[HH] cheapest/simplest way to control a relay from a PC

[Tom Metro]

I'm using a 4-port IR transmitter with my MythTV server to control a
pair of cable boxes. This worked fine for the first 3 or 4 months I had
it deployed, then I changed the high-level code that was calling irsend
and physically moved around the hardware a bit, and I started getting
cross-talk from port 1 to port 2. The consequence is that channel change
commands sent to port 1 are also picked up by the IR emitter on port 2
at least part of the time.

I've documented the particulars of the various experiments I've tried to
troubleshoot the problem in this ticket filed with the hardware vendor:
http://iguanaworks.net/projects/IguanaIR/ticket/273

While the manufacturer says there is a known cross-talk problem with the
hardware, the level off the cross-talk should be down near the noise
floor, yet various experiments attempting to attenuate the signal show
the cross-talk is suppressed at the same level of attenuation as the
primary signal.

Software is pretty much ruled out as a cause. In part because none of
the low-level software (LIRC) was changed at the time the problem
appeared, and in part because the nature of the cross-talk indicated a
degraded signal - signals sent to port 1 are often not perfectly echoed
on port 2 - which seems unlikely to be caused by software.

Hardware failure was the next obvious cause, and so the transmitter was
swapped out, and alternate emitters tried. The transmitter change made
no difference. The emitter change did vary the frequency of cross-talk
occurrence to some degree in testing, but in actual "production" the
cross-talk problem seems to happen just as frequently as it did before
different emitters were used.

The proper next step here is to hook up a scope to observe what is
actually being transmitted from both ports, and look at whether anything
weird is going on with the USB supply voltage. But lacking a scope,
that's not going to happen.

This background is just to set the stage for the brute force solution
I'm considering: routing port 2 through a relay. It would be putty easy
to stick a few lines into my wrapper shell script that sends commands to
the IR hardware to turn on a relay when port 2 was being used.

So my question is, what's a good approach for controlling a relay from a
PC that is a good compromise between cheap and least effort to build.
There are hundreds of ways to accomplish this, and many ready built
relay control boards, but I'm thinking a TTL compatible reed relay, like:
http://www.nteinc.com/relay_web/reed.html

wired to the parallel port (the server has one, and it is unused), and
controlled with some software, like parashell:
http://sourceforge.net/projects/parashell/

(More info on parallel port control here:
http://www.epanorama.net/circuits/parallel_output.html )

You can't get much simpler than a relay, DB25 connector, a 3.5mm jack
and plug, and some wire. The relays above even have an internal diode.

(The other obvious choice is an optocoupler, but for the duty cycle this
will undergo I'm not that concerned, and the mechanical relay provides
the simplest interface to the emitter, where polarity, high-side vs.
low-side, and voltages are irrelevant, and thus more apt to work the
first try.)

Anything simpler? Any skepticism for running the relay directly from the
port without a buffer? Any recommendations to use a serial port control
line instead?

 -Tom