Code Name
Meaning
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 3.17 Modbus Exception Codes
3.9 How to Access Parameters
3.9.1 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
. Example: 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
.
3.9.2 Storage of Data
The coil 65 decimal determines whether data written to
the frequency converter is stored in EEPROM and RAM (coil
65=1) or only in RAM (coil 65= 0).
3.9.3 IND (Index)
Some parameters in the frequency converter are array
parameters, for example
parameter 3-10 Preset Reference
.
Since the Modbus does not support arrays in the holding
registers, the frequency converter has reserved the holding
register 9 as pointer to the array. Before reading or writing
an array parameter, set the holding register 9. Setting
holding register to the value of 2 causes all following read/
write to array parameters to be to the index 2.
3.9.4 Text Blocks
Parameters stored as text strings are accessed in the same
way as the 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.
3.9.5 Conversion Factor
A parameter value can only be transferred as a whole
number. To transfer decimals, use a conversion factor.
3.9.6 Parameter Values
Standard data types
Standard data types are int 16, int 32, uint 8, uint 16, and
uint 32. They are stored as 4x registers (40001–4FFFF). The
parameters are read using function 03 hex
read holding
registers
. Parameters are written using the function 6 hex
preset single register
for 1 register (16 bits), and the
function 10 hex
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 03 hex
read holding registers
and written using
function 10 hex
preset multiple registers
. Readable sizes
range from 1 register (2 characters) up to 10 registers (20
characters).
3.10 Examples
The following examples show various Modbus RTU
commands.
3.10.1 Read Holding Registers (03 hex)
Description
This function reads the contents of holding registers in the
slave.
Query
The query message specifies the starting register and
quantity of registers to be read. Register addresses start at
0, that is, registers 1–4 are addressed as 0–3.
Example: Read
parameter 3-03 Maximum Reference
, register
03030.
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