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Modbus Read, Write and Override Parameters plus Exception Codes
4/07 UDC2500 Universal Digital Controller Product Manual 219
Query
Example: Internal slave error reading 2 registers starting at address 1820h from slave at slave address 02.
02 03 18 20 00 02 CRC CRC
Response
Example: Return MSB in Function Code byte set with Slave Device Failure (04) in the data field.
02 83 04 CRC CRC
Table 10-21 Modbus RTU Data Layer Status Exception Codes
Exception
Code
Definition Description
01
Illegal Function The message received is not an allowable action for
the addressed device.
02
Illegal Data Address The address referenced in the function-dependent data
section of the message is not valid in the addressed
device.
03
Illegal Data Value The value referenced at the addressed device location
is no within range.
04
Slave Device Failure The addressed device has not been able to process a
valid message due to a bad device state.
05, 06
Slave Device Busy The addressed device has ejected a message due to a
busy state. Retry later.
07
NAK,
Negative Acknowledge
The addressed device cannot process the current
message. Issue a PROGRAM POLL to obtain device-
dependent error data.
09
Buffer Overflow The data to be returned for the requested number of
registers is greater than the available buffer space.
Function Code 20 only.