[Elecraft] [OT] K9YC's Serial Cable question

[Don Wilhelm]

  Now that you all understand the situation between Jim and I, you have 
to make some choices.
Do you throw away the bad equipment and get good? or do you try to make 
the best of what you have?

Doing as Jim has suggested will reduce common mode (RF feedback) 
problems, but in some cases will not allow the cable to work due to "bad 
equipment".

Connecting the cable  (without the shield - returns to pin 5) as I have 
suggested will make it work for the RS-232 connection in all cases, but 
in some cases does leave the hardware exposed to common mode current 
(and RF feedback) problems.  Building the cable as Jim suggests (returns 
to the shell) will work with good equipment but will fail to communicate 
with bad equipment.

The ideal is to rid yourself of the bad equipment, but unfortunately, 
manufacturers are not forthcoming with such information, even in their 
detailed specifications to allow us to make a judgement about what is 
"good equipment" and what is "bad equipment" using the specifications - 
it is an "iffy" task.  When is the last time you have seen a line in the 
data for an RS-232 to USB adapter specs that indicated that Signal 
Ground had a connection to the DB9 shell?  Breaking the blister-wrap 
package at your computer store to measure the resistance will be 
severely frowned on by store employees.

As Jim pointed out, a short cable shield can be effective if grounded at 
one end (Faraday shield).  So if you want to be safe for all cases, 
build the shielded cable version of the CAT 5/6 RS-232 cable, but make 
it only as long as necessary - it should function for the RS-232 
connection in all situations.  If you experience "strange behavior" then 
you can investigate your individual grounding situation with your 
computer/adapter/radio for the shield connections.

Sorry, but I cannot address all possibilities with certainty and give a 
"one answer fits all" solution - it all depends ... but I am coming as 
close as I can to that one answer - use the shielded CAT 5/6 solution, 
and then investigate any problems in light of Jim Brown's extensive RFI 
documentation (we used to call these problems "sneak ground paths", but 
Jim has provided enlightenment that is more encompassing).

73,
Don W3FPR

On 6/7/2011 6:13 PM, Jim Brown wrote:
> On 6/7/2011 2:30 PM, Don Wilhelm wrote:
>>    sense that only a few are
>> willing to replace those computers or USB to serial adapters that are
>> classified by Jim's definition of "bad equipment" with good alternatives
>> (BTW, I agree with Jim on the "bad equipment".  Many DO need a signal
>> return connection to pin 5 or they will not work.
> All should understand that Don and I are friends and have the greatest
> respect for each other's experience and advice.  FWIW, I've never seen a
> COMPUTER or ham rig with a DB9 that doesn't have a conductive shell.
> The very nice Quatech PCMCIA 2-port serial adapters also have conductive
> shells that are properly bonded. I have one that I've used on many Field
> Days and other fixed and portable operations since 2003, and  I recently
> bought two more.  I have no doubt that there are cheap USB adapters
> floating around, but because good stuff is also cheap, it's better to
> toss the junk and avoid the problems.
>
> Remember also that a shield that isn't connected to circuit reference or
> to the shielding enclosure of the equipment doesn't work as a shield.
> Cables that are short as a fraction of a wavelength (1/20 wavelength or
> so) can have the shield bonded at only one end if it's not also carrying
> the signal return. This is often done with balanced audio cables as a
> band-aid for pin 1 problems.  A 3-6 ft cable between computer and rig
> bonded at only one end is likely to be just fine below 10M.
>
> Another point that I hope isn't lost on folks -- improper bonding and
> shielding causes RF noise to ESCAPE from equipment and radiate into our
> receivers, just as it allows RF to get INTO equipment and cause
> lockups.  Yet another reason that I'm so bitchy about this stuff. :)
>
> 73, Jim K9YC
>