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8.8.9 How to Control the Frequency

Converter

This section describes codes which can be used in the

function and data fields of a Modbus RTU message.

8.8.10 Function Codes Supported by

Modbus RTU

Modbus RTU supports use of the following function codes

in the function field of a message.

Function Function code

Read coils 1 Hex

Read holding registers 3 Hex

Write single coil 5 Hex

Write single register 6 Hex

Write multiple coils F Hex

Write multiple registers 10 Hex

Get comm. event counter B Hex

Report follower ID 11 Hex

Table 8.18 Function Codes

Function Function

Code

Sub-

function

code

Sub-function

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 bus exception error

count

14 Return follower message

count

Table 8.19 Function Codes

8.8.11

Modbus Exception Codes

For a full explanation of the structure of an exception code

response, refer to chapter 8.8.5 Function Field.

Code Name Meaning

1 Illegal

function

The function code received in the query is

not an allowable action for the server (or

follower). 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 follower) 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 follower). 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

value

A value contained in the query data field

is not an allowable value for server (or

follower). 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 Follower

device failure

An unrecoverable error occurred while the

server (or follower) was attempting to

perform the requested action.

Table 8.20 Modbus Exception Codes

8.9

How to Access Parameters

8.9.1 Parameter Handling

The PNU (Parameter Number) is translated from the

message. The parameter number is translated to Modbus

as (10 x parameter number) DECIMAL. Example: Reading

3-12 Catch up/slow Down Value (16bit): The holding register

3120 holds the parameters value. A value of 1352

(Decimal), means that the parameter is set to 12.52%

Reading 3-14 Preset Relative Reference (32bit): The holding

registers 3410 & 3411 holds the parameters value. A value

of 11300 (Decimal), means that the parameter is set to

1113.00 S.

For information on the parameters, size and converting

index, consult the product relevant programming guide.

Installation and Set-up

Design Guide