Message Header Setup

Before a message can be sent to the SAE-J1939 bus, the message header must be configured to direct the data correctly. The following table shows the bytes that must be set to direct a message to the SAE-J1939 bus.

Table 19. J1939 Message Header

Message Header

B	B	B	B
1	2	3	4
81	21	1F	05

The first two bytes are for the start of message. The third byte (B3) containing hex 1F,1 directs the message to the J1939 bus. (F) hex is the number of bytes in the payload. Byte four (B4) tells the converter in which message slot to put the message (slot 5). Valid slots are 1 through 15, but slot 15 is reserved for receive only.

		Table 20. Valid Codes for Byte 3
Nibble 2	Value		Function
	1		Message directed to SAE-J1939 bus
	2		Internal commands, vender #, baud change
Nibble 1
	0 to 15		Number of bytes to follow byte 4

Table 21. Valid Codes for Byte 4

Function	Nibble 2	Nibble 1
No function (reserved)	Value = 0
Message slot number		Value = 1 to 15 (1 to F hex)

Each message must have a unique slot number. A message sent to the same slot number overwrites the previous message.

14	1939STB4600 Manual
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Message Header Setup

Table 20. Valid Codes for Byte 3

1939STB4600 Manual