Serial Communication; Protocols; Telegram Traffic; Telegram Structure - Danfoss VLT 2800 Design Manual

Programming

4.7 Serial communication

4.7.1 Protocols

All frequency converters are equipped with an RS-485 port
as standard, which makes it possible to select between 2
protocols. The 2 protocols that can be selected in
parameter 512 Telegram Profile, are:
•
4 4
Profidrive protocol
•
Danfoss FC protocol
To select Danfoss FC protocol, set parameter 512 Telegram
Profile to [1] FC protocol.

4.7.2 Telegram Traffic

Control and response telegrams
Telegram traffic in a master-follower system is controlled
by the master. A maximum of 31 followers can be
connected to a master, unless repeaters are used. If
repeaters are used, a maximum of 126 followers can be
connected to a master.
Illustration 4.22 Master-follower System
The master constantly sends telegrams addressed to the
slaves and waits for response telegrams from them. The
slave's response time is a maximum of 50 ms.
Only a slave that has received an error-free telegram,
addressed to that slave can send a response telegram.
Broadcast
A master can send the same telegram simultaneously to all
slaves connected to the bus. During this broadcast
communication the slave does not send any response
telegrams back to the master as to whether the telegram
has been correctly received. Broadcast communication is
set up in address format (ADR), see chapter 4.7.3 Telegram
Structure for more details.
Content of a character (byte)
Each character transferred begins with a start bit. Then 8
data bits are transferred, corresponding to a byte. Each
character is secured via a parity bit, which is set at "1"
when it reaches parity (i.e. when there is an equal number
of 1's in the 8 data bits and the parity bit in total). A
character is completed by a stop bit, thus consisting of 11
bits in all.
106
Design Guide
Start
bit
Illustration 4.23 Character Structure

4.7.3 Telegram Structure

Each telegram begins with a start character (STX)=02 hex,
followed by a byte that denotes the length of the telegram
(LGE) and a byte that denotes the address of the
frequency converter (ADR). Then follows a number of data
bytes (variable, depending on the type of telegram). The
telegram is completed by a data control byte (BCC).
STX
Illustration 4.24 Telegram Structure
Telegram timing
The communication speed between a master and a slave
depends on the baud rate. The frequency converter's baud
rate must be the same as the master's baud rate and be
selected in parameter 501 Baudrate.
After a response telegram from the slave, there must be a
pause of at least 2 characters (22 bits) before the master
can send a new telegram. At a baud rate of 9600 baud,
there must be a pause of at least 2.3 ms. When the master
has completed the telegram, the slave's response time
back to the master is a maximum of 20 ms, and there is a
pause of at least 2 characters.
Illustration 4.25 Telegram Time
The time between the individual characters in a telegram
may not exceed 2 characters and the telegram must be
completed within 1.5 x nominal telegram time. At a baud
rate of 9600 baud and a telegram length of 16 bytes, the
telegram is completed after 27.5 ms.
Danfoss A/S © Rev. May/2014 All rights reserved.
0 1 2 3 4
LGE ADR DATA
•
Pause time, minimum 2 characters
•
Response time, minimum 2 characters
•
Response time, maximum 20 ms
5 6 7 Even Stop
Parity bit
BCC
MG27E402