Serial communication for VLT 2800
Protocols
All frequency converters are equipped with an RS 485
port as standard, which makes it possible to choose
between two protocols. The two protocols that can be
selected in parameter 512 Telegram Profile , are:
• Profidrive protocol
• Danfoss FC protocol
To select Danfoss FC protocol, parameter 512 Tele-
gram Profile is set to FC protocol [1].
Telegram Traffic
Control and response telegrams
Telegram traffic in a master-slave SYSTEM is control-
led by the master. A maximum of 31 slaves can be
connected to a master, unless repeaters are used. If
repeaters are used, a maximum of 126 slaves can be
connected to a master.
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 broad-
cast communication the slave does not send any re-
sponse telegrams back to the master as to whether the
telegram has been correctly received. Broadcast com-
munication is set up in address format (ADR), see
Telegram structure.
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 con-
sisting of 11 bits in all.
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 ad-
dress 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).
Telegram timing
The communication speed between a master and a
slave depends on the baud rate. The frequency con-
verter'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 will be a
maximum of 20 ms, and there will be pause of at least
2 characters.
Pause time, min: 2 characters
Response time, min: 2 characters
Response time, max: 20 ms
The time between the individual characters in a tele-
gram 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 will be completed after 27.5
msec.
MG.27.E2.02 - VLT is a registered Danfoss trademark 111
Programming
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