Voltage problem?

I'm still working on my Retrochallenge entry. There are only a few more days in the month, and I feel like I'm pretty close to the goal. However, it could be that a good ol' hardware issue has me stuck.

I've reverse-engineered some of the code from the "vfloppy" program, and figured out how the PC serial port is used to talk to the PX-8. It's very straightforward, as I thought, based on the specs - 8 bits, no parity, 38400 baud, no flow control, just raw I/O. No magic at all. And that's exactly the way the serial code for the Propeller works, too, by default. So, why do I not have reliable communication?

Based on the trouble I had making the vfloppy program work with one computer and not another, it occurred to me that serial port voltage/current might be an issue. I checked the Epson PX-8 tech specs, and it looks like the serial and RS232 ports on the PX-8 want +/- 8V. Standard RS232 voltage is +/- 12V, and that's what the MAX3232 chip (used on the Propeller development board for serial communications) puts out. So, I'm wondering if there's a sufficient voltage mismatch as to be causing communication issues? Seems unlikely, but I'm kind of at a loss to understand why this is being so difficult to get going.

Anyhow, I'll keep working - perhaps I can put some resistors in line, and see if that improves the situation.