Many of the DirecTV STBes have a serial port (though they come in various flavors) and mostly support one of two major varieties of protocol, and a couple sub-varieties.

You can get some information here:

[Link: dtvcontrol.com]

There is mostly the 'old' and 'new' style commands, but a few specific models don't implement some things and some implement a few model specific commands.

Here is a list of many models and the form of the protocol that they use:

[Link: dtvcontrol.com]