View Full Version : How long of cable for rs232 can be acceptable?

Jorge P
01-25-2010, 11:24 PM
I am just wondering how long of a cable I can get away with using any pin with serin and serout command?

I know after a certain leingth (depending on the wire ratings) there wil be some signal loss.· Is there a math formula to determin this?

thanks in advance

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01-25-2010, 11:33 PM
here is some info found on google (http://www.lammertbies.nl/comm/info/RS-232_specs.html)...

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!

hope this helps.

01-26-2010, 12:57 AM
And note these cable lengths assume RS-232 signalling levels, typically +- 5 volts, sometimes +- 3 volts. But NOT the "TTL" signalling levels of the BS2, you'd need to use a MAX232 driver to get these distances.

Peter KG6LSE
01-26-2010, 01:01 PM
As a side note I know you can get "active baluns" that allow balanced CAT5 to be used for Very long runs . think kM or so .
RS-232 as I know it is a UnBalanced "single ended" medium ...
Me I used Headphone cord with great results up to 100 feet for a 19.2K connection .. and I use 2.5MM jacks as a " Peter standard " for RS232 in my projects .
Right is RX
Left is TX
cause in most new RS-232 apps from what I have found you only meed 3 wires .

Pins 2 3 5 on the Dsub 9

KG6LSE Peter

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