Rs485: Modbus Rtu Message Function Codes; Rs485: Modbus Rtu Parameters - Danfoss VLT HVAC Drive FC 102 Design Manual

Appendix
14.9 RS485: Modbus RTU Message Function
Codes
14.9.1 Function Codes Supported by
Modbus RTU
Modbus RTU supports use of the function codes in
Table 14.17 in the function field of a message.
Function
Read coils
Read holding registers
Write single coil
Write single register
Write multiple coils
Write multiple registers
Get comm. event counter
Report slave ID
Table 14.17 Function Codes
Function Function
code
Diagnostics 8
Table 14.18 Function Codes
14.9.2 Modbus Exception Codes
14 14
For a full explanation of the structure of an exception code
response, refer to chapter 14.8 RS485: Modbus RTU Telegram
Structure.
Code Name
1 Illegal
function
236
®
VLT HVAC Drive FC 102
Function code
1 hex
3 hex
5 hex
6 hex
F hex
10 hex
B hex
11 hex
Sub- Sub-function
function
code
1 Restart communication.
2 Return diagnostic register.
10 Clear counters and
diagnostic register.
11 Return bus message count.
12 Return bus communication
error count.
13 Return bus exception error
count.
14 Return slave message
count.
Meaning
The function code received in the query is
not an allowable action for the server (or
slave). This code 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, for
example because it is not configured and
is being asked to return register values.
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Code Name Meaning
2 Illegal data The data address received in the query is
address 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 A value contained in the query data field
value is not an allowable value for server (or
slave). This code 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 An unrecoverable error occurred while the
failure server (or slave) was attempting to
perform the requested action.
Table 14.19 Modbus Exception Codes

14.10 RS485: Modbus RTU Parameters

14.10.1 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 (10xparameter number) DECIMAL.
14.10.2 Storage of Data
The coil 65 decimal determines whether data written to
the drive is stored in EEPROM and RAM (coil 65=1) or only
in RAM (coil 65=0).
14.10.3 IND
The array index is set in holding register 9 and used when
accessing array parameters.
14.10.4 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.
MG16C302