ProSoft Technology MVI69-MCM User Manual

Page 145

MVI69-MCM ♦ CompactLogix or MicroLogix Platform

Reference

Modbus Communication Module

User Manual

ProSoft Technology, Inc.

Page 145 of 167

March 22, 2011

If the output address does not exist in the slave device, the slave will return an
Exception Response with the Exception Code shown (02). This specifies an

illegal data address for the slave.

Modbus Exception Codes

Code

Name

Meaning

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.

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.

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.

Slave Device Failure

An unrecoverable error occurred while the Slave was
attempting to perform the requested action.

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.

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.

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.

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.

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.