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2016-08-30 11:15

lft

Registered: Jul 2007
Posts: 369

The "same drive" problem

Hi!

Can we talk about why some loaders fail to read disks that weren't written on the same drive?

I can think of two possible reasons: Different alignment and different RPM. But I also read that alignment is not a problem with 1541-II, and that RPM is often very close to 300 rpm. Then what is the problem?

Then there's the matter of *how* this leads to trouble. Are some loaders ignoring checksums? Are there multiple bit errors that cancel so the EOR checksum works out anyway? Or does the loader hang because it does verify the checksum, but keeps getting read errors on the same block?

Has anybody tried to find the root cause of this problem in any kind of systematic way?

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2016-10-07 08:29

Bitbreaker

Registered: Oct 2002
Posts: 510

Did such tests a few months ago and can confirm, depending on the loader used a different range of speed tolerance works. You can even try to turn down speed while loading and see how the loader gets into trouble. However i discovered when spinning too fast it can happen that checksum tests get bypassed at some moment.
The crosstalk stuff/misalignment can also lead to problems on drives. I have one specific 1541-ii where i happen to read chunk on certain tracks, but with valid checksum. I tracked that down to the mechanics, the same mechanic bears the same problems when connected to known good electronics/controllers. Also, there exists a drive that needs 4 cycles extra per transferred byte to not choke. Not sure if it is equipped with a wrong chip type or a broken chip, i can simulate the same problem when exchanging the 74ls14 on the 1541 with a 7414 type.

2016-10-07 09:34

ChristopherJam

Registered: Aug 2004
Posts: 1422

and now you can measure exactly how fast your drive spins with RPM Test 1.0.

Thanks to LFT for testing; I can't reach my hardware at the moment.

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