Simply Modbus logo






RTU Master

RTU Slave

TCP Client





Exception Responses

Following a request, there are 4 possible outcomes from the slave.

  1.  The request is successfully processed by the slave and a valid response is sent.

  2.  The request is not received by the slave therefore no response is sent.

  3. The request is received by the slave with a parity, CRC or LRC error.
      The slave ignores the request and sends no response.

  4. The request is received without an error, but cannot be processed by the slave for another reason.  The slave replies with an exception response.

In a normal response, the slave echoes the function code.  The first sign of an exception response is that the function code is shown in the echo with its highest bit set. All function codes have 0 for their most significant bit.  Therefore, setting this bit to 1 is the signal that the slave cannot process the request.

Function Code in Request

Function Code in Exception Response

01 (01 hex) 0000 0001

129 (81 hex) 1000 0001

02 (02 hex) 0000 0010

130 (82 hex) 1000 0010

03 (03 hex) 0000 0011 131 (83 hex) 1000 0011

04 (04 hex) 0000 0100

132 (84 hex) 1000 0100
05 (05 hex) 0000 0101 133 (85 hex) 1000 0101
06 (06 hex) 0000 0110 134 (86 hex) 1000 0110
15 (0F hex) 0000 1111 143 (8F hex) 1000 1111
16 (10 hex) 0001 0000 144 (90 hex) 1001 0000

Here is an example of a request with an Exception Response:


This command is requesting the ON/OFF status of discrete coil #1186
from the slave device with address 10.

0A 01 04A1 0001 AC63

0A: The Slave Address (0A hex = address10 )
01: The Function Code 1 (read Coil Status)
04A1: The Data Address of the first coil to read
             ( 04A1 hex = 1185 , + 1 offset = coil #1186 )
0001: The total number of coils requested.
AC63: The CRC (cyclic redundancy check) for error checking.


0A 81 02 B053

0A: The Slave Address (0A hex = address10 )
81: The Function Code 1 (read Coil Status - with the highest bit set)
02: The Exception Code
B053: The CRC (cyclic redundancy check).

Following the Function Code is the Exception Code.  The exception code gives an indication of the nature of the problem. The possible codes are shown in the table below.

The exception code shown above 02 is an indication that coil #1186 is an illegal address in the slave.  This coil has not been defined in the slave's modbus map.

The exception codes as explained in the Modbus specification are:

Exception Code



(01 hex)


The function code received in the query is not an allowable action for the slave.  This may be because the function code is only applicable to newer devices, and was not implemented in the unit selected.  It could also indicate that the slave is in the wrong state to process a request of this type, for example because it is unconfigured and is being asked to return register values. If a Poll Program Complete command was issued, this code indicates that no program function preceded it.

(02 hex)

Illegal Data Address

The data address received in the query is not an allowable address for the slave. More specifically, the combination of reference number and transfer length is invalid. For a controller with 100 registers, a request with offset 96 and length 4 would succeed, a request with offset 96 and length 5 will generate exception 02.

(03 hex)
Illegal Data Value A value contained in the query data field is not an allowable value for the slave.  This indicates a fault in the structure of remainder of a complex request, such as that the implied length is incorrect. It specifically does NOT mean that a data item submitted for storage in a register has a value outside the expectation of the application program, since the MODBUS protocol is unaware of the significance of any particular value of any particular register.

(04 hex)

Slave Device Failure

An unrecoverable error occurred while the slave was attempting to perform the requested action.
(05 hex)
Acknowledge Specialized use in conjunction with programming commands.
The slave has accepted the request and is processing it, but a long duration of time will be required to do so.  This response is returned to prevent a timeout error from occurring in the master. The master can next issue a Poll Program Complete message to determine if processing is completed.
(06 hex)
Slave Device Busy Specialized use in conjunction with programming commands.
The slave is engaged in processing a long-duration program command.  The master should retransmit the message later when the slave is free..
(07 hex)
Negative Acknowledge The slave cannot perform the program function received in the query. This code is returned for an unsuccessful programming request using function code 13 or 14 decimal. The master should request diagnostic or error information from the slave.
(08 hex)
Memory Parity Error Specialized use in conjunction with function codes 20 and 21 and reference type 6, to indicate that the extended file area failed to pass a consistency check.
The slave attempted to read extended memory or record file, but detected a parity error in memory. The master can retry the request, but service may be required on the slave device.
(0A hex)
Gateway Path Unavailable Specialized use in conjunction with gateways, indicates that the gateway was unable to allocate an internal communication path from the input port to the output port for processing the request. Usually means the gateway is misconfigured or overloaded.
(0B hex)
Gateway Target Device Failed to Respond Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network.

Home FAQ Enron Download Purchase Contact

Copyright © 2020 Simply Modbus. All rights reserved