XPS-E
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Modbus RTU Master
XPS-E works in Modbus RTU Mode as Modbus Master. Modbus
Slave is not supported. In Modbus RTU Mode the parameters Baud
Rate, Data Mode, XOFF/Modbus timeout, Async. hardware mode
and Baud Rate Multiplier apply. The parameter XOFF/Modbus
timeout is adjusted in multiples of 100 ms and determines the timeout
period for a non-responding Modbus Slave.
The configured PROFIBUS DP IO data sizes (see XPS-E GSD file)
limit the max. size of the Modbus request and response telegrams
exchanged with the PROFIBUS DP Master. The max. possible size
of a Modbus RTU telegram is 237 Bytes (240 Bytes – 3 Byte header
information).
Transmit of a Modbus RTU
request telegram
Byte
Telegram Element
Value Range
(dec)
1
Reserved
2
Send request number
0 – 255 (See Chapter
„S
end Data"
[
}
15] )
3
Data length N of the following user
data
0 – 237 (See
Chapter„Send Data“,
Section „Modbus RTU-
Master“)
4
Adress Modbus RTU Slave
0 – 255
5
Modbus RTU Command
0 – 255
6+n
Modbus RTU Data
0 – 255
Table 9:
Structure of a Modbus RTU Request-Telegram
The telegram buffer consists of 3 byte header and successive
Modbus specific application data (from byte 4).
By the header bytes the transmitting of the request telegram is
controlled. The header bytes are not transmitted to the Modbus
Slave.
– Byte 1
Reserved
– Byte 2
The transmitting of the Modbus request telegram is triggered by
changing the Send request number in Byte 2. The Modbus RTU request
telegram is automatically generated incl. CRC and transmitted to the
Modbus slave.
– Byte 3
Byte 3 must be set to the size of the Modbus specific application data
from byte 4
For the detailed structure of the specific Modbus request telegrams
please refer to the Modbus slave user documentation.
Data exchange
