Re: gEDA-user: basic anti-EMI design q

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Friday 24 March 2006 05:04, Bill Sloman wrote:
>At 05:21 24-3-2006, you wrote:
>>DJ Delorie wrote:
>>>It's not the drivers that mess up, it's the octal latch.
>>
>>if the existing latch is in the interface with ribbon cable to the
>>PC  - there's your problem.
>
>Have you grounded every second wire in the ribbon cable? It sounds
>extravagant, but if you need shielding, that's a commonly used way of
>getting it. It also means that the residual signal wires look like
>50R transmission lines.

Uhh, no.  Its more like 120 ohms, at least for the ribbon cable as built 
for scsi/early ide/floppy cabling.  Hence the original terminating R 
network was a 220 to the 5 volt line and a 330 to ground, which came to 
132 ohms by the usual parallel r formula and was a reasonable facsimily 
of what was needed obtainable by std value r's.  Actives are a much 
better match and can tolerate some loses in their power supply lines, 
unlike the resistor pack designs.  Thats been the major cause of all 
the virgin sacrificing stories the scsi folks are wanting to quote.

This new ultrafine cabling used for the 80 pin ide cabling is as yet an 
unk thing to me so it could possibly be that low.

>You can get shielded ribbon cable, but it is 
>expensive, and grounding the shielding isn't always that easy - you
>need metal (or metallised) connector shells, and they aren't always
>easily available.
>
>>You could probably fix it with something gross like series R in the
>>line and a bypass cap to ground (the ground the latch sees - the
>>chips pin 10) right at the octal latch.
>
>Some logic specifies minimum edge speeds on clock inputs. Too much
>filtering can make the logic do funny things.
>
><snip>
>
>Bill Sloman, Nijmegen

-- 
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