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SAE J1850

•  Introduction to the vehicle on-board diagnostics (OBD) system

PWM uses pulse widths to indicate the symbols in the transmission.

The following timings apply to PWM at 41.6kbps:

Active Phase “1” :                      7 uSec

Active Phase “0” :                      15 uSec

Bit time:                                    24 uSec

SOF/EOD time :                        48 uSec

EOF time :                                72 uSec

IFS Time:                                  96 uSec

Active SOF :                              31 uSec

Active BRK :                              39 uSec

BRK to IFS time:                       120 uSec

OBDKey supports PWM.

The general format of a data frame in a J1850 message is:

Idle::SOF::DATA_0::::::DATA_N::CRC::EOD::NB::IFR_1::::::::IFR_N::EOF::IFS::idle

Where:

idle: Idle Bus (occurs before SOF and after IFS)
SOF: Start of Frame
DATA: Data bytes (each 8 bits long)
EOD: End of Data (only when IFR is used)
CRC: CRC Error Detection Byte (may occur in IFR as well)
NB: Normalization Bit (10.4 Kbps only)
IFR: In-Frame Response Byte(s)
EOF: End of Frame
IFS: Inter-Frame Separation

Note: Break (BRK) can be sent on a J1850 network at any time.

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