GE EPM 9650/9800 manual Function Code 42H Encapsulated DNP for LAN/WAN

Here is an example of a response to the query given earlier:

Function Code 23H Example (Response)

Field Name	Example (Hex)
Slave Address	11
Function Code	23
Byte Count Hi	00
Byte Count Lo	0C
Data Hi (Register 40108, First Read)	02
Data Lo (Register 40108, First Read)	2B
Data Hi (Register 40109, First Read)	00
Data Lo (Register 40109, First Read)	00
Data Hi (Register 40110, First Read)	00
Data Lo (Register 40110, First Read)	64
Data Hi (Register 40108, Second Read)	02
Data Lo (Register 40108, Second Read)	2B
Data Hi (Register 40109, Second Read)	00
Data Lo (Register 40109, Second Read)	00
Data Lo (Register 40110, Second Read)	00
Data Lo (Register 40110, Second Read)	64
Error Check (LRC or CRC)	--

The contents of Register 40108 are shown as the two-byte values of 02 2B Hex or 555 Decimal. The contents of Registers 40109 - 40110 are 00 00 and 00 64 Hex or 0 and 100 Decimal.

1.8.2: Function Code 42H - Encapsulated DNP for LAN/WAN

The existing interface between the Network Card and the Main Unit is in Modbus RTU protocol. In order to maintain all of the existing functionality, it is necessary to pass DNP over IP requests and the resultant responses inside a Modbus wrapper.

DNP has commands of variable length, producing responses of variable lengths. A variable size Modbus request that returns a variable size Modbus response requires a new, non-standard function code 42H.

The creation of different DNP associations over IP requires informing the Main EPM device when a new association is made. Function code 42H requests include a bit to indicate TCP (0) or UDP (1), the Client IP Address, the UDP or TCP Client Port, and a bit to indicate if this is a New Association (1) or a continuation of the Existing Association (0).

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