I have been unable to connect my laptop to my car. My laptop has win98 with tunerpro, winaldl and ecm852. The ecms I have tried to connect to are 165 and 747. I have connected a tech one to both ecms and was able to read data. The cable I created is the two transistors lotus design. The rs232 port pins are 2 receive, 3 transmit, 5 ground, 1,4,and 6 tied together, and 7,8 tied together. I have looked at one and two transistor diagrams. Each one has varous pins counnected to the serial port. I known 2,3,and 5 must be used. I have tried the cable built as the designed and I have tried it with 1,4,6,7, and 8 disconnected. I have check the pin layout on the 9 pin connector. I have varified circuit diagram and measured voltages on the circuit board I built. My truck will idle up when the cable is connected. Tunerpro does not see the cable. Winaldl sees the port on the laptop, but has no data. Ecm852 sents ping but does not receive a echo. For the 165, I have the 10k resistor on a,b and data line on e. The 747, I just use a and e. I use the car for power for the cable and a extention cord for the laptop. I added a 7805 to regulate power for the transistors 2 and added a power led to make sure the cig lighter plug is working. 7805 is working. it has 4.95v transistor 2's collecter. Any Ideas?

 

I found this loop back test for testing the laptop serial port.

http://zone.ni.com/devzone/cda/tut/p/id/3450

I made a serial loop back from a extra connector I have. The laptop passed. I found that the low state must be below 3v. I added the 7805 to keep from putting 12v into the serial port. I think with the 10k resistor the voltage drop is too low and it is unable to transmit a high state. My 10k resistor is 9.62k I will try to remove the 7805 and put 12v into the circuit next and then add some resistance to the 10k across all pins a,b. The 9.62k might be out of tolerates.