Farallon Electronics in California has a plug and play cable to interface the PTC-IIe with the 746. It is part #9090 and costs $42.00. The cable will also supply power from the radio to the modem. There number is 415-331-1924. The pin out wiring diagram is available on the website www.sailmail.com
Go to the Sailmail primer and look at application notes for radios. The wiring is the same as for the M700PRO or M710 Marine radios.