Modbus Protocol; Electrical Interface; Data Format & Data Rate - GE Power Quality Meter Series Instruction Manual

7 MODBUS COMMUNICATIONS 7.1 OVERVIEW
7 MODBUS COMMUNICATIONS 7.1 OVERVIEW

7.1.1 MODBUS PROTOCOL

The GE Power Management PQM implements a subset of the AEG Modicon Modbus RTU serial communica-
tion standard. Many popular programmable controllers support this protocol directly with a suitable interface
card allowing direct connection of the PQM. Although the Modbus protocol is hardware independent, the PQM
interface uses 2-wire RS485 and 9-pin RS232 interfaces. Modbus is a single-master multiple-slave protocol
suitable for a multi-drop configuration provided by RS485 hardware. In this configuration, up to 32 slaves can
be daisy-chained together on a single communication channel.
The PQM is always a Modbus slave; it cannot be programmed as a Modbus master. Computers or PLCs are
commonly programmed as masters. The Modbus protocol exists in two versions: Remote Terminal Unit (RTU,
binary) and ASCII. Only the RTU version is supported by the PQM. Monitoring, programming and control func-
tions are possible using read and write register commands.

7.1.2 ELECTRICAL INTERFACE

The electrical interface is 2-wire RS485 and 9-pin RS232. In a 2-wire RS485 link, data flow is bi-directional and
half duplex. That is, data is never transmitted and received at the same time. RS485 lines should be connected
in a daisy-chain configuration (avoid star connections) with a terminating network installed at each end of the
link, i.e. at the master end and the slave farthest from the master. The terminating network should consist of a
120 Ω resistor in series with a 1 nF ceramic capacitor when used with Belden 9841 RS485 wire. The value of
the terminating resistors should be equal to the characteristic impedance of the line. This is approximately
120 Ω for standard #22 AWG twisted-pair wire. Shielded wire should always be used to minimize noise. Polar-
ity is important in RS485 communications: each '+' terminal of every device must be connected together for the
system to operate. See Section 2.2.9: RS485 SERIAL PORTS on page 2–18 for details on serial port wiring.
7.1.3 DATA FORMAT & DATA RATE
One data frame of an asynchronous transmission to or from a PQM consists of 1 start bit, 8 data bits, and 1
stop bit, resulting in a 10-bit data frame. This is important for high-speed modem transmission, since 11-bit
data frames are not supported by Hayes modems at bit rates greater than 300 bps. The Modbus protocol can
be implemented at any standard communication speed. The PQM supports operation at 1200, 2400, 4800,
9600, and 19200 baud.
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GE Power Management PQM Power Quality Meter 7-1