First you need to know the correct Baud rate, Parity and stop bits then we need to know if its expecting an ASCII GCode line or something else?

The humming from your steppers is normal for stationary steppers currently holding. I would stick with the Paralell connection and use Mach/EMC.

If you really want to use your PDA look at the RepRap forums as they use a serial(via usb) controller to supply step/dir signals to the paralell connection on your board. You just send it a line of GCode via serial in ascii and it does it. I wrote a program for Windows Ce /Pocket PC to controll a Router/Mill this way.

Are you sure its RS-232 on the board you have and not 422 or some other bus?

Its possible modbus for which Mach have a plugin.

Michael.