Re: Reading serial port lines
From: Alf Christophersen (alf.christophersen_at_basalmed.uio.no)
Date: 10/14/04
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Date: Thu, 14 Oct 2004 09:49:05 +0200
On 13 Oct 2004 10:43:22 GMT, vbdis@aol.com (VBDis) wrote:
>Sometimes ground loops can cause such effects. Once I found it helpful to cut
>the cable shields (protective ground) at one end of the cables. This is
>essential when the devices have separate power supplies, and recommendable
>whenever a connection between the shield and the logic ground exists.
It might also function as a radio antenna and induce lot of noice.
I remember about 20years behind when we had a 4 user Altos 8000/7 with
Z80A 4.66 MHz and a 4 bank RAM card, connected to about 15 terminals
using switches to connect the different stations and also hooking up
two printers to the MPM 1.12 system (using a hacked printer spooler
based on ideas from DEC 10 printer spooler.
I had one of the longest distances from the computer, and being
responsible for every side of the system, I had often too grumble what
was riddling the computer now and then. Suddenly my terminal got
flares of random signs on the screen meanwhile the computer almost
knealed due to enormous flow of bits entering the computer on one side
and my (or others) on the other side.
Between floors we had a 50 pair cable connecting all floors to the
computer.
The reason was partly what was mentioned above, but the other problem
was unused pairs not shortcutted in one end. Being open on both ends
of the building, they functioned as an antenna, and picked up radio
signals and noise from other emittors. Maybe early versions of mobile
telephones.
If you have open pairs that are not used, also shortcut these on one
end, and break earthing of shield on one end.
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