Some stuff I worked on used a pic directly without buffer so it needed the logic inverted in the code. It sounds like this is the case here. You can often get away with this but the signals are 0 to 5 VDC and non compliant but often works. This is especially true on the desk or bench but when you get to volume you will usually get into trouble. There is a reason for the +/- swings used.
Larry

> 
> From: Jinx <joecolquitt@clear.net.nz>
> Date: 2005/10/06 Thu AM 08:11:03 EDT
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Subject: Re: [PIC] Serial pins to MAX232
> 
> > a MAX232 will do a logic inversion. You either need to use
> > another inverter between the PIC and the MAX232 or change
> > the pic code to invert the signals
> 
> I've never used an inverter + MAX232. All the RS232 I've
> ever worked with has been negative logic
> 
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