Modbus Protocol; Electrical Interface; Modbus Rtu Description - GE 489 Communications Manual

GE
Grid Solutions
CG.1 Modbus Protocol
CG.1.1 Electrical Interface
CG.1.2 Modbus RTU Description
489 GENERATOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE
489 Generator Management Relay
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 489 must be connected together for the system to operate. Refer to the
489 Instruction Manual for correct serial port wiring.
The 489 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 489 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.
CG–1