1-10
In the request or response PDU, Link Status bit 0 indicates TCP(0) or UDP (1). In the request
PDU, Link Status bit 1 indicates a continuation of the Current Association (0) or the start of a New
Association (1).
Here is an example of a pair of requests where a DNP over TCP connection was established with a
client at IPAddress 1.2.3.4, port 20000. The first TCP frame transmitted 7 bytes in the stream, con-
taining part of a DNP frame. The first request indicates the start of new association and passes the
TCP connection information and passes the 7 bytes received. The first response includes no
response meant for the network. The second TCPframe transmitted 5 additional bytes in the stream,
containing the remainder of the first DNP frame and the start of a second. The second request indi-
cates the continuation of the association, passes the TCPconnection information and the additional 5
bytes received. The second response acknowledges the second request and contains the response for
the first DNP frame, which is meant to be returned in the TCP.
Field Name (Hex) DNP Field
Name
Address 0x001
Function 0x042
Status 0x002
IPAdd Quad 1 0x001
IPAdd Quad 2 0x002
IP Add Quad 3 0x003
IPAdd Quad 4 0x004
Client Port Hi 0x04E
Client Port Lo 0x020
Byte Count Hi 0x000
Byte Count Lo 0x007
DNP octet 0x005 Start
DNP octet 0x064 Start
DNP octet 0x005 Length
DNP octet 0x0C0 Control
DNP octet 0x001 Destination Lo
DNP octet 0x000 Destination Hi
DNP octet 0x00A Source Lo
Checksum (CRC1) 0x05A
Checksum (CRC2) 0x045
Request PDU Response PDU
Field Name
Address
Function
Status
IPAdd Quad 1
IPAdd Quad 2
IPAdd Quad 3
IPAdd Quad 4
Client Port Hi
Client Port Lo
Byte Count Hi
Byte Count Lo
Checksum (CRC1)
Checksum (CRC2)
(Hex)
0x001
DNP Field
Name
0x042
0x000
0x001
0x002
0x003
0x004
0x04E
0x020
0x000
0x000
0x0A4
0x01B