RS-232 AND RS-485 SERIAL NETWORKING
SNAP PAC S-Series User’s Guide
20
Guidelines for Grounding SNAP PAC Brains and Controllers
Power supply side
: The DC output of the power supply to the I/O unit (or controller) should be "floating,”
which means the negative output terminal is not tied to ground.
RS-485 serial cable side
: Connect the signal common from the controller to the signal common on each of
the I/O units using an insulated wire that is part of the serial cable. Though sometimes hard to find, there are
cables that have a twisted pair for serial communication plus an additional insulated wire for common.
However, it might be easier to obtain a cable that has an extra twisted pair and then use one of the wires from
this extra pair for the signal common connections.
NOTE: Make sure there are enough wires in the cable for a separate insulated signal common wire. Do not connect
the signal common wire to chassis ground. Do not use the overall cable shield drain wire as the signal common.
Connect the cable shield drain wire of the serial cable to chassis ground at one location only. If the shield is not
continuous from one I/O unit to the next, then the shield of each segment will need to be tied to chassis
ground at one location only per segment.
Use one of the wires from an
extra twisted pair for the signal
common
Connect the cable
shield drain wire to
chassis ground
RS-485 data pair
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