Need help with Advantech PCL-740 serial card

I have an ancient (AMD K6, 300 MHz, Win XP) desktop PC that I am trying to use to download RL02 diagnostics to my PDP-8/A via the TTY console port (20 ma current loop). I bought an Advantech PCL-740 card (ISA slot) which has one port that can be configured for RS-232, 422, 485 or current loop. But I cannot get it to transmit any data no matter what, and I'm pulling my hair out! I downloaded the drivers on Advantech's website and they do show I can change the status of the RTS, CTS, etc. control lines. On transmit test, the status box keeps displaying "Tx Pending" once a second, but nothing comes out on the line. I scoped directly on the 16550 UART chip and I can see the various interrupt and chip select lines moving, and the baud rate clock, but the transmit data pin never changes state.The seller shipped me another card, and it behaves identically. The software requires the user to install the COM3: port under Win XP, then separately configure the board to that port with the Advantech program. At the command prompt or in Device Manager, MODE COM3: shows that the port is getting set up with the appropriate parameters. While in the DOS box, attempting to copy a file directly to COM3: results in a "The handle is invalid. No files were copied" message. Don't know the significance of that? Is there some BIOS-level problem wiith the motherboard I should be looking for? I have used this PC with vtserver to send data via the onboard COM1: port and it does work. There are no IRQ or port address conflicts. thanks Charles