The "BAD DMA" - The Myth , The Legend, The lies.

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The "BAD DMA" - The Myth , The Legend, The lies.

Post by exxos » Tue Aug 22, 2017 10:46 pm

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The "Bad DMA" has been circulating around the Internet since the dawn of time. Where the -38 is apparently the route of all evil and every STE that has one will have hard drive problems. Well, not exactly, the chip is far from faulty as many sites lead you into falsely believing. Just about every site on the Internet will tell you that – 38 DMA in a STE is a recipe for disaster. Well, that may be so, but in general the DMA chip is not to blame for hard drive faults. The situation is actually very complicated and it is not easy to summarise the whole situation.

I personally have spent near on 25 years debugging and fault finding equipment, not just Atari ST's. I can tell you that you should be reading my current research into the issues, not grandma's good old tech doc's from the 1950's. If you happen to stumble on such a site, I urge you to not go there again and burn it into the eternal fires of hell for all eternity.

For those who are not technically minded, or just too darn lazy to read my research on it, let me explain it as simply as possible. Designing a motherboard working in MHz ranges is very difficult. Atari could only do so much with a 2 layer PCB and regardless of how well routed it may or may not be, there are going to be issues with noise on signals. This is a very common problem in all electronics equipment. So our poor old DMA chip (or indeed any chip) can see signals which are not really there and basically goes a bit mental.

In reality the issues are much more complex and a lot more involved than that. A more in-depth count of the progress along the way is documented on my site, even so, that is only really a summary of tests and results. I unfortunately do not have time to document every single test and explanation to go with it. A lot of manufactures app-notes can give a much more in-depth view of many issues.

Swapping the -38 with another -38 will likely work as well. Just grab one from an STF and you may find that works. But there really isn't any point in changing the DMA unless it is actually faulty, in which case , screwing around trying to fix your hard drive problems likely blew it up. In which case this article won't help you in the slightest :)

We can however try something simple which research by my readers has had very good results. This is simply change the CPU to a 68HC000 type. This reduces the current pulled by the CPU and lessens the load on many signals. All this reduces this "noise" and increases stability a great deal. Oh how simple awesome and easy that was!

But I hear you cry "It still hasn't solved my problem".. Well, I never said it would. I said it solves this particular issue. I mean if your hard drive is faulty causing data corruption (it happens!) then changing the CPU and/or DMA isn't going to do a darn thing is it. Of course, you may actually have a faulty DMA chip, but just because it is a – 38 chip, does not automatically mean it is faulty!

Even so, we can't blame every data corruption on a "bad DMA" as there can be multiple causes and all need to be ruled out. Swapping the CPU is a very easy cheap fix to try and I would personally change the CPU regardless if you have "DMA issues" or not. Swapping the CPU is a whole lot easier and cheaper than changing the DMA chip! You will also need a upgraded or new power supply otherwise your machine will not be running properly to start with!

There can be many ways a hard drive can fail from what I have seen. This can be anything from the PSU going bad to the values of pull up resistors to even the brand of the CPU & ROMs! Working though every issue takes a lot of time to investigate and likely there is still a lot more work to be done yet.

One thing which is still WIP, is that STE's with Motorola CPU's are likely to have "DMA issues". Whereas machines with a SGS / ST CPU may not. I only have a limited number of STE's here, but that so far seems to be the pattern!

I should also mention here, if you have one of my upgraded or new power supplies, and have changed CPU from one in my web store, and you still have issues, then please send the DMA chip to me for testing. The jury is still out on if there really is a faulty batch of – 38 DMA chips or not. Work can only continue if people send me the DMA to test and catalogue their batch numbers.

DMA research page.
http://www.exxoshost.co.uk/atari/last/DMAfix/index.htm

HC CPU STORE
https://www.exxoshost.co.uk/atari/last/storenew/#0092

Please considering buying the CPU from my store. As said in another post, while things might be cheaper "elsewhere", such places/people are not investing huge amounts of time into Atari ST projects. All revenue generated from my store goes into funding all my Atari related work.

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