Rs 485 Modbus Rtu Technology; Modbus Protocol; Modbus Telegram - aqua metro CALEC ST En Complément Des Instructions De Montage Et D'utilisation

RS 485 Modbus RTU technology

Modbus is an open, standardized field bus system which is used in the areas of manufacturing automation, process automation and buil-
ding automation. RS 485 Modbus RTU (Remote Terminal Unit) allows the heat calculator CALEC
PLC or SCADA systems.
The Modbus RS 485 distinguishes between master and slave devices. The CALEC
• Master devices:
Master devices determine the data traffic on the field bus system. They can send request telegram to one (Standard) or all (only Broad-
cast Address = 0) slaves.
• Slave devices:
Slave devices are able to send their data only in response to a request of a master.

Modbus protocol

The protocol defines the way in which messages will be transmitted between CALEC

Modbus telegram

The data is transferred between the master and slave by means of a telegram. A request telegram from the master contains the following
telegram fields.
• Slave Address:
The bus address of the CALEC
means of the slave address 0 (Broadcast Message).
• Function Code:
The function code determines which read, write and test operations should be executed by means of the Modbus protocol.
• Data:
Depending of the function code, the following values are transmitted in this data field:
- Register start address (from which data are transmitted)
- Number of registers
- Read Data
- Data length
• Check sum:
The telegram check sum forms the end of the telegram.
If an error occurs during data transfer or if the slave cannot execute the command from the master, the slave returns an error telegram to
the master.
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ST has to be in an address range from 1 to 247. The master talks to all the slaves simultaneously by
® ST to be easily integrate to DDC, BMS,
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ST of Aquametro AG works as a slave station.
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ST and a Modbus master.