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Code Name
Meaning
1
Illegal
function
The function code received in the query is
not an allowable action for the server (or
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
server (or slave) is in the wrong state to
process a request of this type, for
example because it is not configured and
is being asked to return register values.
2
Illegal data
address
The data address received in the query is
not an allowable address for the server
(or 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 generates
exception 02.
3
Illegal data
value
A value contained in the query data field
is not an allowable value for server (or
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,
since the Modbus protocol is unaware of
the significance of any particular value of
any particular register.
4
Slave device
failure
An unrecoverable error occurred while the
server (or slave) was attempting to
perform the requested action.
Table 9.7 Modbus Exception Codes
9.2.3.4 Data Field
The data field is constructed using sets of two hexadecimal
digits, in the range of 00 to FF hexadecimal. These are
made up of one RTU character. The data field of messages
sent from a master to slave device contains additional
information that the slave must use to take the action
defined by the function code. This can include items such
as coil or register addresses, the quantity of items to be
handled and the count of actual data bytes in the field.
9.2.3.5 CRC Check Field
Messages include an error-checking field, operating based
on a Cyclical Redundancy Check (CRC) method. The CRC
field checks the contents of the entire message. It is
applied regardless of any parity check method used for the
individual characters of the message. The CRC value is
calculated by the transmitting device, which appends the
CRC as the last field in the message. The receiving device
recalculates a CRC during receipt of the message and
compares the calculated value to the actual value received
in the CRC field. If the two values are unequal, a bus
timeout results. The error-checking field contains a 16-bit
binary value implemented as two 8-bit bytes. When this is
done, the low-order byte of the field is appended first,
followed by the high-order byte. The CRC high-order byte
is the last byte sent in the message.
9.2.4 Register Addressing
In Modbus, all data are organized in coils and holding
registers. Coils hold a single bit, whereas holding registers
hold a 2-byte word (i.e. 16 bits). All data addresses in
Modbus messages are referenced to zero. The first
occurrence of a data item is addressed as item number
zero. For example: The coil known as ‘coil 1’ in a
programmable controller is addressed as coil 0000 in the
data address field of a Modbus message. Coil 127 decimal
is addressed as coil 007EHEX (126 decimal).
Holding register 40001 is addressed as register 0000 in the
data address field of the message. The function code field
already specifies a ‘holding register’ operation. Therefore,
the ‘4XXXX’ reference is implicit. Holding register 40108 is
addressed as register 006BHEX (107 decimal).
RS-485 Installation and Set...
AF-650 GP
TM
Design and Installation Guide
DET-767A
9-3
9
9
Summary of Contents for AF-650 GP Series
Page 3: ...Safety AF 650 GPTM Design and Installation Guide DET 767A ...
Page 9: ...Contents AF 650 GPTM Design and Installation Guide DET 767A ...
Page 19: ...Introduction AF 650 GPTM Design and Installation Guide 1 10 DET 767A 1 1 ...
Page 35: ...Installation AF 650 GPTM Design and Installation Guide 2 16 DET 767A 2 2 ...
Page 41: ...Start Up and Functional Tes AF 650 GPTM Design and Installation Guide 3 6 DET 767A 3 3 ...
Page 55: ...About Programming AF 650 GPTM Design and Installation Guide 5 10 DET 767A 5 5 ...
Page 91: ...Application Setup Examples AF 650 GPTM Design and Installation Guide 6 36 DET 767A 6 6 ...
Page 103: ...Status Messages AF 650 GPTM Design and Installation Guide 8 4 DET 767A 8 8 ...
Page 123: ...Warnings and Alarms AF 650 GPTM Design and Installation Guide 10 10 DET 767A 10 0 ...
Page 127: ...Basic Troubleshooting AF 650 GPTM Design and Installation Guide 11 4 DET 767A 11 1 ...
Page 129: ...Terminal and Applicable Wir AF 650 GPTM Design and Installation Guide 12 2 DET 767A 12 2 ...
Page 161: ...Specifications AF 650 GPTM Design and Installation Guide 13 32 DET 767A 13 3 ...