Dragon 64 repair

[dublevay]

You can see the tell-tale sticker on the SAM chip!

[Kepler]

So...the question is, what is Bob's secret "switch" to make our machines work again?!

[dublevay]

And that is a question that I sat for several days tracing the circuit and building a schematic for. But still it wouldn't work. Yours is a different version to mine though - it might be a Mark 1, as it has less onboard.

If you want yours back to stock, however, it's really going to be a case of making sure that there are absolutely no mods left on the board.

[bluearcus]

Hi Kepler,

As John's pretty much updated you on these machines history, there's not a huge amounr I can add.

Bob designed 3 or perhaps more different MMU modification versions. They were documented in a series of fragmented, error-strewn (thanks Ed) and rather difficult to follow articles in Dragon Update, the National Dragon User Group newsletter.

The Mk I was a 2k paged Motorola 6829-style MMU add on. After getting this working, Bob realised that he didn't have a sane, legal or even plausible route to getting a version of OS9 Level 2 ported for it, so although sound in principle, the design languished and was never used.

Around the time of the complete publication of that series of articles, Tandy released the CoCo 3 and as hardware details of that began to emerge and the release of OS9 level 2 for the machine occurred, Bob retargeted his efforts on designing a CoCo3-like MMU setup using the knowledge he'd acquired in designing the Mk I. The intention was then that the Tandy release of OS9 Level 2 would only require relatively small changes to 'port' it to this new design.

The second part of his design series then detailed the Mk II, which used 8k pages and a high ram 'trampoline / vector / os code' page just like the CoCo3. Two main variants of the design were discussed, one for 128k machines (Dragon 64s would naturally upgrade this way with the piggybacking arrangement you saw, requiring only 8 4164 class RAM chips) and one for 256k machines (Dragon 32s with 16x16k RAMs which could not be reused were the more likely target of this mod which would take 8 41256 class RAMs in a similar style to the Tandy CoCo 'The Banker' mod, but with MMU paging).

He also detailed a long and rather arduous series of patches to get Tandy OS9 L2 to boot and run on the machines.

Bob had at least 2 Mk II MMU machines running at one stage - one 256k and one 128k, and demonstrated them at at least one Dragon show in the very late 80s. Whether they were ever terribly reliable, I'm not sure.

The Mk II design articles were republished elsewhere (68' Micro users journal, allegedly, and the OS9 Users Group, definitely). There was even talk of a small run of MMU PCBs being designed and produced, though we have no idea if that ever actually happened, and none has ever been seen.

Unlike other expansions like the DragonPlus which intercede between the mainboard and the SAM, Bob's designs sit between the CPU and the mainboard, and they need to modify some mainboard signals in ways that just aren't possible from the CPU socket. So there are permanent mainboard mods in the mix. You may well be struggling with a hard to find one of these (I guess in the RAM addressing area), or it may be some other thing entirely. As Ciaran said earlier - your 'it failed and looks like this' pictures don't look like a standard Dragon failure mode that any of us are familiar with.

Hope that fills in a little detail! Bob's Dragon MMUs are one area I've been interested in for a long time.

Kind regards,

Mike

[Kepler]

More fascinating information on this now apparent rare gem of a machine. Thank you.

[Go4Retro]

Is there a handy link to the respective articles and any errata available?

I ask because I have a CoCo1/2/Dragon MMU solution that replicates the CC3 MMU config, and running OS9L2 on it is I think the true test of how well it works. Pere's asked to test a unit, but since there's prior art on this, making it compatible might have value is the code's available.

[pser1]

Hello Jim,
I had found some articles related to these RAM upgrades with MMU by Bob Hall in the magazines DRAGON UPDATE
You could download them from this link
https://archive.worldofdragon.org/brows ... n%20Update
If I am not wrong, the issues concerned are:
- 1987 Nrs 10-11-12 (RAM-MMU)
- 1988 Nrs 01-05-12 (RAM-MMU)
- 1989 Nrs 01-02-03-04-05-06-07-12 (all dedicated to OS9-L2)
If anyone knows of other magazine numbers related to RAM upgrades with MMU and how to modify OS9 Level II
I would really appreciate if that info could be shared here!
thx a lot

[Go4Retro]

Wow, you weren't kidding. That was hard to read (up to the 1989 articles, but have not waded through them yet.

[Kepler]

I have had a quick read of them. There is a fourth and final article in the next issue. I was hoping to find some information about the changes Bob made to the actual motherboard(s) but all the discussion seems to concentrate on how the MMU works. Nevertheless, really quite fascinating. I wonder what Bob's actual job was?

[pser1]

There is a fourth and final article in the next issue. I was hoping to find some information about the changes Bob made to the actual motherboard(s) but all the discussion seems to concentrate on how the MMU works. Nevertheless, really quite fascinating.

Hello,
I haven't found anything related to RAM expansions/OS9L2 in next issue (1990 01)
Could you, please, tell me wich issue number (or Year-Month) is the one you are referring to?
thanks a lot in advance!
cheers!