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Cable max length using RS232

I read it somewhere but can't remember. What's AMX recommendation on MAX length on RS-232 control? I couldn't find it in Tech Notes.




  • Spire_JeffSpire_Jeff Posts: 1,917
    I believe they say 50ft, but it is very dependant on the speed of the connection, the type of cable used and the presence of interference. If you need to get further, you can find adapter that will let you connect via IP or extend the run via RS485.

  • KouyaKouya Posts: 60
    Cable max length using RS232

    Thanks Jeff,

    Actual run is about 35 - 40ft, Figured it's shouldn't give me any problems, Just one of our sales guys asked the question and I couldn't put my finger on the answer

  • jjamesjjames Posts: 2,908
    50ft is a reccomendation . . . just like stop signs that have white borders. :)

    I know we've gone as far as 200ft and it's worked like a charm . . . although I wouldn't reccomend it.
  • banobano Posts: 173
    I have a 295 ft run controlling a sony plasma (4800 baud). When the installer was landing the wire at the device he asked me if I wanted to give serial control a shot. I said go ahead. I almost fell out of my chair when I sent a power on commmand and it worked!
  • TrikinCurtTrikinCurt Posts: 158
    I assume there isn't anything AMX specific to this? If not, it is all based on the cable you use. It is one of these funny situations where heavy gauge won't get you further, cat5 will get you as far as you typically need to go.

    I had found a great geeky site once upon a time that explains the distances (and how to calculate). Can't find it now, but do some searching and suffice it to say, I am sure many of us have gone over 100' on cat5 with no problems, me included.

  • Spire_JeffSpire_Jeff Posts: 1,917
    I just did a quick search for the info and found this:
    Maximum cable lengths

    Cable length is one of the most discussed items in RS232 world. The standard has a clear answer, the maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. The latter rule is often forgotten. This means that using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. If for example UTP CAT-5 cable is used with a typical capacitance of 17 pF/ft, the maximum allowed cable length is 147 feet.

    The cable length mentioned in the standard allows maximum communication speed to occur. If speed is reduced by a factor 2 or 4, the maximum length increases dramatically. Texas Instruments has done some practical experiments years ago at different baud rates to test the maximum allowed cable lengths. Keep in mind, that the RS232 standard was originally developed for 20 kbps. By halving the maximum communication speed, the allowed cable length increases a factor ten!

  • TrikinCurtTrikinCurt Posts: 158
    Ya, that's the one :)

  • Be aware that you can go farther with the signal from a "true" RS-232 port than with the signal from an IR port.
    Also if you have to run greater lengths, you could use the RS-422 wiring on the AMX device and use a RS422-to-RS232 converter close to the device to be controlled. Since RS-422 is basically a "balanced" version of RS-232, the protocol will be the same (without hardware handshaking options though) and should allow you to send and receive at about 3000 ft / 1 km.
    RS-485 might also work, but you have to be careful about traffic control as the transmit and receive signals run over the same 2 conductors (485+ and 485-).
  • Chip MoodyChip Moody Posts: 727
    Jeff, that's the most informative post I think I've seen in my short career as a CSP... Can't wait for Wednesday to email it to my company's engineering staff.

    - Chip

    Spire_Jeff wrote:
    I just did a quick search for the info and found this:

  • MurphMurph Posts: 3
    Long 232 Runs

    You can run low-ish speed (9600 or less) Rs232 reasonable distances (up to say 50m). However the big problem is that RS232 is very vulnerable to casuing device failure during power spikes because it is unprotected and can pass electrical disturbances from one location dirtectly in to your processor.

    We have over 60 AMX systems in a university and often have problems becasue projectors are powered from different distribution points to the processor. Optical isolation fixed the issues.

    At the very least, if you're running RS232 some distance, also run the power from the controller to the periperal device and run the whole system it all off a good mains filter.

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