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 |
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| Nibble 1 |
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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 |
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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 |
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| Data Payload |
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Check | Check | Control | Control |
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1 | 2 | 1 | 2 |
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| << Count Number of Bytes for byte 3 nibble 1 >> |
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Nibble 1 of control byte 3 can be set to any value between 0 and 16 (0 to F hex).
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| Table 6. Byte 3 Nibble 1 | ||
| Bit Number | Value |
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| Bit 3 | 1 |
| Number of bytes in data payload (MSB) | |
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| 0 |
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| Bit 2 | 1 |
| Number of bytes in data payload |
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| 0 |
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| Bit 1 | 1 |
| Number of bytes in data payload |
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| 0 |
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| Bit 0 | 1 |
| Number of bytes in data payload (LSB) |
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| 0 |
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6 |
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| 1939STB4600 Manual | |
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph
B&B Electronics Ltd – Westlink Commercial Park – Oranmore, Galway, Ireland – Ph +353 91 792444 – Fax +353 91 792445