DNP 3

DNP3 (Distributed Network Protocol) is a set of communications
protocols used between components in process automation systems. Its main
use is in utilities such as electric and water companies. Specifically, it
was developed to facilitate communications between various types of data
acquisition and control equipment. It plays a crucial role in SCADA
systems, where it is used by SCADA Master Stations, Remote Terminal Units
(RTUs), and Intelligent Electronic Devices (IEDs). It is used only for
communications between a master station and RTUs or IEDs. ICCP, the
Inter-Control Centre Protocol, is used for inter-master station
communications.

This DNP3 MORSE interface support DNP3 message carry only over
RS232/422 or RS485 standart, TCP/IP transport should be carried via
IP-MORSE – IP mode.

DNP3 is based on the standards of the International Electrotechnical
Commission (IEC) Technical Committee 57, Working Group 03 who have been
working on an OSI 3 layer “Enhanced Performance Architecture” (EPA)
protocol standard for telecontrol applications.

An FT3 frame is defined as a fixed length header block followed by
optional data blocks. Each block has a 16-bit CRC appended to it. The IEC
specifies that the header fields consist of 2 start bytes, 1 byte length,
1 byte control, a destination address and an optional fixed length user
data field. In this implementation the fixed length user data field is
defined as a source address.

Fig. 1: DNP3 frame

DNP3 Header description:

SYNC [0x0564]	0x0564The Start field is 2 bytes in length. The first byte is a 05 hexadecimal and the second byte is a 64 hexadecimal too.
LENGTH	The length field is 1 byte in length and specifies the count of user bytes in the frame. The CONTROL,DESTINATION and SOURCE field sizes are included in this count. The minimum value for this field is 5 and the maximum value is 255.
CONTROL	The control field contains the direction of the frame, type of frame and flow control information. Figure 2 defines the fields of the control byte. Station A is defined as the designated master station. Station B is not a master station. The primary station is the originator of the message, the source of the message. The secondary station is the destination station. Fig. 2: DNP3 CONTROL DIR Physical transmission direction 1 = station A to station B 0 = station B to station A PRM Primary Message 1 = frame from primary (initiating station) 0 = frame from secondary (responding station) FCB Frame count bit FCV Frame count bit valid 1 = Frame count bit is valid 0 = ignore frame count bit DFC Data flow control bit RES Reserved = 0 FUNCTION CODE The function code identifies the type of frame. The definition of the values placed in this field are different between primary and secondary stations. The following tables define the implemented codes and associated FCV. PRM = 1 Function Code Frame Type Service Function FCV Bit 0 SEND – CONFIRM expected RESET of remote link 0 1 SEND – CONFIRM expected Reset of user process 0 2 SEND – CONFIRM expected TEST function for link 1 3 SEND – CONFIRM expected User Data 1 4 SEND – NO REPLY expected Unconfirmed User Data 0 9 REQUEST – RESPOND expected REQUEST LINK STATUS 0 Secondary PRM = 0 Function Code Frame Type Service Function 0 CONFIRM ACK – positive acknowledgement 1 CONFIRM NACK – Message not accepted, Link busy 11 RESPOND Status of Link (DFC = 0 or DFC = 1)

Terminology used:

Primary station — CU ... CU — Secondary station

Both radiomodems (CU) work in the same way.

Primary station transmiting:

0564 0BC4 0400 0300 E42B E5C0 0102 0006 985C

Response (Secondary station):

18:33:26.058 tx     17 | S01
0564 0A44 0300 0400 7CAE E0C0 8100 00C1 F0

DNP3 parameters:
(m)ode :RADIOSLAVE
(q)uit