Electrical Interface; Modbus Rtu Protocol - GE 469 Communications Manual

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Modbus Protocol

Electrical Interface

Modbus RTU Protocol

469 MOTOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE
469 Motor Management Relay
Communications Guide
Communications Guide
The hardware or electrical interface is one of the following: one of two 2-wire RS485 ports
from the rear terminal connector or the RS232 from the front panel connector. In a 2-wire
RS485 link, data flow is bidirectional. Data flow is half duplex for both the RS485 and the
RS232 ports. 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 at 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. Polarity is important in RS485 communications. Each '+'
terminal of every 469 must be connected together for the system to operate. See Chapter
2 of the 469 manual for details on correct serial port wiring.
The 469 implements a subset of the AEG Modicon Modbus RTU serial communication
standard. Many popular programmable controllers support this protocol directly with a
suitable interface card allowing direct connection of relays. Although the Modbus protocol
is hardware independent, the 469 interfaces include two 2-wire RS485 ports and one
RS232 port. Modbus is a single master, multiple slave protocol suitable for a multi-drop
configuration as provided by RS485 hardware. In this configuration up to 32 slaves can be
daisy-chained together on a single communication channel.
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