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StarTalk™ DTM Help for StarPac 3 System
© Flowserve Corporation
Version 1.0.0 – Printed November 1, 2013
StarPac 3 Wiring and Grounding Guidelines
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 3 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 3 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 3 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.
Flowserve'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.
4-20 mA Command Input, Auxiliary Input, and Feedback Output
These signals are isolated but shielded twisted pair wire should be used to reduce crosstalk from
other signals. Again, the shield should be connected only at the source.
Discrete Input and Output
These signals are isolated, yet because they are frequently used to switch high voltage (120
VAC); they should be run in separate shielded wire paths away from the other StarPac signals.