Finally....I've got GPS sync working on the bench using receivers purchased from eBay...
The receiver has RS232, 1PPS and 10MHz outputs. On the encoder side, RS232 is connected to the serial port on the ST computer. On the decoder computers, RS232 is connected to the serial port and the 1PPS output is connected to the left audio input of the ASI5810 sound card. 10Mhz is not used. Everything locks up and looks right. Been running several hours and seems like it's going to hold up.
Only problem I'm having is that the ASI5810 ASIO driver does not show up as a device on uMPX. Had to use ASIO4ALL (not sure how this will affect timing between transmitters).
Will update again when we get everything installed at our towers, and get the additional GPS licenses purchased for the encoder and decoders.
Thanks Hans and everyone for the info!!!