Woodward 2301E, Инструкция по установке и эксплуатации, Страница: 148 / 182

2301E Digital Control Manual 26641
138 Woodward
Modbus Communication Mode
The 2301E control supports RTU Modbus transmission mode.
Characteristic RTU
Coding System 8-bit binary
Start Bits 1
Data Bits per Character 8
Parity none
Stop Bits 1
Baud Rate 9600, 19200, 38400, 57600, 115200
Error Checking CRC (Cyclical Redundancy Check)
Table 8-1. RTU Modbus
In RTU mode, data is sent in 8-bit binary characters and transmitted in a
continuous stream.
The Modbus protocol allows one master and up to 247 slaves on a common
network. Each slave is assigned a fixed, unique device address in the range of 1
to 247. With the Modbus protocol, only the network master can initiate a
transaction. A transaction consists of a request from the master to a slave unit
and the slave’s response. The protocol and Modbus device number are set in the
Program Mode and can be adjusted in Service Mode.
The 2301E control is programmed to function as a slave unit only. As a slave
unit, the 2301E will only respond to a transaction request by a master device.
The 2301E can directly communicate with a DCS or other Modbus supporting
device on a single communications link, or through a multi-dropped network. If
multi-dropping is used, up to 246 devices (2301Es or other customer devices)
can be connected to one Master device on a single network. The control address
is programmed under the 2301E
’s communications block.
Each message to or from a master has a defined structure called the message
“frame”. A frame consists of the slave device address, a code defining the
requested data and error checking information (see Table 7-2).
Beginning
of Frame
SLAVE
Address
Function
Code
DATA Error
Check
Code
End of
Frame
RTU 3-Char
Dead
Time
1 Char
8 bits
1 Char
8 bits
8 bits
Data per
Char
1 Char
8 bits
3-Char
Dead
Time
Table 8-2. Modbus Frame Definition
The Modbus function code tells the addressed slaves what function to perform.
The following table lists the function codes supported by this control.