GE EPM 9650/9800 manual Address, Link Status, Client IP, Client Port, Byte Count, Data, Checksum

While the DNP over TCP Connections may be closed at the decision of the Network Card, as would be conveyed by the New Association bit with the next received connection, the Main device might also determine that the current DNP over TCP Connection should be closed. The request to close the current DNP over TCP Connection will have to be held in Modbus registers in the Main device, polled by the Network Card, and cleared by the Network Card overwriting the registers. If Modbus register 0x0E040 contains the value 0x0AAAA, then the current DNP over TCP Connection (the IP address will be in Modbus registers 0x0E041-0x0E042, the client port number in 0x0E043) must be closed by the Network Card and a value of 0x05555 should be written back to register 0x0E040.

DNP requests and responses may have more than 255 bytes. Function code 42H requests and responses include a 2-byte byte count. The Modbus Function Code 42H, DNP over Modbus is only supported on the internal connection made with a Network Card. It is not available on external communication ports.

This function code is used to transport DNP protocol requests and responses between a Modbus master with a DNP over IP Connection to a Modbus slave with built in DNP functionality. The request PDU (Protocol Data Unit) specifies: New Association or Previous Association, UDP or TCP, IP Address, Client Port Number and a byte count of DNP octets in the UDP datagram or TCP stream being transmitted.

The response returns the Function Code, UDP or TCP indication, IP Address, Client Port Number and a byte count of DNP octets for a possible response.

Examples of a Request and				Response PDU (*N = Byte Count):
Address	1	Byte	0x001 - 0x0F7	Address	1	Byte	0x001 - 0x0F7
Function Code	1	Byte	0x042	Function Code	1	Byte	0x042
Link Status	1	Byte	0x000 to	Link Status	1	Byte	0x000 to
			0x003				0x001
Client IP	4	Bytes	0x00000000 to	Client IP	4	Bytes	0x00000000 to
Address			0x0FFFFFFF	Address			0x0FFFFFFF
Client Port	2	Bytes	0x00000 to	Client Port	2	Bytes	0x00000 to
			0x0FFFF				0x0FFFF
Byte Count	2	Bytes	0x00001 to	Byte Count	2	Bytes	0x00000 to
			0x0FFFF				0x0FFFF
Data	N*		Value	Data	N*		Value
			1 Byte LRC				1 Byte LRC
Checksum	1 or 2 Bytes		(ASCII)	Checksum	1 to 2 Bytes		(ASCII)
			2 Byte CRC				2 Byte CRC
			(RTU)				(RTU)

Error Example:	Address	1 Byte	0x001 to 0x0F7
	Error Code	1 Byte	0x0C2
	Exception Code	1 Byte	01 or 03
	Checksum	1 or 2 Bytes	1 Byte LRC (ASCII) or 2 Byte CRC (RTU)

1-9