Max RS232 length? and noise issues on cable. 1

[stepfrog]

What is the max cable length for rs232 communications? As well what is the best shielded cable to use for rs232. I want to use db-9 connectors. I need to drop the noise to it's ablsolute lowest and I need to see what my maximum length I can go with it. thx

 

[JDMcB]

Max recommended RS232 Cable length is 15M. Anything above this should be RS422 or RS485.

 

[daronwilson]

According to "Serial Port Complete", a book by Jan Axelson covering more about serial that you EVER want to know, the maximum length for RS-232 is 50-100 feet. I've seen many different ideas on this, I personally don't want to go look up the standard and find out what it says. I have had good success well over 100' at lower (9600 baud) speeds, some higher. If you have to go more than 100 feet you can easily convert the signal to RS-422 and go 1000 feet easy.

What is your noise concern? RS-232 is pretty robust if run in a shielded cable, and making sure the shield is grounded to the frame ground. Also the speed you are using can play a part, and whether or not you are implementing hardware flow control.

Good Luck!
It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com

 

[daronwilson]

I dug a bit deeper, apparently the earlier versions of the RS-232 standard recommended limiting the maximum cable length to 50 feet. Later versions of the standard changed the limit to a maximum capacitance of 2500 picofarads at the receiver (including the capacitance in the reciever). Based on that, single unshielded twisted pair should get you about 133 feet, and shielded twisted pair should get you about 66 feet. (calculations based on formulas presented in the serial port complete book). Those distances should let you do 20k or so for a speed. As always, your individual mileage may vary.

With regards to twisted pair, this book suggests using a pair for a signal and ground. For RS-232 you would put transmit on one wire of the pair, and ground on the other wire of the pair.

Anyway, twisted pair is most likely the way to go to keep the noise out and the signal in. Shielding also helps accomplish this if grounded properly.

Good Luck! It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com

 

[stepfrog]

Thank you DaronWilson, this helped me very much. -stepfrog