Eaton Power Xpert PXM 4000, User And Installation Manual

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IM02601004E
Page 43
3 Installation
3.13. RS485 Network
The following simplified rules apply to a given system consisting of master and slave de-
vices. For more complex configurations please refer to standard wiring specification rules
for the RS485 network.
The maximum system capacity is 4,000 feet of communication cable and 32 devices.
Make sure the twisted pair wire is recommended for RS485 network use. For reference,
review the RS485 wiring spec for wiring specifications.
Tie the communication cable shield to ground only once at the RS485 master device.
3.14. RS485 Cable Characteristics
Make sure the twisted pair wire is recommended for RS485 applications:
• Single twisted pair for data 18-24AWG
• Shield consiting of aluminized Mylar & partial coverage braid with drain wire
• Characteristic impedance ~ 120 ohms
• Data L-L capacitance ~12pF/ft.
• Typical ratings are 75C and 300V NEC CM
• Belden Data Tray series 3074F is 600V NEC TC
3.15. RS485 Wiring Basics
The maximum system capacity is 4000 ft. of cable and 32 devices. The baud rate can
restrict the maximum cable length (see the table on pg. 3-14).
• Wiring is done in a strict daisy chain without taps or stars
• 120 ohm 1/4W end of Line Termination Resistors (EOLTR) are required at
each end of the cable
• Attach like terminals to like terminal types for each RS485 port being net-
worked.
• Tie the communication cable, shield to ground, at one location only.
• Data line definition - As per the RS485 standard - In an idle marked state (logic
1) Data B (+) will be electrically > than Data A (-)
Note: When connecting to third party RS485 devices their data line nomenclature may be
inverted. If the RXD LED is on continuously, this indicates a crossed data pair.
• The use of ferrules to terminate the cable ends is strongly recommended to
minimize problems with frayed wires and to strengthen the terminal block con-
nection, when daisy chaining wires use a dual wire ferrule.
Phoenix Contact and many other vendors carry ferrules and crimping tools for this pur-
pose.
For more information about RS485 wiring, please refer to TD 17513, Eaton Electrical field
Devices Communication Wiring Specification.
Fail safe Pull up/down Bias resistors (Rb) are used to insure that the RS485 link goes to
a marked state when no node is driving the line. The bias resistors are applied at only the
Master node of each RS485 link and not at each slave. The display is always a master