I'm trying to hook up my serial arduino to my car's OBDII system to read information from it. It's a J1850 PWM system, which runs a differential bus, so I have two wires (bus+ and bus-) where I need to send the signal on one and the complement of that signal on the other (right?), but essentially, I need to be able to read and write on the same wire (it's not seperate tx and rx wires). I noticed for digitalwrite and digitalread, you have to set the pinmode to input or output, so would I have to switch the pinmode on the fly in the middle of the program in order to read and write on the same line? seems like things could get messy...keep in mind I'm a noob when it comes to the arduino, so if I'm completely off base, please correct me. Thanks!
I really don't know if these previous discussion can help you:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1208458349
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1188495479
You can send a pm to the people on those forum.
yeah, unfrotunately they're using the elm chips, I'm trying to drive it directly without additional hardware, might be a futile attempt, but I think I have this point figured out. I do have another question however. I am a noob when it comes to serial as well, so this is probably a dumb question. How can I drive -5v? From the diagrams I've seen, a high is +5v and a low is -5v. Can the arduino do this without additional hardware? thanks!
You can read and write on the same pin, just not at the same time. You can change pin mode on the fly using pinmode, but it's a little slow, read the atmega 168 datasheet for register names, setting these is a lot faster. You can get -5V regs and or build a +-5V supply from a 10V supply, either way you need at least a 10V supply initially.
If I understand the bus correctly, the - signal is the inverse of the + signal, this can be replicated using a dual opamp and 1 output pin. Split the output into an opamp configured as a non-inverting voltage follower and another configured as an inverting voltage follower. When the output goes high one opamp will output +5V and the other -5V (using voltage followers also allows you to source much more current).
For input run the -ve line through an inverting voltage follower, and then run both into an AND gate, when the +ve line is high AND the -ve line is low the input pin will be high. A dummy load in parallel with the input pin may be necessary.
Remember to use diodes to ensure the current doesn't run backwards through the opamps!
This is how I'd do it, can anyone think of a simpler method?
You can get a negative supply by using a driver chip that produces it from a voltage inverter. This will also produce the signal processing necessary to convert from a single ended to differential input / output.
There are many such chips around here is one to look at: http://www.farnell.com/datasheets/62857.pdf. That one doesn't generate the negative rail but relies on pulling the voltages on the line already. Just hunt round for driver chips. There is no way of doing it without extra hardware.