Danfoss VLT 2800 Design Manual

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Danfoss VLT 2800 Design Manual Manual pdf 108 page image

4.7 Serial communication4.7.1 ProtocolsAll frequency converters are equipped with an RS-485 portas standard, which makes it possible to select between 2protocols. The 2 protocols that can be selected inparameter 512 Telegram Profile, are:• Profidrive protocol• Danfoss FC protocolTo select Danfoss FC protocol, set parameter 512 TelegramProfile to [1] FC protocol.4.7.2 Telegram TrafficControl and response telegramsTelegram traffic in a master-follower system is controlledby the master. A maximum of 31 followers can beconnected to a master, unless repeaters are used. Ifrepeaters are used, a maximum of 126 followers can beconnected to a master.Illustration 4.22 Master-follower SystemThe master constantly sends telegrams addressed to theslaves and waits for response telegrams from them. Theslave'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.BroadcastA master can send the same telegram simultaneously to allslaves connected to the bus. During this broadcastcommunication the slave does not send any responsetelegrams back to the master as to whether the telegramhas been correctly received. Broadcast communication isset up in address format (ADR), see chapter 4.7.3 TelegramStructure for more details.Content of a character (byte)Each character transferred begins with a start bit. Then 8data bits are transferred, corresponding to a byte. Eachcharacter is secured via a parity bit, which is set at "1"when it reaches parity (i.e. when there is an equal numberof 1's in the 8 data bits and the parity bit in total). Acharacter is completed by a stop bit, thus consisting of 11bits in all.0 1 32 4 5 6 7195NA036.10StartbitEven StopParity bitIllustration 4.23 Character Structure4.7.3 Telegram StructureEach 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 thefrequency converter (ADR). Then follows a number of databytes (variable, depending on the type of telegram). Thetelegram is completed by a data control byte (BCC).STX LGE ADR DATA BCC195NA099.10Illustration 4.24 Telegram StructureTelegram timingThe communication speed between a master and a slavedepends on the baud rate. The frequency converter's baudrate must be the same as the master's baud rate and beselected in parameter 501 Baudrate.After a response telegram from the slave, there must be apause of at least 2 characters (22 bits) before the mastercan send a new telegram. At a baud rate of 9600 baud,there must be a pause of at least 2.3 ms. When the masterhas completed the telegram, the slave's response timeback to the master is a maximum of 20 ms, and there is apause of at least 2 characters.Illustration 4.25 Telegram Time• Pause time, minimum 2 characters• Response time, minimum 2 characters• Response time, maximum 20 msThe time between the individual characters in a telegrammay not exceed 2 characters and the telegram must becompleted within 1.5 x nominal telegram time. At a baudrate of 9600 baud and a telegram length of 16 bytes, thetelegram is completed after 27.5 ms.Programming Design Guide106 Danfoss A/S © Rev. May/2014 All rights reserved. MG27E40244