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OBD Reader - what protocol?


GreigM

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I want to buy an OBD reader for various reasons (including reset of CEL if I ever need it) and the company are asking me what kind of connector it uses - they pointed me to this page:

http://www.ukobd.co.uk/forum/forum_posts.asp?TID=15&PN=1

Which indicates for an ISO connector, pins 1 and 15 should be in use, but the connector on the car doesn't seem to match as it looks like that attachment (pin 15 empty)....any ideas?

post-1608-1140362365_thumb.jpg

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Don't worry that Porsche don't have a pin allocated to slot 15.

A great deal of car manufacturers are using the 'standard' 16 pin J1962 connector for diagnostic connections. It was all born from trying to standardise fault diagnosis mandated by OBDII. It was thought that if a vehicle complied, it would be able to be connected to any generic scan tool for fault reading. However, the J1962 connector became a bit like the SCART socket on the back of our TVs and DVD players, where you have the option of using RGB, composite video or S-Video - all completely different signals on different pins.

Getting back on topic, VW, Audi, Seat, Skoda and Porsche use the ISO9141 hardware layer for their comms. Ford use PWM etc. So for ISO9141, you use pin 16 for Battery +, pin 4 or 5 for Ground and pin 7 for the communication or 'K-Line'. On the very earliest (1994-1996) cars, the control module would have been 'woken up' on the 'L-Line', with comms continuing on the K-Line once established. The L-Line was in pin 15, hence the reference from the site you mentioned.

ISO9141 is the physical hardware of the interface, it differs completely from PWM or VPW - think of it like RS232 and RS422 or RS425. So although VAG and Porsche use the same ISO9141 hardware, the protocol that is used with it is different, like thousands of computer applications use an RS232 interface, they each will have a different protocol.

So, the manufacturers that utilise the ISO9141 hardware layer, support a generic OBD protocol that allows communication with the engine electronics only, they have their own protocols for the other modules in the car. For the VAG group, they use Key Word Protocol 1281 and KWP2000. I'm not yet sure what the KWP is that Porsche use.

With more control modules wanting to interact in the vehicle, manufacturers are increasing the use of Controller Area Networks. The CAN bus offers a much more robust communications network. I believe there is a provision to connect to the CAN bus at the J1962 diagnostic connector, but again you will require a device with the propriety hardware and software to interrogate this connection.

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