[Elecraft] Kio2 cablength

[Margaret Leber]

W3FPR - Don Wilhelm wrote:

> ...what I learned in large system computer design and testing
> experience was that shields are only DC grounded at one end, and that
> end is the 'driving' device or in the RS-232 world, that would be the
> box defined as the DCE - in the DTE devices, the shield would be
> grounded through a  capacitor, or alternately left open... 
> Differences in the potential of the chassis...of a box on one side of
> a raised floor computer room as compared with a box on the other side
> could be substantial enough to cause current to flow

When I was working for a very large IBM mainframe shop in the mid-1970s, 
we had the Mother Of All Serial Devices: an IBM 3705 communications 
controller. This beast was rolled into our raised-floor machine room and 
left standing on its shipping wheels next to the Comten controller it 
was intended to replace.

The controller, a computer in it's own right, woul not run for more than 
a day or so without halting due to a hardware-detected fault...denoted 
by the illumination of one or more of the dreaded "red light" 
indications on it's Star Trek TOS-like front panel (and the immediate 
sessation of any data flow though it from the modem bank to the 
processor complex it was attached to).

After field-replacing every circuit board in the box (and it was a very 
big box) and scheduling the replacement of the 370 Motherboard Itself 
(which was more of a "passive backplane" than what we call a 
"motherboard" in a PC today), IBM Field Engineering began to notice that 
every time they went to place a scope probe on signal lines in the box, 
it would crash. This was odd enough, until the time their tech reached 
out to place the scope probe on a line, and the box crashed just 
*before* he touched it.

Subsequent investigation disclosed that the weight of the 3705 had 
caused the alignment of the panels of the raised floor to shift slighly, 
  creating two huge insulated sections of floor in a room that was 
easily 1,000 sq ft. Standing on one of the floor panels on the boarder 
of the two sections could cause the panel to rock slightly in place like 
a table with uneven legs (which is effectively what it was), closing a 
circuit between two huge capacitor plates (which is effectively what 
*it* was), and causing a surge of static to race into the surprisingly 
delicate device. Of course one of the "landmine" floorplates was right 
in front of the light-bedecked console...

  73 de Maggie K3XS

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-----/___.   _)Margaret Stephanie Leber CCP, SCJP/"The art of progress /
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