[Coco] Disk Cable Length

[Gene Heskett]

On Tuesday 14 July 2009, Ries, Rich (NY80) wrote:
>Could someone with a working physical CoCo set up with drives please
>measure the length of the ribbon cable that runs from the controller to
>the end drive? I do not have the original cable, or I would do it
>myself.
>
>Thanks a lot,
>--Rich

If the drives are properly terminated, its almost immaterial.  I bought a 10 
foot spool of 34 wire flat cable in 1987 when I was setting up my coco3 
system, and I put the controller connector on one end of it, and 3 drive 
connectors at 6" intervals on the other end of it, making sure drive 0 was 
terminated AND it was on the last, end connector.  Haven't had a burp in 22 
years.  The last connector gets moved from a 5.25" to a 3.5" and back 
occasionally, it seems that all 3.5" drives must be terminated well enough to 
work most of the time too.

ISTR reading someplace that the cable could be up to 39 meters long but can't 
name the src of that info.  But all cables need terminated to control the 
unwanted ringing on signal edges.  Floppies, using ttl level interfacing, with 
term pullups are considerably more immune to that as the noise margins are 
twice as high as they are for a scsi setup.  The diff is that pullups on a 
floppy are to the 5 volt line, whereas for scsi its 3 volts.  But scsi has a 
protection diode to prevent cross-coupling the 5 volt lines in case the drive 
enclosure has its own 5 volt supply, and that diodes drop means the logic one 
voltage on a scsi cable is around 2.65 volts.  That is only 2 or 3 hundred 
millivolts of noise margin above the ttl specs of 2.4 volts, whereas the 
floppy has over 2.5 volts of noise margin.  That makes it very forgiving of 
impedance miss-matches since the impedance of this type cable is nominally 120 
ohms, and scsi systems MUST be terminated very accurately.

That explains why we often wind up sacrificing virgins to make scsi work...

-- 
Cheers, Gene
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