4.5.3 Shielding
The use of shielded cables is normally not demanded but offers high protection against electro-
magnetic interferences, especially high frequencies. However, the effectiveness of the shield depends
on careful installation of the line.
If shielded cables are used, the shield should be placed at "PE" on at least one side (preferably on the
master connection). The occurrence of compensating currents may have to be considered if the shield
is contacted on both sides.
Picture 11: Shield connection of lines
5 MODBUS transmission layer
5.1 Introduction
MODBUS is a data transfer protocol of the application layer for the master-slave communication
between the devices connected to the network.
Requests can only be initiated by the master, the reply comes only from the addressed slave. None of
the slaves replies to a broadcast request (request to all slaves).
Structure of the MODBUS frame:
The MODBUS application protocol defines a simple Protocol Data Unit (PDU).
Adressfeld Funktionscode Daten CRC
PDU für serielle MODBUS-Übertragung
MODBUS PDU
Picture 12: General MODBUS frame
The address field only contains the slave address in the serial MODBUS transmission. The CRC
value (Cyclic Redundancy Check) is calculated by the transmitting device and appended to the
message to be sent. The receiving device also calculates a CRC value during reception of the
message and then compares this with the original value received in the CRC field. There is an error if
these two values are not identical.
Attention
It must be ensured that all devices have different addresses otherwise abnormal, unforeseeable states
may occur on the serial bus so that communication of the master with the slaves on the bus is no
longer possible.
The set address is retained voltage failure.
Technical information MODBUS RTU System – model series MODBUS transmission layer
R-TIL08_01-GB 1215 Part.-No.
10/16
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