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CHAPTER 2: INSTALLATION
Serial communications
SCADA, PLC, OR
PERSONAL COMPUTER
(*) TERMINATING IMPEDANCE AT EACH END
CAUTION:
CAUTION
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
–
Output Relay 7 (fail-safe, self-reset): Critical Failure
•
For general purpose:
–
Output Relays 5 to 6: Can be programmed as fail-safe or non-failsafe, as well as
self-reset or latched.
Figure 23: RS485 wiring diagram
Z T (*)
OPTOCOUPLER
DATA
COM
GROUND THE SHIELD AT THE
SCADA/PLC/COMPUTER ONLY
OR THE 339C ONLY
(typically 120 ohms and 1 nF)
One two-wire RS485 port is provided. Up to 32 IEDs can be daisy-chained together on a
communication channel without exceeding the driver capability. For larger systems,
additional serial channels must be added. Commercially available repeaters can also be
used to add more than 32 relays on a single channel. Suitable cable should have a
characteristic impedance of 120 ohms (for example, Belden #9841) and total wire length
should not exceed 1200 meters (4000 ft.). Commercially available repeaters will allow for
transmission distances greater than 1200 meters.
Voltage differences between remote ends of the communication link are not uncommon.
For this reason, surge protection devices are internally installed across all RS485 terminals.
Internally, an isolated power supply with an optocoupled data interface is used to prevent
noise coupling.
To ensure that all devices in a daisy-chain are at the same potential, it is imperative
that the common terminals of each RS485 port are tied together and grounded only
once, at the master or at the slave. Failure to do so may result in intermittent or failed
communications.
The source computer/PLC/SCADA system should have similar transient protection devices
installed, either internally or externally. Ground the shield at one point only, as shown in the
figure above, to avoid ground loops.
TWISTED PAIR
SHIELD
UP TO 32 339C
OR OTHER IEDs,
MAXIMUM CABLE
LENGTH OF
Z T (*)
1200 m (4000 ft.)
ELECTRICAL INSTALLATION
339C IED
F3
RS485 +
OPTOCOUPLER
F4
RS485 -
F5
COMMON
IED
RS485 +
RS485 -
COMMON
IED
RS485 +
RS485 -
LAST
COMMON
DEVICE
896732.CDR
DATA
2–19
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