Modbus Rtu Protocol - Siemens SENTRON ATC6300 Manual

MODBUS communication

11.2 MODBUS RTU protocol

11.2 MODBUS RTU protocol
If the MODBUS RTU protocol is used, the following structure must be observed:
T1
T2
T3
Address: Contains the serial address of the slave target device.
Function: Contains the code of the function that is performed by the slave.
Data: Contains the data that is sent to the slave, or the data of a query received by the slave
(maximum length 80, 16-bit register).
CRC: This is used for an integrity check. If a message is corrupted by electric noise or
interference, the CRC field enables the ATC6300 to detect the error and thus ignore the
message.
T1,T2,T3: Corresponds to the time during which the data may not be exchanged via the
communication bus. The connected devices use this to detect the end of one command and
the beginning of a new one. This time must be at least 3.5 times longer than the time
required for sending one character.
Note
The ATC6300 measures the time between receiving one character and receiving a new one.
If this value exceeds the time required for transmitting 3.5 characters at the selected baud
rate, then the next character is regarded as part of a new message.
Example:
In order, for example, to read out the number of alarms of an ATC6300 with the serial
address 1 for changeovers of breaker 1 (Register: 3Ah9), the following command must be
sent:
01
01 = Slave address
04 = MODBUS function 04 Read Input Register
00 39 = Register address of the query (number of alarms for changeovers of Breaker 1);
reduced by 1 (see chapter Data library (Page 157))
0002 = Number of registers to be read, starting at Address 22
A1 C6 = CRC Checksum
148
Address Function
(8 bit) (8 bit)
04 00
Data
(N x 8 bit)
39 00
CRC T1
(16 bit) T2
T3
02 A1
3KC ATC6300 transfer control device
Manual, 03/2018, L1V30535632002A-01
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