MBS Protocol
PLX30 Series
User Manual
Ethernet and Serial Gateways
Page 150 of 200
ProSoft Technology, Inc.
September 19, 2012
If the output address does not exist 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
Meaning
01
Illegal Function
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.
02
Illegal Data Address
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.
03
Illegal Data Value
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.
04
Slave Device Failure
An unrecoverable error occurred while the server was
attempting to perform the requested action.
05
Acknowledge
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 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 server is engaged in processing a long-
duration program command. The Master should retransmit
the message later when the server 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 server
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 server device.
0a
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 that the gateway
is misconfigured or overloaded.
0b
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.
