PCI Quad 422/485 Card
Chapter 10 Optional RS422/485 Operation
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Note: The receiver end of MAIN line terminated in characteristic impedance, stubs off
the main not terminated.
Optional Grounding Arrangements.
Proper operation of the cable circuit, according to TIA EIA: 485 A (1995) requires
that the cable ground shield is not connected directly to the equipment ground shield. A
current limiting resistor should be used in series with the shield to avoid possible large
current flow due to differences in ground potential. Any one of the methods shown
below can do this.
Configuration A)
GWG
100
Ω
(1/2)
W
SG
Configuration B)
GWG
SG
SG
=
Signal Ground Interchange circuit.
GWG =
Green wire ground of power system.
=
Protective ground of frame ground.
=
Circuit ground or circuit
common.
Configuration A)
The circuit common of the equipment is connected to protective ground, at one point only by a
100
Ω, ±20%
resistor with a power dissipation rating of 1/2W. An additional provision may be made for the resistor to be
bypassed with a strap to connect signal common and protective ground directly together when specific installation
conditions necessitate.
