characteristic impedance of 120 ohms 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
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 MM300. Failure to do so may result in intermittent or
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.
Correct polarity is also essential. The MM300 IEDs must be wired with all the positive (+)
terminals connected together and all the negative (–) terminals connected together. Each
relay must be daisy-chained to the next one. Avoid star or stub connected configurations.
The last device at each end of the daisy-chain should be terminated with a 120 ohm ¼
watt resistor in series with a 1 nF capacitor across the positive and negative terminals.
Observing these guidelines will ensure a reliable communication system immune to
The uncovered communications cable shield connected to the common terminal should
not exceed 1" (2.5 cm) for proper EMC shielding of the communications cable.
The following protection modules are available for the MM300.
Table 2-4: MM300 protection modules
Three-phase current metering and thermal overload, undercurrent, and single-
phase underpower protection.
Three-phase voltage metering and three-phase underpower, undervoltage,
overvoltage, and phase reversal protection.
E, F, G, H Bank of three RTDs (100 ohm Platinum).
MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
CHAPTER 2: INSTALLATION