(DG) P70S connection to Sea Talk and SPX30 computer

[sailcrazy]

My P70 control head suddenly would not turn on, after several weeks of good performance. It was connected to my Sea Talk network and SPX30 computer by stripping back the P70 pigtail, and connecting the red(+12vDC), bare(-12vDC), and yell(signal) wires directly to my SPX30 computer, per RayMarine Tech Advice and direction. I removed and sent the P70 in for warranty repair, and was eventually told it was OK, nothing was wrong with it, and was being returned. I had since bought a P70S control head. But when I connect either of them, the display remains blank and nothing works. There is 12v power avail at the P70 cannon plug, and continuity for the 3 wires is checked good to the computer. The Power/Standby key has no effect. My old (and partially faded screen) ST7001, however, works fine-except half of the screen is sunburned and unreadable. How do I get either the P70 or P70S to work in my system? How do I turn it On?