Siemens SENTRON ATC6500 - Function 04: Read Input Register

MODBUS communication
10.3 MODBUS functions
3KC ATC6500 transfer control device
Manual, 05/2019, L1V30538268002A-01
187
10.3.1 Function 04: Read input register
The MODBUS function 04 permits the reading of one or more consecutive registers from a
memory of the slave. The register addresses are contained under the subsection Data library
(Page 192). As in the case of MODBUS as standard, the effective register address from the
data library must also be reduced by the factor 1 in the case of queries. If the register
address is not contained in data library 11.5, or the number of registers specified exceeds
the accepted number, an error message is displayed in accordance with the section "Error
Messages".
Querying of the master:
Slave Address
08h
Function
04h
MSB address
00h
LSB address
0Fh
Number of MSB registers 00h
Number of LSB registers
08h
LSB CRC
C1h
MSB CRC
56h
In the previous example, 8 consecutive registers are requested by the slave. Starting with
register 10h, the registers 10h to 17h are output. The message is concluded by the CRC
Checksum.
Response of the slave:
Slave Address
08h
Function
04h
Number of bytes
10h
MSB register 10h
00h
LSB register 10h
00h
-
-
MSB register 17h
08h
LSB register 17h
C1h
LSB CRC
56h
MSB CRC
B1h
The response always consists of the address of the slave, the code of the function that was
queried by the master, and the content of the queried register. The message is concluded by
the CRC Checksum.