ProSoft Technology MVI46-MCM - Manual (Page 95)

MVI46-MCM ♦ SLC Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 95 of 108
March 29, 2011
In this example, the Master 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 Slave.
Modbus Exception Codes
Code Name Meaning
01 Illegal Function 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.
02 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 Illegal Data Value A value contained in the query data field is not an allowable
value for Slave. 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.
04 Slave Device Failure An unrecoverable error occurred while the Slave was
attempting to perform the requested action.
05 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 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.
08 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 record file, but detected a parity error in
the memory. The Master can retry the request, but service
may be required on the Slave device.