Cable Selection for serial data transfer 1

[gmh265]

Hello, I am looking to extend the cable length between a radio modem and the serial port of a copmputer to about 30m, What type of data cable is suitable for this application? I have had a quick browse through the RS catalouge but there is many different cable options which has made the choice even harder.

Thankyou

 

[BrianG]

Important questions: what kind of serial interface do the modem and computer have - USB, RS232, RS422? What data rate are you expecting to run at?

If you have an RS232 interface between the computer and the modem it's not good for more than about 10 metres at a maximum data rate of 9600 bps, as - in theory - the line drivers in each device can't drive longer cables at high data rates without errors. If your speed is lower or you can find VERY low capacitance cable you may have better results.

If you have an RS422 interface you can easily drive several hundred metres of cable. The best cable type for this is, in order of preference, screened twisted pair, followed by twisted pair. Connect the screen to ground at one end only.

 

[jloram]

If you are using Rs232C, then you want to use the lowest capacitance cable you can fine. Avoid twisted and shielded cable if possible. Hopefully you are opperating in an electirically 'quiet' environment. I have personally run 9600 baud over a few hundred feet using 8 wire 22 guage unshielded Belden cable using standard Rs232 drivers/receivers in an office environment. As you go up in speed the length will drop. Ocassional a designer will roll his own drivers/receivers either to save manufacturing cost or to improve performance. In that case all bets are off.

regards, -john-

 

[gmh265]

Thanks for the replies, I am using the RS232 Interface and unfortunately I am also working in what I guess would be a rather noisy environment, (Motorsport pit area, where there is heaps of electrical equipment) How do I determine if a cable has a low capacitance? Is that something that should be listed in the specs of the cable?

Thanks again

 

[BrianG]

One alternative to trying to fing a low capacitance cable would be to use an "interface converter" at each end of the cable.

You can buy packaged RS232 to RS422, and the complementary RS422 to RS 232, signalling converters. These are packaged into a D-type connector to plug straight into the modem or computer sockets resepectively and draw a small amount of power from the RS232 socket. This means that the long cable run in between uses the robust RS422 (differential signalling) standard, but both modem and computer see a normal RS232 connection.

Thay way you get the best of both worlds - high noise immunity without having to modify modem or computer interfaces.

 

[analogkid2digitalman]

I concur with Brian, 422 would provide better immunity. BlackBox makes a wide variety of converters and cabling.

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