Lumel KD8 Service Manual
Also see for KD8: User manualUser manual
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3port communication parameters (baud rate, information unit) during the configuration of anydevice.In the MODBUS system, transmitted messages are placed into frames that are not related to serialtransmission. These frames have a defined beginning and end. This enables for the receiving deviceto reject incomplete frames and to signal related errors with them.Taking into consideration the possibility to operate in one of these two different transmission modes(ASCII or RTU), two frames have been defined.Explanation of some abbreviations:ASCII = American Standard Code for Information InterchangeRTU = Remote Terminal UnitLRC = Longitudinal Redundancy CheckCRC = Cyclic Redundancy Check CR = Carriage Return LF = Line-Feed (character)MSB = Most Significant BitChecksum = Control Sum2.1. ASCII framingIn the ASCII mode, each byte of information is transmitted as two ASCII characters. The basicfeature of this mode is that it allows to long intervals between characters within the message(to 1 sec) without causing errors.A typical message frame is shown below.Start beginningindexAddress Function Data LRCcheckEndindex1 char“:”2 chars 2 chars n chars 2 chars 2 charsCR LFIn ASCII mode, messages start with a colon character (“:” -ASCII 3Ah) and end with CR and LFcharacters. The frame information part is protected by the LRC code (Longitudinal RedundancyCheck).2.2. RTU FramingIn RTU mode, messages start and end with an interval lasting minimum 3.5 x (lasting time ofa single character), in which a silence reigns on the link.The simplest implementation of the mentioned time interval character times is a multiple measureof the character duration time at the set baud rate accepted on the link.The frame format is shown below:Start beginningindexAddress Function Data CRC check End indexT1-T2-T3-T4 8 bits 8 bits n x 8bits 16 bits T1-T2-T3-T4 |
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