IEEE 1815-2012 pdf download IEEE Standard for Electric Power Systems Communications— Distributed Network Protocol (DNP3)
The purpose of this state table is to specify an outstation’s behavior with regard to fragmment reception andtransmmission.
The outstation needs to examine the FIR, FIN, and UNS bits and the SEO value in the application controoctet of received fragments. lt shall also inspect the Application Layer function code. In addition, it needsto remember the SEO value and all of the octets in request fragrments that it accepts and in the responscfragments that it transmits.
The outstation accepts request fragments if they contain a valid request and meet the criteria in the table.otherwise it discards the fragments and remains in the same state. The outstation does not need toremember application control octet bits or values or any of the octets from fragments that it discards.
In this subclause, the term “valid request refers to a fragment received with a function code that has value in the range of 1 to 128 and that is listed in Table 6-1, and the FIR, FIN, and UNS bits in theapplication control octet have the following conditions:
FIR =1
– FIN =1
UNS =0
The outstation shall maintain several local variables for each master with which it communicates
FirstValidRequestAccepted, This Boolean variable is used to synchronize the processing of SEOvalues in valid requests. The value of this variable is cleared to false at startup, immediatelyfollowing a restart. It is set to true when the first valid request is received as shown in the table.
ECSN.26 This variable’s name stands for Expected Confirm Sequence Number. (The name is shortso that it fits in the state table.) If the outstation is expecting a confirmation to a solicited response.the value of this variable contains the sequence number that appears in the application controloctet of the solicited response awaiting confirmation: otherwise, it has a value of NE, whichstands for Nothing Expected. Only solicited confirmations that are received with a sequencenumber matching ECSN are processed; all others are discarded. This variable does not have anysignificance for unsolicited responses, although its value may be set or examined while awaiting aconfirmation to an unsolicited response
The SEQ value from its most recently accepted valid request fragment
The Application Layer octets from its most recently accepted valid request fragmmentThe FIR, FIN, and CON bits and the SEO value in its most recently transmitted solicited responsefragment.
The Application Layer octets from its most recently transmitted solicited response fragment.The FIR, FIN, and CON bits and the SEQ value from its most recently transmitted unsolicitedresponse fragment.
The Application Layer octets from its most recently transmitted unsolicited response fragment
Outstation software requires three states for proper reception.