[sci.electronics] How to connect grounds when using RJ11 for RS232

mdf@tut.cis.ohio-state.edu (Mark D. Freeman) (02/14/88)

Yes, I know how stupid this question is.

We are starting to use multiport boards in our multi-user microcomputer
systems.  The boards have 8 6-wire modular phone-type jacks on them.
We are running cable through the walls which has 2 twisted pairs and a
shield.

At the terminal end, we attach a male RJ11 to the wire coming from the
comuter, and plug it into the terminal through an RJ11->DB25 adapter.

Most of the time, we only need Send Data and Receive Data connected, as
we are using XON/XOFF handshaking.  The rest of the time, we need to
add Clear-to-send.  So that takes care of two or three wires.
RS232 provides for a pin 1 ground and a pin 7 ground.  The cable's
shield ought to be connected to something, but all the connectors are
plastic.

The question, what do I connect the cable's shield to?  If I can only
connect one ground, which should I hook up?  If I can hook up both,
should I bother?


-- 
Mark D. Freeman						  (614) 262-1418
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cgs@umd5.umd.edu (Chris Sylvain) (02/16/88)

In article <6561@tut.cis.ohio-state.edu> mdf@tut.cis.ohio-state.edu (Mark D. Freeman) writes:
<
< The cable's shield ought to be connected to something, but all the connectors
< are plastic.
<
< The question, what do I connect the cable's shield to?  If I can only
< connect one ground, which should I hook up?  If I can hook up both,
< should I bother?
<

Not knowing if all your terminals are all one the same AC circuit, nor if they
all live on the same phase of the AC service in the building, and you
definitely don't want to CREATE a ground-loop, then I suggest you connect the
shield to earth ground AT ONE END ONLY.

With the single ground connection you can at least have the electrostatic
shielding of the cable's shield, without having a connection which would
provide a path for ground currents.
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