Dräger Polytron 8000 Series Technical Manual page 14

Basic principles of the Modbus technology
– Function code
– Error code (Exception Code)
– CRC (checksum)
To mark the error message, the leading bit of the returned function code is set. The
error code (Exception Code) is used to transmit the error cause to the Master.
Excep-
tion code
01
02
03
04
Addressing of the registers
Register
Discrete Input Bit
Coils
Input Register 16-bit word Read only
Holding Reg-
ister
* The register address is transmitted as a 16-bit word. The first register is assigned
the address value 0.
Please note: another notation, where the register addresses are counted starting
with 1, is also common in Modbus applications. In that case, this address (register
address - 1) is transmitted in the Modbus telegram.
14
Description
ILLEGAL_FUNCTION
The function code sent by the Master is not supported by the Slave.
ILLEGAL_DATA_ADDRESS
The register addressed by the Master is not assigned (does not exist)
or the length of the requested data is too long.
ILLEGAL_DATA_VALUE
The Master is attempting to write to a register that allows only read
access. The value appearing in the data field is not permissible: e.g.,
range limits exceeded or wrong data format.
SLAVE_DEVICE_FAILURE
The Slave did not respond to the request telegram from the Master or
an error occurred when processing the request telegram.
Data type Access
Read only
Bit Read/write
16-bit word Read/write
Technical Manual
Address Information
range*
1XXXX Not used by the device
2XXXX Not used by the device
3XXXX This data can only be
read by the device
4XXXX This data can be read
by the device and
changed by the appli-
cation
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Digital Communication - Modbus RTU