GE Power Quality Meter Series, Инструкция по эксплуатации

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2-18 PQM Power Quality Meter GE Power Management
2.2 ELECTRICAL 2 INSTALLATION
2
2.2.9 RS485 SERIAL PORTS
A fully loaded PQM is equipped with three serial ports. COM1 is a RS485 port available at the rear terminals of
the PQM which is normally used as the main communications interface to the system. COM2, which is also a
rear RS485 port, can be used for data collection, printing reports, or problem analysis without disturbing the
main communications interface. COM3 is a front panel RS232 port that can be used for setpoint programming
or recording using the PQMPC software.
A serial port provides communication capabilities between the PQM and a remote computer, PLC, or distrib-
uted control system (DCS). Up to thirty-two PQMs can be daisy chained together with 24 AWG stranded,
shielded, twisted-pair wire on a single communication channel. Suitable wire should have a characteristic
impedance of 120
Ω
(such as Belden #9841). These wires should be routed away from high power AC lines
and other sources of electrical noise. The total length of the communications wiring should not exceed 4000
feet for reliable operation. Correct polarity is essential for the communications port to operate. Terminal (485+)
of every PQM in a serial communication link must be connected together. Similarly, terminal (485–) of every
PQM must also be connected together. These polarities are specified for a 0 logic and should match the polar-
ity of the master device. If the front panel RX1 or RX2 lights are flashing, this indicates that the PQM is receiv-
ing data. If the front panel TX1 or TX2 lights are flashing, this indicates that the PQM is transmitting data. Each
PQM must be daisy-chained to the next one as shown in Figure 2–14: RS485 COMMUNICATION WIRING.
Avoid star or stub connected configurations. If a large difference in ground potentials exists, communication on
the serial communication link will not be possible. Therefore, it is imperative that the serial master and PQM
are both at the same ground potential. This is accomplished by joining the 485 ground terminal (Terminal 46 for
COM1; Terminal 49 for COM2) of every unit together and grounding it at the master only.
The last PQM in the chain and the master computer require a terminating resistor and terminating capacitor to
ensure proper electrical matching of the loads and prevent communication errors. Using terminating resistors
on all the PQMs would load down the communication network while omitting them at the ends could cause
reflections resulting in communication errors. Install the 120
Ω
, ¼ watt terminating resistor and 1 nF capacitor
externally. Although any standard resistor and capacitor of these values are suitable, these components can
also be ordered from GE Power Management as a combined terminating network.
Each communication link must have only one computer (PLC or DCS) issuing commands called the master.
The master should be centrally located and can be used to view actual values and setpoints from each PQM
called the slave device. Other GE Power Management relays or devices using the Modbus RTU protocol can
be connected to the communication link. Setpoints in each slave can also be changed from the master. Each
PQM in the link must be programmed with a different slave address prior to running communications using the
S1 PQM SETUP \ COM1 RS485 SERIAL PORT \ MODBUS COMMUNICATION ADDRESS setpoint. The PQMPC software,
a communications program developed by GE Power Management, may be used to view status, actual values,
and setpoints. See Chapter 6 for more information on the PQMPC software.