC for sockets installation.





Still in the process of testing more CPU's and hopefully some CPU upgrades.





Region switch:


Adding another 8-bit ISA slot so my sound card and my ISA XT CF V4 board can coexist:









IC wiring so I can use both PC and Megadrive video on a modern compatible screen:

 

I am very impressed.

 

Wow. That's freaking awesome.

 

Thanks for the comments guys!

I really need to find out about this simms wiring mod if indeed ever existed. Perhaps someone in its infinite wisdom was able to save it somewhere so I can complete the circle. Worst case scenario I guess I can just trace everything but that would take time, time that I don't have .

 

The highest address line of these SIMMs is on Pin 24, but I suspect the one that is relevant for you is actually Pin 19, because you want to use just 4MB modules and not 16MB ones (they exist, they are extremely rare and the memory controller on your board wouldn't like them).

Purely guesswork and very shortened version: I would guess that the connection from RA10 of the memory controller to A10 of the SIMM socket is missing. The "proper" way to do it would be to trace the connection from RA9 to A9 of the SIMM so see if there are any additional components on the signal and copy them, the simple way would be to just try to use a wire from the WD76C10 to the SIMM socket. Make sure you use 9-chip 4MB SIMMs, the 3-chip-ones may or may not get refreshed properly.

There were a few 286-to-386SX-Upgrades on the market since both chips have a very similar external bus. In theory the fastest CPU you could get for the system would be such an upgrade that uses a Cyrix/TI 486SLC processor, but I have no idea if one was ever made.

 

Awesome! great information. That's a good start, I'll go back to to the drawing board and see what I find with these pins thanks!. I do have some 8-chip 4MB sticks (I wonder what's the difference ??) some other PC's didn't like them thou. I also momentarily tested some Apple 2MB sticks on each slot, they worked on it but obviously the Teradrive could only see 1MB per slot.

Yes, I 've seen these CPU upgrades (only in pics ) . Hopefully I can get a hold of one for further testing. Thanks for your reply!.

 

3- or 9-chip SIMMs have a chip for parity, 2- or 8-chip SIMMs don't.

For 4MB SIMMs, the 3-chip variant uses two 4Mx4 DRAM chips for the data, which usually are organized as a 12x10 array - the memory controller seen in your pictures only supports 11 address lines for the RAM though, so it can't handle chips that are organized as 12x10 (rows/columns). A 9-chip SIMM uses nine 4Mx1 DRAM chips, which are usually 11x11. As far as I know, this difference exists because 12x10 allows for a slightly more power-efficient design than 11x11.

 

Hmm I see, I tested these 4X 4MB 8-chip sticks I have in both the Teradrive and my Amstrad Mega PC and the pc sides didn't even turn on unlike the 2MB Apple Mac sticks, what's the deal?. Both pc's are also based around the same WD chips.

I did a bit of poking around and the Teradrive is indeed missing the pin 19 (A10) wiring to the slots. It might be just a matter of patching it up ???.

 

Here's an awful video I recorded a while back when doing some testing (excuse all of the shakiness, it was a quick type of thing). You can see the ISA XT-CF-Lite LED light going if you pay close attention. Everything running in Composite video with Sound Blaster Pro2 sound :

 

Do ypu know the NS rating of the included ram?