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12.2. Function codes
HEX
DEC
CODE
MODBUS NAME DEFINITION COMMENT
0x01
1
Read Coils Reading of Digital Inputs
This function allows the master to read the desired
number of bits from a concrete address. Each
address corresponds to a register of 1 bit with the
state of the digital input
0x03
3
Read Holding Registers Reading of Analog Inputs
The master indicates the base address and number
of words to read from this address, while the
equipment indicates in the response the number of
returned bytes, followed by these values.
0x05
5
Write Single Coil Writing
This function allows writing the state of a Digital
Output of the relay. The master specifies the
address of the bit or command to modify followed
by 0x00 to put a 0 or 0xFF to put a 1. The relay
responses with a similar frame indicating the
modified address and the set value in the bit or
command.
12.3. Exemptions and error answers
The error codes defined by the ModBus protocol are as follows:
01 ILLEGAL FUNCTION The slave does not support any function with the function code received in
this message.
02 ILLEGAL DATA ADDRESS The master is trying to do an operation in a wrong address.
03 ILLEGAL DATA VALUE The slave has detected that the value sent by the master is not valid.
04 SLAVE DEVICE FAILURE Indicates an error occurred in the slave while trying to execute the request of
the master.
05 ACKNOWLEDGE Generic recognition.
06 SLAVE DEVICE BUSY The slave is busy and unable to perform the required operation.
07 NEGATIVE ACKNOWLEDGE Generic non-recognition.
12.4. Data type
TYPE LENGTH DESCRIPTION
BIT 1 1 Bit per address
UINT16 2 Integer eithout sign of 2 bytes (lsw-msw)