Modbus Rtu Error Codes - Siemens SIMOCODE pro Function Manual

2.3.1.7

Modbus RTU error codes

Exception responses
Operating principle
On recognition of an error in the request frame from the master (illegal register address, for
example), the slave sets the highest value bit in the function code of the response frame
(that is, the requested function code + 80h). This step is followed by transmission of a byte
with the exception code that describes the cause of the error.
Typical exception code frame
The exception code frame from the slave has the following structure, for example: slave
address 5, requested function code 5, exception code 2.
Response frame from slave:
Slave address
05H
Error codes supported by SIMOCODE pro
Error Meaning in
code accordance with
Modbus
specification
1 Illegal function
2 Illegal data
address
3 Illegal data value
4 Failure in
Associated
Device
6 Busy, rejected
message
SIMOCODE pro - Communication
Function Manual, 11/2018, A5E40508495002A/RS-AC/003
Function code
85H
Cause Brief description
Illegal function code The requested function code is not supported. It is not included in the list
of function codes supported by SIMOCODE pro (see Modbus RTU
function codes (Page 98)).
Illegal bit or register Address does not exist. For functions that work with an addressing
address on the slave range, all addresses affected by the request are checked.
Slave has illegal data The number of addresses is not correct. The number of parameters for
value the requested function was too high (or 0)
Slave has internal There is an unspecified server error that prevented execution of the
error request.
Slave is not ready to The device is busy and unable to process the request at this time. This
receive can occur following a parameterization operation via Modbus when the
new parameter values are transferred to the device.
2.3 Modbus communication
Error code
02H
Communication
CRC
0x....
109