Class 2 (J1850) Transceiver Bit Timing: The Impact of Propagation Delay & Output Voltage Slew Rate 2000-01-0147
Transceiver propagation (also known as wrap-around, loop-back, or round-trip) delay time and output voltage slew rates have a profound impact upon bit timing. Excessive propagation delay produces an error in the generated symbol time that may cause a bit timing error in a receiving node. Any difference between the compensation time of the protocol handler and the transceiver propagation delay will be seen in the transmitted pulse time. The larger the gap, the longer the transmitted bit time. The bit distortion increases even further if the receiving node is operating in a ground offset voltage with respect to the transmitter. Bit distortion is exacerbated as well when the output voltage slew rates are too slow. When the slew rates are too slow, the receiving bit time may be too short in a positive ground offset voltage condition and too long in a negative ground offset voltage condition. Either of these issues may also contribute to the inability of the protocol handler to successfully arbitrate the bus. The transmitting node may diagnose a short-to-ground condition and cause glitches on the bus.
Transceiver propagation delay must be designed to be compatible with the compensation capabilities of the intended protocol handler(s). Output voltage slew rates must also be understood so that the total contribution of these two errors does not result in the inability to transmit successfully into maximum ground offset conditions.