ISO 14230 KWP2000 (Keyword Protocol 2000)
pin 7: K-line
pin 15: L-line (optional)
Physical layer identical to ISO 9141-2
Data rate 1.2 to 10.4 kbit/s
High signal voltage level: +12V (min/max 9.60 to 13.5)
Message may contain up to 255 bytes in the data field
RS-232 to ISO-9141 cable scheme
DB9M connector X1 goes to car OBDII port and supplies +12V to this circuit. Use J1962M to DB9F, Type D Cable (P/N 143301), or build an equivalent. DB25F connector X2 is RS-232 serial port to connect to PC or laptop. A DB9F could be used here instead, with appropriate pin number changes.
See also PC RS232 interface details.