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The next table contains a list of default exceptions, defined by the Modbus protocol specification (version 1.1b).
Exceptions are registered in Driver's log, whenever detected, and they can be read by an application by Reading the Last Exception Code.
Notice that, in addition to the exceptions listed here, a device may define other proprietary exceptions. In this case, they are supposed to be described on manufacturer's documentation of that device.
Exception codes standardized by the Modbus protocol
Code (in Hexadecimal)
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Name
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Meaning
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01
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ILLEGAL FUNCTION
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The function code received is not valid. This may indicate that this function is not implemented yet or the slave is in an inadequate status to process it
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02
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ILLEGAL DATA ADDRESS
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The data address received is not a valid address. More specifically, the combination of a reference address and the amount of data to transfer is invalid
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03
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ILLEGAL DATA VALUE
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The current value on Master's request is not valid. This indicates a failure on the remaining data structure of a complex request, such as when the informed size for a data block is not correct. This exception does not indicate that values submitted for writing are out of the expected scope by an application, as such information is not accessible to this protocol
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04
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SLAVE DEVICE FAILURE
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An irrecoverable error occurred during the processing of the requested function
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05
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ACKNOWLEDGE
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Used with programming commands. The Slave accepted this message and it is processing it. However, this processing demands a long time. This exception prevents a Master time-out. The end of this request must be tested by a polling process
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06
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SLAVE DEVICE BUSY
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Used with programming commands. Indicates that the Slave is processing another long-lasting command and that this request must be sent again later, when the Slave is available again
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08
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MEMORY PARITY ERROR
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Used with functions 20 and 21, reference type 6, to indicate the extended file failed on a consistency test. The Slave device may need maintenance
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0A
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GATEWAY PATH UNAVAILABLE
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Used together with gateways, to indicate that a gateway was unable to allocate an internal path to process this request. It usually indicates that a gateway is not correctly configured or it is overloaded
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0B
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GATEWAY TARGET DEVICE FAILED TO RESPOND
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Used together with gateways, to indicate that no response was received from the destination device. It usually indicates that such device is not available on the network
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