Practical Industrial Data Networks:Design, Installation and ...

Data Highway Plus/DH485 overview 127Symbol Type DescriptionDLE STX Control symbol Sender symbol that indicatesthe start of a message.DLE ETX BCC/CRC Control symbol Sender signal that terminatesa message.DLE ACK Control symbol Responses symbol whichsignals that a message hasbeen successfully received.DLE NAK Control symbol Response symbol whichsignals that a message wasnot received successfully.DLE ENQ Control symbol Sender symbol that requestsretransmission of a responsesymbol from the receiver.APP DATA Data symbol Single character data valuesbetween 00–OF and 11–FFincludes data from theapplication layer includinguser programs and commonapplication routines.DLE DLE Data symbol Symbol that represent thedata value 10 Hex.Table 9.2Symbols used for full-duplex modeFormat of a messageNote that response symbols transmitted within a message packet are referred to asembedded responses.From a User Application ProgramDST CMD STS Command DataApplication Layer Datagram (Layer 7)DSTSRCCMDSTS TNS Data (From Application Program)Data Link Layer Frame (Layer 2)DLESTX Data (From Application Layer) DLEETXCRCFigure 9.1Protocol structureThe CRC-16 calculation is done using the value of the application layer data bytes andthe ETX byte. The CRC-16 result consists of two bytes.