Hmm. I'm currently using a hack job in my interface. I didn't have any MAX chips around, and could be bothered to order one, so used a 75188 Quad RS232 receiver that I had lying around to handle both directions. Works fine over the short distance that I need. Useful that most RS232 chipsets accept TTL signalling

I'm also using a 7805 regulator bolted to the nice thick metal case. Power is now drawn directly from 12V, switched through a RatShack relay by the Amp Remote line. The Amp Remote line is more than capable of providing enough juice, but I was on a noise elimination quest recently, and that got changed. In retrospect I think that the noise was because of grounding issues rather than this, but the deed is done know.