been there...
In a word, NO.
I've used long CAT-5 cables for serial... and parallel, too!
I make my own NON-crossover RJ45 cables with boots.
They do need to be wired to spec, not just straight-through.
Crossover cables get RED boots, and I have very few - they are pretty useless to me!
When you get the RJ45 to DB9 adapters, you have to wire them.
They don't come pre-wired. At least, the ones I used did not.
You might want to search out some good prices on Monoprice dot com , as I think you'll find 'em hard to beat.
As I recall, we tested serial to 100 feet, and at 115k baud, just as a stress test.
Ran reliably... probably for days.
And I was doing some stuff with the parallel port over similar connections, and I'm sure we stressed that to 50 feet, at least.
Maybe I used the same 100-foot roll, I just don't remember.
Both distances were well beyond what I was expecting to use, so I figured there was plenty of wiggle room (safety margin).
In fairness, I was talking to hardware I built, not an STB, but I would have some expectation that you would succeed.
