[mythtv-users] I'm about to give up on this entire thing...

[Ray Olszewski]

At 10:03 PM 6/10/2003 +1000, Christian Hack wrote:
[...]
>The null part simply means you need to connect transmit on one box to
>receive on the other box - and then recieve on the first box to transmit on
>the second box. You probably already are, but I was just checking to make
>sure.

While this is an accurate description of "null", the underlying advice to 
try it in this application (connecting a DirecTV device to a PC) may not be 
good.

RS232 conections come in two "flavors": DTE (terminals; PC ports are wired 
this way) and DCE (communications devices; modems are wired this way). 
These wiring rules mean something when the devices have DB25 or DE9 
connectors ... a DTE device transmits on the TX line and receives on the RX 
line, while a DCE device transmits on the RX line and receives on the TX 
line. The various handshaking ports are correspondingly reversed between 
the two ends.

A "null" connection (the usual term is "null modem cable") needs to be made 
when both devices are DTE or both are DCE. Swapping the RX-TX pairs is the 
big change, but if the devices use any of the handshaking lines, they have 
to be either swapped or looped back as well, a requirement that (because of 
the "or") causes null-modem cables to come in at least 3 distinct varieties 
I can think of. It is a headache to get right ... but, fortunately, a 
headache that does not apply to this situation.

Since the DirecTV device uses an RJ22 connector and does not use 
handshaking lines, there is no *standard* definition for its pinouts, 
making the concept of a "straight" or a "null" connection meaningless in 
this context.