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Basically, what you're dealing with is a serial communications port. You need to identify which of the pins in the OBD2 access port do what. There should probably be 3-7 active pins minimum and potentially more. I had an SCT programmer for a while (best programmer on the market hands down). It used a cable that was a DB9 on the programmer end and a standard OBD2 adapter on the other side. That shows that we only need 9 pins max to access the computer.
After you cover the physical layer, you need to attend to the protocols and packaging and voltages. The ECU and OBD2 systems are on 12V FWIR and serial is 5V. You'll have to do some work stepping up and down the voltages.
After you've gotten base communication out of the way, you'll have to worry about power consumption and error handling on a half duplex connection.
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Linear Mode
