466 Practical Modern SCADA Protocols: DNP3, 60870.5 andRelatedSystems Variable length messages Variable length messages LPCI APDU LPCI S L L S C A A CS E 1. ASDU Hex example 6 1 1 6 7 0 0 67 01 06 00 00 00 00 55 69 10 0D 13 D2 8 1 1 8 3 1 0 01 01 LPDU S Start character – always 0x68 in variable length messages L Length character which specifies the length in octets (bytes) of the ASDU+C+A+A L Repeat of previous S Repeat of previous C Link control character A 16 bit Outstation address, LSB first – this example is 0x0001 CS Checksum, a single byte accumulation of ASDU+C+A+A Control Field: Interpret using details in Chapter 18.104.22.168 (Page 218 onwards) ASDU Structure: 1 6 ASDU Data Unit Identifier Information Object 1 Type ID Variable Structure Qualifier Cause of Transmission Common Address of ASDU Information Object Address Information Elements Time Tag Information Object For each Information Object Type (Type ID) the sequence of bytes, Variable Structure Qualifier, Causes of Transmission, Information Object Addresses and the Information Elements are described as shown in the following examples:
22.214.171.124.1 Appendix D: Practicals 467 126.96.36.199.2 Examples of Various Type Codes 188.8.131.52.3 Type 1 INFORMATION OBJECT TYPE: 1 CODE: M_SP_NA_1 DESCRIPTION: Single point information without time tag VALID WITH SQ: 0, 1 Information Object for SQ = 0 (Sequence of Information Objects) Information Object Address IV NT SB BL SP SIQ I Information Object for SQ = 1 (Sequence of Information Elements) Information Object Address 1 IV NT SB BL SP SIQ, Information Element 1 I ….. Information Object Address N IV NT SB BL SP SIQ, Information Element N I Valid Cause of Transmission Codes Background scan Spontaneous Requested Return of information caused by remote command Return of information caused by local command Interrogated by station interrogation Interrogated by group G interrogation, G= Notes SIQ – Single-point information with quality descriptor.
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