5–6 EPM 9450/9650 ADVANCED POWER QUALITY METERING SYSTEM – INSTRUCTION MANUAL
RS485 COMMUNICATION CHAPTER 5: COMMUNICATION WIRING
5.3 RS485 Communication
RS485 communication allows multiple devices to communicate on a bus. The EPM 9450/
9650 meter’s Ports 1 to 4 are RS485 terminals, viable for a distance of up to 4000 feet
(1219 meters). (Port 1 can be switched between RS232 and RS485.) The
following figure shows wiring detail of a 2-wire RS485 port.
Figure 5-5: 2-Wire RS485 Port Detail
All of the EPM 9450/9650 meter’s RS485 ports have the following connections:
• +V- (Voltage terminals for power connections): use with EPM I/O Modules and
Displays only. The EPM 9450/9650 meter supplies 17V DC through the +V-
terminal connections.
Note
Do not connect these pins to devices that receive power from another source—e.g., a
computer—or to devices that do not require power to operate.
• S (Shield): the Shield connection is used to reference the meter’s port to the same
potential as the source. It is not an earth-ground connection. You must also
connect the shield to earth-ground at one point. Do not connect the shield to
ground at multiple points, as this will interfere with communication.
• +/- (Two-wire, RS485 communication terminals): connect the + terminal of the EPM
meter’s port to the + terminal of the device; connect the - terminal of the EPM
meter’s port to the - terminal of the device.
Note
RS485 Communication:
• Use a shielded twisted pair cable 22 AWG (0.33 mm2) or larger, grounding the
shield at one end only.
• Establish point-to-point configurations for each device on an RS485 bus: connect
(+) terminals to (+) terminals; connect (-) terminals to (-) terminals.
• Protect cables from sources of electrical noise.
• Avoid both “star” and “tee” connections (see Figure 5.6). No more than two cables
should be connected at any one point on an RS485 network, whether the
connections are for devices, converters or terminal strips.
• Include all segments when calculating the total cable length of a network. If you
are not using an RS485 repeater, the maximum length for cable connecting all
devices is 4000 feet (1219 meters).
• RT EXPLANATION: Termination Resistors are generally used on both ends of longer
length transmission lines. The value of the Termination Resistors is determined by
the electrical parameters of the cable. Use RTs only on Master and Last Slave when
connecting multiple meters in a Daisy Chain.
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