ProSoft MVI56E-MNETCR User Manual page 135

MVI56E-MNETCR ♦ ControlLogix Platform
Modbus TCP/IP Multi-Client Enhanced Communications Module for Remote Chassis
Request
Starting Address Hi
Starting Address Lo
Quantity of Outputs Hi
Quantity of Outputs Lo
In this example, the Client addresses a request to server device. The function
code (01) is for a Read Output Status operation. It requests the status of the
output at address 1245 (04A1 hex). Note that only that one output is to be read,
as specified by the number of outputs field (0001).
If the output address is non-existent in the server device, the server will return
the exception response with the exception code shown (02). This specifies an
illegal data address for the server.
Modbus Exception Codes
Code Name
01 Illegal Function
02 Illegal Data Address
03 Illegal Data Value
04 Slave Device Failure
05 Acknowledge
06 Slave Device Busy
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Response
04 Exception Code
A1
00
01
Meaning
The function code received in the query is not an allowable
action for the server. 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 server 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.
The data address received in the query is not an allowable
address for the server. 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.
A value contained in the query data field is not an allowable
value for server. This indicates a fault in the structure of the
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, because the
Modbus protocol is unaware of the significance of any
particular value of any particular register.
An unrecoverable error occurred while the server was
attempting to perform the requested action.
Specialized use in conjunction with programming
commands. The server 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 Client. The Client can next issue a
poll program complete message to determine if processing
is completed.
Specialized use in conjunction with programming
commands. The server is engaged in processing a long-
duration program command. The Client should retransmit
the message later when the server is free.
Reference
User Manual
02
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