[b3lha]

Originally Posted by cdvs

Phil, I've been holding back on this whole connector issue. You have made a tremendous effort which I respect greatly. So, I didn't want to say that a good chunk of this work probably wasn't necessary. I especially didn't want to say this without having proved it myself. I haven't done this. I've been preoccupied with other things and while I find this very interesting I just can't give it very much of my time. However, it now appears that perhaps if I share what I think I know, it will be helpful in achieving your goals. Specifically by making it easier for others to join in the fun.

I do have historical experience with RS232 DTE (data terminal equipment) and my suggestions are based on this.

1. Unless there is a requirement for one device to be powered by the other, no positive voltage connection is required or recommended. This is not the case here, so deleting the positive connection eliminates one perceived incompatibility.

2. The data line standard for modern computer equipment is +3 to +12 volts to signal 'space' or zero and 0 volts to signal 'mark' or one. Both the ECU and your laptop are 'modern' computers and should operate fine as long as the signal voltages are in this range. It doesn't(shouldn't) matter if they are different. There is every reason to expect that the Subaru Select Monitor is no different in this regard.

3. We are left with the infamous 3 wire connector. Receive on one end connected to transmit on the other (times 2) and ground. No special conversion electronics required.

Remember, I haven't tested this and free advice is worth what you paid for it.

Charl

Hi Charl,

Thanks for your input. Every bit of advice is appreciated.

What you said about RS232 is correct, but the ECU does not use RS232. It has a UART, but the signalling is done at TTL levels. I think the logic may also be reversed, ie. 0 is 0v (ground), 1 is +5v (floating). Admittedly, the RS232 port on the PC will understand the ECU signal, and the bits can be flipped in software if necessary. However, as I understand it, the signal from the PC to the ECU needs to be dropped down from RS232 to TTL levels. That is what the MAX232 (clone) in the interface does. The MAX232 requires a 5v supply and that is why there is a voltage regulator connected to the power line.

The guys on the Legacy forum are using a simpler interface made from a couple of resistors, a diode and a transistor. See http://www.graphics.cornell.edu/~v/b10scan/ under "Serial Port". But I figured that a proper RS232-TTL converter IC was probably a better option. So I looked at the datasheets, took the example circuit for the IC and the example circuit for the voltage regulator and joined them together to create my circuit. Although I was reassured by its similarity to the circuit on the VWRX site, I was still quite surprised when it actually worked.

I don't know whether your idea of just connecting the wires will work, I suspect it probably won't for the reasons outlined above. My circuit has been tested and proven to work with an SVX, but you don't have to use it if you don't want to. By all means design one of your own and join in the fun. Or try the one from the Legacy site or the VWRX site, or the original one from Hitoshi Kashima's site. He appears to be an electronics guru, so I'd trust his circuit over my own or any other. http://kaele.com/~kashima/car/index-e.html (Look under Subaru ECU)

Seriously, if you think you have a better way, please try it and report back the results so we can all benefit from your experience.

Phil.