B&B Electronics 1939STB manual Control Byte, Bits of Byte, Byte 3 Nibble, Transmit Data Format

Control Byte 3

Byte 3 directs the data packet to the proper destination. The setting of this byte will affect the rest of the message. That is to say changing this byte changes the meaning of the following data bytes.

Table 3. Bits of Byte 3

	Nibble 2				Nibble 1
Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0

Byte 3 has two parts, Nibble 1 and Nibble 2. Nibble 2 (bits 4 – 7) sets the control code as follows:

Table 4. Byte 3 Nibble 2

Bit Number	Value	Function
Bit 7	1	Reserved	not used
	0	Reserved not used
Bit 6	1	Reserved	not used
	0	Reserved	not used
Bit 5	1	Internal commands (Baud rate, Version number)
	0	No function
Bit 4	1	External commands (Messages to J1939 bus)
	0	No function

Nibble 1 (bits 0 – 3) of control byte 3 defines the number of bytes in the data payload of the message. The byte count does NOT include the message header (bytes 1 through 4). The message payload is limited to 16 bytes. This limit allows a complete J1939 packet to be sent or received from the bus.

Table 5. Transmit Data Format

Byte 1	Byte 2	Byte 3	Byte 4		Message Information	Data Payload
Check	Check	Control	Control
1	2	1	2
				<< Count Number of Bytes for byte 3 nibble 1 >>

Nibble 1 of control byte 3 can be set to any value between 0 and 16 (0 to F hex).

			Table 6. Byte 3 Nibble 1
	Bit Number	Value		Function
	Bit 3	1		Number of bytes in data payload (MSB)
		0
	Bit 2	1		Number of bytes in data payload
		0
	Bit 1	1		Number of bytes in data payload
		0
	Bit 0	1		Number of bytes in data payload (LSB)
		0
6				1939STB4600 Manual

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