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1 GENERAL
This document specifies a subset of the Modbus serial communications protocol used to transfer data
between a master computer station and the PM171. The document provides the complete information
necessary to develop a third-party communications software capable of communication with the Series
PM171 Powermeters. Additional information concerning communications operation, configuring the
communications parameters, and communications connections is found in "Series PM171
Powermeters, Installation and Operation Manual".
IMPORTANT
1. The voltage parameters throughout the protocol can represent line-to-neutral or line-to-line voltages
depending on the wiring mode selected in the instrument. When the 4LN3 or 3LN3 wiring mode is
selected, the voltages will be line-to-neutral; for any other wiring mode, they will be line-to-line voltages.
In 4LN3, 4LL3, 3LN3 and 3LL3 wiring modes, harmonic voltages will represent line-to-neutral voltages.
In a 3-wire direct connection, harmonic voltages will represent line-to-neutral voltages as they appear on
the instrument's input transformers. In a 3-wire open delta connection, harmonic voltages will comprise
L12 and L23 line-to-line voltages.
2. In 3-wire connection schemes, the unbalanced current and phase readings for power factor, active
power, and reactive power will be zeros, because they have no meaning. Only the total three-phase
power values can be used.
3. Most of the instrument's advanced features are configured using multiple setup parameters that can be
accessed in certain contiguous registers. When writing the setup registers, it is recommended to write
all the registers at once using a single request, or to clear (zero) the setup before writing into separate
registers.