Re: [OT] K9YC's Serial Cable question

Posted by Jim Brown-10 on Jun 07, 2011; 8:57pm
URL: http://elecraft.85.s1.nabble.com/OT-K9YC-s-Serial-Cable-question-tp6450273p6451412.html

On 6/7/2011 12:13 PM, Don Wilhelm wrote:
>  Jim,
>
> I could not assure anyone that the Signal Ground (pin 5) is connected
> to the shell of the DB9 in each and every computer or USB to RS-232
> adapter.  In fact I am holding an adapter right now that has no
> continuity between pin 5 and the shell.

No question that some poorly built junk is made like this.

> That adapter will not work with your suggested connections because
> there is no signal return path.  What about those who use plastic
> bodied connectors?  It will not have a return path through the shell
> for those either.

Any equipment with a DB9 connector that does not have a conductive
shell, and with system common bonded to that shell at some point is very
likely to have issues with hum, buzz, or RFI.  Yes, some poorly designed
crap is made that way, but we should avoid it like the plague that can
easily become.

>
> The signal return path for RS-232 must always be on pin 5 - whether
> the shell is grounded or not, it will always work.

It may work without RF present, but it won't "work" if it couples hum,
buzz, or RFI. I've seen RFI so bad that it shuts the computer down --
that's how this cable came to be developed!   I'm sticking with my
original advice. Common mode current on signal cables is one of the most
common causes of hum, buzz, and RFI.  We don't want that current to go
to the common bus on the PC board, we want it to go to a ground plane,
grounding star point, or a shielding enclosure. That's why I'm so
insistent about this.  Equipment that does not allow proper wiring
techniques is BAD equipment, and should be avoided.

73, Jim K9YC

> 73,
> Don W3FPR
>
> On 6/7/2011 2:35 PM, Jim Brown wrote:
>> On 6/7/2011 9:06 AM, Brian - N5BCN wrote:
>>> For UNSHIELDED twisted pair CAT5:
>>> The K2 instructions use Pin 5 for the signal return lines.  Using
>>> K9YC's
>>> method, would I connect the return lines to the DB9 chassis, and
>>> then bond
>>> Pin 5 (or Pin 1, or both) to the chassis also?
>> Do not use pin 5 for the return, use the shell of the DB9.
>>
>

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