Examples - Danfoss vlt aqua Design Manual

7 RS-485 Installation and Set-up

7.10 Examples

The following examples illustrate various Modbus RTU commands. If an error occurs, please refer to the Exception Codes section.
7.10.1 Read Coil Status (01 HEX)
Description
This function reads the ON/OFF status of discrete outputs (coils) in the adjustable frequency drive. Broadcast is never supported for reads.
Query
The query message specifies the starting coil and quantity of coils to be read. Coil addresses start at zero, i.e., coil 33 is addressed as 32.
Example of a request to read coils 33-48 (Status Word) from slave device 01:
Field Name
Slave Address
Function
7
Starting Address HI
Starting Address LO
No. of Points HI
No. of Points LO
Error Check (CRC)
Response
The coil status in the response message is packed as one coil per bit of the data field. Status is indicated as: 1 = ON; 0 = OFF. The LSB of the first data
byte contains the coil addressed in the query. The other coils follow toward the high order end of this byte, and from 'low order to high order' in subsequent
bytes.
If the returned coil quantity is not a multiple of eight, the remaining bits in the final data byte will be padded with zeros (toward the high order end of
the byte). The Byte Count field specifies the number of complete bytes of data.
Field Name
Slave Address
Function
Byte Count
Data (Coils 40-33)
Data (Coils 48-41)
Error Check (CRC)
NOTE!
Coils and registers are addressed explicit with an off-set of -1 in Modbus.
For example, Coil 33 is addressed as Coil 32.
7-20
Example (HEX)
01 (adjustable frequency drive address)
01 (read coils)
00
20 (32 decimals) Coil 33
00
10 (16 decimals)
-
Example (HEX)
01 (adjustable frequency drive address)
01 (read coils)
02 (2 bytes of data)
07
06 (STW=0607hex)
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VLT AQUA Drive Design Guide