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
--
-----/___. _)Margaret Stephanie Leber CCP, SCJP/"The art of progress /
----/(, /| /| http://voicenet.com/~maggie SCWCD/ is to preserve order/
---/ / | / | _ _ _ ` _ AOPA 925383/ amid change and to /
--/ ) / |/ |_(_(_(_/_(_/__(__(/_ K3XS / preserve change amid/
-/ (_/ ' .-/ .-/ ARRL 39280 /order."-A.N.Whitehead/
/________________(_/_(_/_______AMSAT 32844_/<[EMAIL PROTECTED]>/
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