Function code 0x03 (Read Holding Registers)
The function code 03 is a query to read holding register of the slave system. Broadcasting is not supported.
(1) Sending data area format
Sending data area requires 4 words. Please set 6 bytes for head word.
Figure 6.35 Sending data area format (Function code 0x03)
1) Device No.: Set the slave number by hexadecimal number. (H01 to HFF. H00 cannot be used.)
2) Function code: Set the function code to H03.
3) Starting address: Set the starting address of holding register by hexadecimal number. (H0000 to HFFFF)
4) Number of holding registers: Set the number of holding registers by hexadecimal number. (H0000 to H00FE)
Ex.) Read the value of 12 holding registers from the address 40129 of slave No.07.
The head address of Sending data area WR100 WR100 = H0006
WR101 = H0703
WR102 = H0080
WR103 = H000C
(2) The state of receiving data area
1) Completed properly
Figure 6.36 Receiving data area format (Function code 0x03)
When a response is received properly from the slave, completion flag (t+1) will be ON, and the holding register
information is stored in the address specified by the query from head word.
2) Receiving exception response
When an exception response is received, error flag (t+2) turns on, and the receiving data area is as follows.
Figure 6.37 Receiving data area format (Function code 0x03 / Exception response)
3) Other errors (except for exception response)
When there is no response from a slave or an abnormal factor is detected in the response, error flag (t+2) turns on,
an error code is stored in the return code area of the s parameter, and the number of receiving data is cleared to
H0000. (Data other than the number of receive data bytes maintains previous values.)
Please refer to section “6.3.5 Error detection in modbus master mode” for more information on error codes.
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