Specifications

7.8.11 Modbus Exception Codes
For a full explanation of the structure of an exception code
response, refer to 7.8.5 Function Field.
Cod
e
Name Meaning
1 Illegal
function
The function code received in the query is
not an allowable action for the server (or
slave). This can 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, because it is
not configured and is being asked to return
register values, for example.
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, 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 specif-
ically 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 7.21 Modbus Exception Codes
7.9 Parameter Access
7.9.1 Parameter Handling
The PNU (Parameter Number) is translated from the
register address contained in the Modbus read or write
message. The parameter number is translated to Modbus
as (10 x parameter number) DECIMAL.
7.9.2 Storage of Data
The Coil 65 decimal determines whether data written to
the frequency converter are stored in EEPROM and RAM
(coil 65 = 1) or only in RAM (coil 65 = 0).
7.9.3 IND
The array index is set in Holding Register 9 and used when
accessing array parameters.
7.9.4 Text Blocks
Parameters stored as text strings are accessed in the same
way as other parameters. The maximum text block size is
20 characters. If a read request for a parameter is for more
characters than the parameter stores, the response is
truncated. If the read request for a parameter is for fewer
characters than the parameter stores, the response is space
filled.
7.9.5 Conversion Factor
Since a parameter value can only be transferred as a whole
number, use a conversion factor to transfer decimals.
7.9.6 Parameter Values
Standard Data Types
Standard data types are int16, int32, uint8, uint16 and
uint32. They are stored as 4x registers (40001 – 4FFFF). The
parameters are read using function 03HEX "Read Holding
Registers." Parameters are written using the function 6HEX
"Preset Single Register" for 1 register (16 bits), and the
function 10HEX "Preset Multiple Registers" for 2 registers
(32 bits). Readable sizes range from 1 register (16 bits) up
to 10 registers (20 characters).
Non-standard Data Types
Non-standard data types are text strings and are stored as
4x registers (40001 – 4FFFF). The parameters are read
using function 03HEX "Read Holding Registers" and written
using function 10HEX "Preset Multiple Registers." Readable
sizes range from 1 register (2 characters) up to 10 registers
(20 characters).
7.10 Examples
The following examples illustrate various Modbus RTU
commands. If an error occurs, refer to 8 General Specifi-
cations and Troubleshooting.
Installation and Set-up VLT® HVAC Drive FC 102 Design Guide
174 MG16C102 - Rev. 2013-08-20
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