5.8.9 How to Control the Frequency
Converter
This section describes codes which can be used in the
function and data fields of a Modbus RTU telegram.
5.8.10 Function Codes Supported by
Modbus RTU
Modbus RTU supports use of the following function codes
in the function field of a telegram:
Function
Function code (hex)
Read coils
1
Read holding registers
3
Write single coil
5
Write single register
6
Write multiple coils
F
Write multiple registers
10
Get comm. event counter
B
Report slave ID
11
Table 5.19 Function Codes
Function
Function
code
Subfunction
code
Subfunction
Diagnostics 8
1
Restart communication.
2
Return diagnostic
register.
10
Clear counters and
diagnostic register.
11
Return bus message
count.
12
Return bus communi-
cation error count.
13
Return slave error count.
14
Return slave message
count.
Table 5.20 Function Codes
5.8.11 Modbus Exception Codes
For a full explanation of the structure of an exception code
response, refer to
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
succeeds, while 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 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 value of any register.
4
Slave device
failure
An unrecoverable error occurred while the
server (or slave) was attempting to
perform the requested action.
Table 5.21 Modbus Exception Codes
5.9 How to Access Parameters
5.9.1 Parameter Handling
The PNU (parameter number) is translated from the
register address contained in the Modbus read or write
telegram. The parameter number is translated to Modbus
as (10 x parameter number)
decimal
.
Examples
Reading
parameter 3-12 Catch up/slow Down Value
(16 bit):
The holding register 3120 holds the parameters value. A
value of 1352 (decimal), means that the parameter is set to
12.52%.
Reading
parameter 3-14 Preset Relative Reference
(32 bit):
The holding registers 3410 and 3411 hold the parameters
values. A value of 11300 (
decimal
), means that the
parameter is set to 1113.00.
For information on the parameters, size, and conversion
index, see the
VLT
®
Midi Drive FC 280 Programming Guide
.
RS485 Installation and Set-...
VLT
®
Midi Drive FC 280
64
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