ISO 9141-2. This protocol has an asynchronous serial data rate of 10.4 kbit/s. It is somewhat similar to RS-232; however, the signal levels are different, and communications happens on a single, bidirectional line without additional handshake signals. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
- pin 7: K-line
- pin 15: L-line (optional)
- UART signaling
- K-line idles high, with a 510 ohm resistor to Vbatt
- The active/dominant state is driven low with an open-collector driver.
- Message length is Max 260Bytes. Data field MAX 255.
R1,2,6,7,9,10 - 560 ohm, 0.25W R3,4,5,8 - 4K7 OHM, 0.125w T1 - BC556 or 2N3906 T2,T3 - BC546 or 2N3904
One of our users reports that from collector of t1, r4 has to go to GND.
See also PC RS232 interface details.
Tx is -12V when inactive. therefore the diagram correctly shows that r4 goes to tx because it uses the -12V to pull down Rx when RS232 is in Rx mode.