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2
Protocol description
2.1
Master-slave principle
Communication between a PC (master) and a device (slave) with Modbus takes place according to the
master/slave principle in the form of data request/instruction – response.
Slave 1
The master controls the data exchange; the slaves only have a response function. They are identified
by their device address.
2.2
RTU transmission mode
RTU mode (Remote Terminal Unit) is used as the transmission mode. Data is transmitted in binary for-
mat (hexadecimal) with 8 bits. The LSB (least significant bit) is transmitted first. The ASCII operating
mode is not supported.
Data format
The data format describes the structure of the characters that are transferred. The following options are
provided for the data format:
Data word
8-bit
8-bit
8-bit
8-bit
2.3
Device address
The device address must lie in the range 1 to 255.
Address 0 is reserved for broadcast messages, i.e., the instruction is executed by the IPC 300 but not
answered.
If an instruction with the device address 255 is received in the device, it will always be processed, even
if a different device address has been configured.
If a device address of 0 is configured, the power controller will not signal a bus error
if it has not received an instruction for 10 s.
NOTE!
A maximum of 31 slaves can be addressed via the RS485 interface.
Two different options are available for data exchange:
Query
Data request/instruction by the master to a slave via the corresponding device address.
The addressed slave responds.
Master
Slave 2
Parity bit
-
even
odd
-
2 Protocol description
Stop bit
1/2 bit
1
1
1
2
Slave n
Number of bits
9
10
10
10
9
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