18 Jun
2013
18 Jun
'13
11:33 a.m.
Does anyone know what's inside the hash filter
that mounts in the sheet metal
on the back of the 20 mA bulkhead (I forget whether the EIA version has
something similar -- probably)? It just looks like a straight-through Berg
connector, but evidently it's not. I'm figuring it's probably just
feedthrough
The EIA one has a pair of 50 pin filters mounted ona sub-panel that fits
behind the normal distribution panel. There's a whort ribbon cable that
links one side of the device to the header on the distribution panel PCB,
the cable from teh Unibus card goes on the other side.
Bew warned that the short cable from fiter to distribution panel has a
strange conencotr on one end that swaps the rows of pins. That way, the
filer can be straight through without swaping anything (think aobut it).
Just a little trap for those who don't think and who either make up their
own cables from filter to distribution panel or try to use said DEC
cables elsewhere.
caps or something, but can't figure out how to
check that or find the value (my
DMM's "capacitance" mode makes a nice random-number generator).
AFAIK it's just 50 feethrough capacitors (one for each pin) to the metal
casing and thus to mains ground. The valuse of each capcitor is goign to
be fairly low -- a few hundred pF? If I can find one I wil ltry to
measure it.
With that sort of value, stray capcitance will affect the measurement,
and most DMM capcitance ranges aren't that good anyway. I have a good (if
old) digital LCR meter that should be able to do it.
I'd like to make sure my multiport RS232/20mA
thingy solves whatever problem
the filters are there for (in cases where it's not plugged directly into a real
H317F which has the filter).
I believe the only reason for the filters was to remove RF noise from the
interface cables to meet FCC requirements (part 15?)
-tony