Application and Installation Guide EMCP 4 SCADA Data Links
©2013 Caterpillar
All rights reserved. Page 7
F
IGURE 2-4: POSSIBLE WIRING TO FULL-DUPLEX RS-485 DEVICE
Full
Duplex
RS-485
Device
EMCP
RS-485
SCADA
Tx- (A)
Tx+ (B)
Rx- (A)
120 120
REF (or
GND)
Rx+ (B)
MB-
MB-REF
MB+
BATT-
SHIELDING
It is also recommended to connect all the shields (if daisy-chaining more than two devices, you will have
multiple cables) together and ground them at a
SINGLE Battery negative point, to drain noise while
preventing ground loop currents.
TERMINATION
RS-485 requires termination on the extreme ends of the bus, to prevent signal reflections. Although the
communication may work properly on short buses or under certain conditions, it is good practice to
always use termination. There is
NO termination internal to the EMCP 4.
NOTE: Some devices, such as PLCs or converters, may already have termination built-in.
Consult your device documentation.
EXTENDING RANGE
The RS-485 SCADA data link distance limitations may be overcome by using repeaters. However,
depending on the design of the Modbus Master, the performance may decrease. The key factor here is the
Modbus Master timeout, which is the amount of time the Modbus Master will wait until it thinks that the RTU
(the EMCP) will not respond. Consult the documentation for your Modbus Master device for more details.
2.3 SOFTWARE CONFIGURATION
Before devices will communicate, there are some software configuration steps on the EMCP 4, on the
Modbus Master, and perhaps on any other RTU device. Consult the documentation for your Modbus
Master or third-party RTU device for details on configuring their setpoints. Consult the EMCP 4
Application and Installation Guide for more details on configuring setpoints via the display or ET. The
EMCP 4 setpoints are located in the menu structure under CONFIGURE > ALL SETPOINTS >
NETWORK > RS-485 SCADA.
NOTE: The RS-485 SCADA port on EMCP 4.3 and EMCP 4.4 controllers is a dual purpose
port. Therefore this port will be disabled and unable to communicate over Modbus
when the setpoint “ENGINE CONTROL DATA LINK CONFIGURATION” is
configured to “CAT DATA LINK.” This setpoint is found under MAIN MENU >
CONFIGURE > ALL SETPOINTS > CONTROL > AUTO START/STOP.
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