🚀 go-pugleaf

(Howard Chu) writes:
>Hugh writes:
>>3)	This isn't really a grump, more of a puzzle.  Occasionally in the past I
>>have booted my machine with hard disk and laser turned off - I just want to
>>copy some floppies or some such simple minded operation.  If I do this, it
>>is then impossible to format or copy floppies.  Why so? .....
>>   -- Hugh
>
>I noticed this on my Mega. The funny thing is, this didn't happen on my 1040.
>(At least, I don't remember it happening...) I used to run the system just
>fine with my hard drive plugged in but turned off (too noisy, and I suspect
>that iot overheated easily...). Now, if I want to access the floppy drive at
>all, I have to remember to power up the hard drive as well. (I still keep it
>turned off most of the time, 'cause it's still noisy, though the new case has
>a fan...) It would seem that some type of signals are getting lost out the
>DMA port, and they were probably sufficiently buffered in the 1040 to not be
>a problem...

	Howard you were almost right in your last comment about this problem.
The problem is not in the buffering because there is NO buffering at all.  This
is true for all of the ST's.  The DMA chip is used to drive both the floppy
drive controller and the DMA port connector in the back.  So if one were to
ground any of the signals on the DMA port the floppy drives would also cease
to work.
	When you have the power turned off on the Hard drive it leaves the
buffer chips unpowered.  This can create a load on the signals coming from
the ST.  There is no easy way to get around the problem,  unless you what to
disconnect the Hard drive.  You could add a switch to dissable the host
adapter,  we did this on our new host adapter.  If there is some interest I
can look into finding out how to do this on the SH204.
	By the way you might be interested in why the DMA cable is so short.
Since the DMA port has no buffers this also means it has no termination.
Without termination there is crosstalk between the signals in the DMA cable.
The crosstalk is caused by stray capacitance between wires in the cable.  With
longer cables the capacitance increases to an unacceptable level.

	Hope this helps.....

Vance Chin  @ Berkeley Microsystems

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