[mythtv-users] Changing channels on a new D11 directv receiver

Jeff Simpson jeffsimpson at alum.wpi.edu
Fri Dec 30 12:13:10 EST 2005

On 12/30/05, Brian Wallen <brian_wallen at hotmail.com> wrote:
> Ok, I've read the guide you wrote up on mythtv.info.  My problem is, I
> can't
> envision how this works.  First off, what is the difference between a null
> modem cable and a serial cable, and when I'm looking at one, how can I
> tell
> which kind it is?

The difference is that a normal serial cable goes straight through: ie the
Rx goes to Rx, Tx goes to Tx, CTS goes to CTS, etc. They are intended to be
used with a device that is expecting that, like a modem or an old-school
mouse. Since these are straight-through, they are often used as extension
cables, since you can string together as many as you like, assuming it's not
so long that you have issues with propagation delay and signal attenuation.

A null modem cable (or adapter) swaps the RX and TX, as well as CTS/RTS.
This is roughly equivalent to the tin-can telephone, where the string coming
from the can at my mouth has to go to the tin can at your ear in order for
it to work (and vice versa, the one from your mouth has to go back to my

You can't tell by looking at it (although most null-modem cables/adapters
will be labelled as such), but you can tell by using a voltmeter set to
continuity and check the pins to see if they are straight-through or not.
The easiest thing to do is just buy a cable that is what you are looking

Secondly, where does the usb->serial adapter plug in?  The way I'm seeing it
> now is I plug a usb cable into my receiver.  Then on the other end of that
> cable is the usb->serial adapter.  Then on the serial side of the
> usb->serial adapter goes a null modem cable that will plug into the serial
> port of my backend.  Is this anywhere close to being right?

Exactly right. There are two options for this:

1) Serial to USB: The serial port of your computer connects to a null-modem
cable. That cable in turn connects to the USB->serial adapter, which plugs
into the D11
2). USB to USB: one of the USB->Serial adapters plugs into your PC, the
other plugs into the D11. The null-modem adapter goes between the two

> And lastly, how would I change channels using one backend and 2+ directv
> receivers?

you'd need two of these cables going to two different serial ports. The
script can be modified to talk to a specific serial port or take a serial
port as a parameter (I meant to write it that way initially, but never got
around to it). The simplest way is to make two copies of the script
"directv_1.pl" and "directv_2.pl" and edit each of them to hard-code the
serial port you intend to use (/devttyS0 and /devttyS1). If you use USB,
it's a little more complicated, as the serial ports are assigned when the
adapter is connected, you may not be able to easily tell which is which, so
you'll have to test it out.
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