Flowserve StarPac II Control Valves - Manual (Page 57)

StarPac Wiring and Grounding
Guidelines
This guideline will help you in achieving maximum noise rejection and
performance with a StarPac Intelligent Control System. This guide must
NOT be used to supersede local electrical code or plant safety wiring
practices.
Shielding Versus Grounding
All signals to the StarPac system should be in shielded cables. Shields
must be tied to a ground at only one end of the cable to provide a place
for environmental electrical noise to be removed from the cable. A
ground wire, unlike a shield, is attached at both ends to provide a
continuous path for electrical conductivity.
Grounding Screw
The grounding screw by the user interface terminal block should be
used to provide the unit with an adequate and reliable earth ground
reference. Either one of the mounting screws holding the terminal block
may be used as a grounding screw. This ground should be tied to the
same ground as the electrical conduit. Additionally, the electrical
conduit connecting to the StarPac unit should be earth grounded at both
ends of its run. The StarPac II grounding screw should not be used to
terminate any signal shield wires.
24 VDC Power
The 24 VDC connection points will work best with shielded twisted pair
wire with the shield wire connected only at the source. The input power
is isolated within the StarPac II system and may be referenced to
whatever level is necessary. For best performance the 24 VDC power
supply should not be connected to earth ground.
RS-485 Communication
RS-485 wiring requires shielded twisted pair wire. Maximum
performance will be attained when using cable with a characteristic
impedance of 120 ohms. The shield should be connected only at the
source, not in the StarPac unit. The StarPac II internal system ground
is isolated and not earth ground referenced. The RS-485 port can float
to whatever common mode voltage appears at its input terminals.
These signals are referenced to the StarPac internal system ground,
and because of this it is the main fault path when one of the isolation
points fail. For this reason special care must be taken to ensure that
the RS-485 cable is wired correctly. The RS-485 allows only a -7 to
12V common mode voltage differential between stations. This means
that an RS-485 network connected to multiple devices must not have
more than one grounding point. Valtek's RS-232/RS-485 converter is
not a grounded connection, it is fully isolated and is not a ground point.
However, PC's with internal RS-485 cards are often earth grounded and
if another communication device is on the network that also has an
earth ground, a fault condition will almost certainly exist due to transient
and steady state differences in ground potential.
StarPac II User Interface Manual 51