User manual

IEC61131 User and Reference Manual
April 22, 2008
388
Slave ID
The slave ID is a variable length message containing controller specific information. It is
returned in response to the Report Slave ID function (see the Slave Mode section).
A C language application program can define the information and the length of the message.
The modbusSlaveID function sets the information. Ladder logic programs cannot set this
information.
Extended Station Addressing
The TeleBUS RTU and ASCII protocols support two type of Modbus station addressing.
Standard Modbus addressing allows a maximum of 255 stations and is compatible with
standard Modbus devices.
Extended Modbus addressing allows a maximum of 65534 stations. Extended Modbus
addressing is fully compatible with standard Modbus addressing for addresses between 0
and 254.
Theory of Operation
The address field of a Modbus message is a single byte. Address 0 is a broadcast address;
messages sent to this address are sent to all stations. Addresses 1 to 255 are station
addresses. Figure 1 shows the format of a standard Modbus message.
Field
Address
Function
...
Size
1
1
N
Figure 45: Standard Modbus Message
The address field extension adds a two-byte extended address field to the message. Figure
2 shows the format of an extended address Modbus message.
Field
Addres
s = 255
Extended
Address
(high)
Extended
Address
(low)
Function
...
Size
1
1
1
1
n
Figure 46: Extended Address Modbus Message
Messages for addresses 0 to 254 use the standard format message. The station address is
stored in the address byte.
Messages for stations 255 to 65534 use the extended address format message. The
address byte is set to 255. This indicates the extended address format is used. The actual
address is stored in the two extended address bytes.
Station address 65535 is reserved and cannot be used as a station number. This station
address is used in store-and-forward tables to indicate a disabled station.
Slave, master and store-and-forward stations treat the addresses in the same manner. The
application program controls the use of the extended addressing format. It may enable or
disable the extended addressing.
Slave Mode
The TeleBUS protocols operate in slave and master modes simultaneously. In slave mode
the controller responds to commands sent by another device. Commands may be sent to a
specific device or broadcast to all devices.