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EXCEPTION CODES
MODBUS TCP provides reserved codes used for exceptions. These codes provide error information relating to
failed requests. Exceptions are signalled by hex 80 being added to the function code of the request, followed
by one of the codes listed in Figure 108.
Code
Dec Hex
01
01
02
02
03
03
04
04
09
09
10
0A
11
0B
7.2.2 Data types
The following data types are supported:
1.
2's complement signed 16-bit analog values with implied decimal point. The decimal point position must
be configured in both the recorder and the host computer.
2.
16, 32 and 64 bit signed integers.
3.
16-bit unsigned integer values.
4.
32 bit IEEE Floating point values.
5.
Strings of limited size, can be transferred across Modbus TCP in Unicode format using a single non-
multiplexed set of consecutive registers.
DATA ENCODING
MODBUS uses what is called a 'Big endian' representation for addresses and data items. This means that when
a numerical quantity larger than a single byte is transmitted, the most significant byte is sent first. For example
a 32-bit hex value of 12345678 would be transmitted as 12, followed by 34, followed by 56 and finally 78.
7.2.3 Invalid multiple register writes
When a recorder receives a multi-register write request, it is possible that one or more requests will be rejected.
Under such a circumstance, the recorder accepts all valid write requests and ignores any invalid writes. No error
response is produced.
7.2.4 Master communications timeout
Whilst the instrument is archiving, it is possible that communications responses slow sufficiently to cause
communications timouts. The Modbus master device should be configured with a timout value large enough to
ensure against nuisance timeouts during archiving.
7.2.5 Non-volatile parameters in EEPROM
Caution: The parameters in the following list must not be written-to on a continuous basis as
Page 116
FEC920: USER GUIDE
Modbus definition
Illegal function
Illegal Data Address
Illegal Data Value
Slave Device Failure
Illegal Sub Function
Gateway path unavailable Gateway misconfigured or overloaded
Gateway target device
failed to respond
Figure 108 Exception codes
to do so will damage the EEPROM, greatly shortening its useful life.
Description (see Modbus specification for full details)
An invalid function code was received
An invalid data address was received
An invalid data value was received
An unrecoverable error occurred in the instrument
An invalid sub function was received
Device not present on the network
A5E45696052A Rev-AA
September 18
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