Danfoss VLT AQUA Drive FC 202 Design Manual

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7.8.3 Start/Stop FieldMessages start with a silent period of at least 3.5 characterintervals, implemented as a multiple of character intervalsat the selected network baud rate (shown as Start T1-T2-T3-T4). The first transmitted field is the device address.Following the last transmitted character, a similar period ofat least 3.5 character intervals marks the end of themessage. A new message can begin after this period. Theentire message frame must be transmitted as a continuousstream. If a silent period of more than 1.5 characterintervals occurs before completion of the frame, thereceiving device flushes the incomplete message andassumes that the next byte is the address field of a newmessage. Similarly, if a new message begins before 3.5character intervals after a previous message, the receivingdevice will consider it a continuation of the previousmessage, causing a time-out (no response from the slave),since the value in the final CRC field is not valid for thecombined messages.7.8.4 Address FieldThe address field of a message frame contains 8 bits. Validslave device addresses are in the range of 0–247 decimal.The individual slave devices are assigned addresses in therange of 1–247. (0 is reserved for broadcast mode, whichall slaves recognise.) A master addresses a slave by placingthe slave address in the address field of the message.When the slave sends its response, it places its ownaddress in this address field to let the master know whichslave is responding.7.8.5 Function FieldThe function field of a message frame contains 8 bits. Validcodes are in the range of 1-FF. Function fields are used tosend messages between master and slave. When amessage is sent from a master to a slave device, thefunction code field tells the slave what action to perform.When the slave responds to the master, it uses thefunction code field to indicate either a normal (error-free)response, or that an error has occurred (called anexception response). For a normal response, the slavesimply echoes the original function code. For an exceptionresponse, the slave returns a code that is equivalent to theoriginal function code with its most significant bit set tologic 1. In addition, the slave places a unique code into thedata field of the response message. This code tells themaster what error occurred, or the reason for theexception. See 7.8.9 Function Codes Supported by ModbusRTU.7.8.6 Data FieldThe data field is constructed using sets of two hexadecimaldigits, in the range of 00 to FF hexadecimal. Thesesequences are made up of one RTU character. The datafield of messages sent from a master to slave devicecontains more information, which the slave must use to dowhat is defined by the function code. This information caninclude items such as coil or register addresses, thequantity of items, and the count of actual data bytes inthe field.7.8.7 CRC Check FieldMessages include an error-checking field, operating basedon a Cyclical Redundancy Check (CRC) method. The CRCfield checks the contents of the entire message. It isapplied regardless of any parity check method used for theindividual characters of the message. The transmittingdevice calculates the CRC value then appends the CRC asthe last field in the message. The receiving devicerecalculates a CRC during receipt of the message andcompares the calculated value to the actual value receivedin the CRC field. If the two values are unequal, a bus time-out results. The error-checking field contains a 16-bitbinary value implemented as two 8-bit bytes. After error-checking, the low-order byte of the field is appended first,followed by the high-order byte. The CRC high-order byteis the last byte sent in the message.RS-485 Installation and Set... VLT® AQUA Drive FC 202 Design GuideMG20Z102 - VLT® is a registered Danfoss trademark 1737 7