2–20
PQM POWER QUALITY METER – INSTRUCTION MANUAL
ELECTRICAL
CHAPTER 2: INSTALLATION
A serial port provides communication capabilities between the PQM and a remote
computer, PLC, or distributed 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
polarity of the master device. If the front panel RX1 or RX2 lights are flashing, this indicates
that the PQM is receiving 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:
. 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 Multilin 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 Multilin 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 EnerVista PQM Setup software, a communications program developed by GE
Multilin, may be used to view status, actual values, and setpoints. See Chapter 6 for more
information on the EnerVista PQM Setup software.
Содержание Multilin PQM
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