LIAN 98(en) : Protocol MODBUS serial, Telegram structure MODBUS RTU-mode

MODBUS serial : Telegram structure MODBUS RTU-mode

The LIAN 98 manual can and may not contain the complete MODBUS documentation, caused by legal position. Only all knowledge that is essential necessary for the LIAN 98 software operation is specified in parts in this manual.

Detailed information to the protocol you will find in the MODBUS documentation
on the webpage of the "Modbus Organization"
<Modbus Specifications and Implementation Guides>

Telegram format

Control- and monitoring direction

ADU : Application Data Unit
PDU : Protocol Data Unit

Error message : Monitoring direction only ( slave to the master )

Slave address :

The serial MODBUS protocol is a master - slave protocol. Only one master is connected with one or more slaves ( maximum 247 ) to the same serial bus. The master itself has no specific address. Only the slaves must have an unambiguous address at this bus. The master interrogates a particular slave directly by prefixing the address of this slave to a PDU. Alike, the slave sends its address in the response to the master to indicate the sender of the message.

Function code :

Standard function codes used on MODBUS application layer protocol are described in details in the documentation
"MODBUS Application Protocol Specification" on the webpage of the "Modbus Organization".

Code	Function	Explaination
1	Read Coils	Single bit indication ( read/ write possible )
2	Read Discrete Inputs	Single bit indication ( read only )
3	Read Holding Registers	16 bit analogue value ( read/ write possible )
4	Read Input Registers	16 bit analogue value ( read only )
5	Write Single Coils	e.g. single bit command
6	Write Single Register	e.g. 16 bit analogue value/ bit pattern
7	Read Exception Status	only serial line
8	Diagnostics	only serial line

11	Get Comm Event Counter	only serial line
12	Get Comm Event Log	only serial line

15	Write Multiple Coils	e.g. single bit command
16	Write Multiple Register	e.g. 16 bit analogue value/ bit pattern
17	Report Slave ID	only serial line

20	Read File Record
21	Write File Record
22	Mask Write Register
23	Read/ Write Multiple Register	e.g. 16 bit analogue value/ bit pattern
24	Read FIFO Queue

43	Encapsulated Interface Transport

Exception code :

The function code of an exception response correlates to the function code of the request plus 128. The exception code is provided to indicate the reason of the error and is described in the documentation "MODBUS Application Protocol Specification - Function codes descriptions" on the webpage of the "Modbus Organization".

Checksum :

The RTU mode includes an error–checking field at the end of the message that is based on a Cyclical Redundancy Checking ( CRC ) method performed on the message contents. The CRC field contains a 16–bit value appended as two 8–bit bytes. The CRC value is calculated by the sending device, which appends the low–order byte of the field first, followed by the high–order byte. The receiving device recalculates a CRC during receipt of the message, and compares the calculated value to the actual value it received in the CRC field. If the two values are not equal, an error results.

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Manual LIAN 98