[mythtv-users] Maximum length of cables for USB<>Serial DirecTV control?
mythtv-users at westbrook.com
Mon Apr 5 02:18:07 UTC 2010
On Sun, Apr 4, 2010 at 7:19 PM, <bhaskins at chartermi.net> wrote:
> USB to RS-422<>RS-422 to USB ?
> Or even USB to fiber<>fiber to USB.
> 422 is much better that 232 on anything more than three feet or so.
> There are converters available for either/both.
I do precisely this in my work all the time, although for applications of
serial control signals other than TV. :)
On Sun, Apr 4, 2010 at 7:47 PM, Jean-Yves Avenard <jyavenard at gmail.com>wrote:
> It really depends on the cable.
Certainly true. The way I do it, though, I always plan for (and get) <=
300ft with RS422, <= 50ft with RS232. That's about as much as you can
expect to get with either using any cable; there's too much capacitance in
the line after that.
Here's how I consistently get those results, if you're curious:
I use the Yost convention for connectors and cables, exclusively, for both
RS232 and RS422. Forgive me if this goes astray, but if you're interested
in easy, optimal serial comms, you'll be interested.
I love the Yost system. All your serial cables are the same, with the same
connectors (RJ45s) on each end. Make your own and cut them to length to be
tidy, and save money in the process. Even go ahead and use EIA-568-B pin
layouts if you like (and I do) so they're even the same cables you use for
ethernet... as long as the pins are connected 1-to-1 by pin number, you're
good to go. You need to wire up an adapter connector for each serial port
you're converting, but the parts are super cheap (I can look up where I get
them if you'd like), and once you screw your adapter on to the serial port,
you forget all about DB9 forever. It boggles my mind that everyone doesn't
use Yost. Maybe cause one of "those weird unix guys" invented it, lol.
So that's always what I recommend. To return to topic, though... I've
always wondered whether the twisted-pair fidelity of the cat5 boosts my
transmission range in and of itself, but I've never wondered enough to run
50ft of 18-2 and compare measurements. :) And I get as much distance in my
runs as anyone else I know of. So my recommendation is, do consider using
twisted pair, whether Yost or not, but your mileage may vary.
As for computer interfacing, though... is it just me? I find converters for
anything-to-RS422 to be expensive. Between 50 and 100 bucks or more! A
cheap USB-to-RS232 adapter goes for $7.99 at Newegg. So to get to RS232 is
cheap, RS422 is pricier, at least in my frustrated experience. (If anyone
knows different, please advise!)
PC => USB => RS232/RS422 <=> <=> RS422/RS232 <= USB <= PC
So here's my $0.02:
- Use Yost;
- Use cat5/568b;
- Use RS232 for runs under 50ft, enjoy the cheaper adapters;
- Convert RS232 to/from RS422 for runs up to 300ft.
I don't believe you can do better. Unless you can find the cheaper RS422
adapters that elude me. :)
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