Hmmm ... I might try that but I'm a little worried. I tried the Tivo cable and it did not work and when I looked at the picture in the ProntoNEO manual it showed the serial cable with a 4 pole mini-jack, not a 3 pole jack like the one you're describing and what the Tivo cable had.

So to appropriate your diagram, there would need to be 4 connections like so

/ \ = A
| = B
| = C
| = D

I think the diagram in the link below (the top pin layout) is probably what is needed but I'm not sure. Speaking of whcih, can anyone who has the actual serial cable take a look and see how many poles the mini-jack end has. A 3 pole jack will have 2 black rings creating 3 independent contact areas while a 4 pole jack will have 3 black rings creating 4 independent contact areas. Thanks.