Transmission Media For Modbus; Structure Of A Modbus Telegram; Function Codes; Reading N Words - ECOLAB Versatronic Modbus Beschreibung

3.2

Transmission media for Modbus

Serial interface
For data communication via a serial interface, the Modbus specification provides the
transmission modes RTU mode (Remote Terminal Unit) and ASCII mode (transmission of
data in ASCII format). The Versatronic supports only the RTU mode. Here, the data are
transmitted over the serial bus in binary format (RS422/485).
Ä Chapter 6 'Modbus over a serial interface' on page 37.
Ethernet
Use of an Ethernet network is possible with the aid of Modbus/TCP. The Modbus data
are encapsulated in the form of Modbus/TCP telegrams in TCP segments of the TCP/IP
protocol family. In this way, it is possible to transmit Mod bus/TCP telegrams via Ethernet.
Ä Chapter 7 'Modbus over Ethernet' on page 39.
3.3

Structure of a Modbus telegram

Modbus telegrams have the following structure:
Slave address
1 byte
Every telegram has four fields:
Slave address
Function code
Data field
Checksum
3.4

Function codes

The functions described in the following (from the Modbus standard) are available
for extracting measured values, device and process data, and for writing data.
Function code
Hex
03 or 04
06
10
NOTICE!
If the device does not respond to these functions or generates an error
code, these can be evaluated.
Ä Chapter 3.8 'Error messages' on page 25
Function code
1 byte
Device address of a specific slave
Function selection (read/write words)
Contains the information (depending on the function code)
word address / bit address
n
n number of words / number of bits
n word value(s) / bit value(s)
Detection of transmission errors
Function code
Dec. Function
3 or 4

Reading n words

6

Writing one word

16 Writing n words
Modbus protocol description
Data field
x bytes
Limit
Max. 127 words (254 Bytes)
Max. 1 word (2 Bytes)
Max. 127 words (254 Bytes)
17
Checksum CRC
2 bytes
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