Modbus Modbus Protocol Overview - GE D90 Plus Communications Manual

D90
Chapter 2: Modbus communication
Modbus protocol overview
Physical layer
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D90 LINE DISTANCE PROTECTION SYSTEM – COMMUNICATIONS GUIDE
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Line Distance Protection System
Modbus communication
This chapter outlines the Modbus protocol. Use the memory map at the end of the chapter
when designing a client to access the D90
Modbus is a standard protocol for communication among devices. Modbus is available via
serial links (Modbus remote terminal unit, or RTU) or Ethernet (Modbus/TCP). The D90
supports Modbus RTU.
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The D90 acts as a slave device. It listens and responds to requests issued by a master
computer. It never initiates communications.
The following description is intended for users who want to develop their own
communication drivers and applies to the serial Modbus RTU protocol.
The Modbus RTU protocol is hardware-independent, allowing the physical layer to be any
standard hardware configuration. The D90
communications ports that can be configured as RS485, 10/100Base-T, or 100Base-F.
Data flow is auto-negotiated to half-duplex or full-duplex in all configurations.
Each data byte is transmitted in an asynchronous format consisting of one start bit, eight
data bits, one stop bit, and possibly one parity bit. This produces a 10 or 11-bit data frame.
This can be important for transmission through modems at high bit rates (11 bit data
frames are not supported by many modems at baud rates greater than 300).
The baud rate and parity are independently programmable for each communications port.
Baud rates of 300, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 33600, 38400, 57600, or
115200 bps are available. Even, odd, and no parity are available.
The master device in any system must know the address of the slave device for
communication. The relay does not act on a request from a master if the address in the
request does not match the relay slave address, unless the address is the broadcast
address.
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device using Modbus.
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includes a front USB port and rear
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