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rs485 length limit
Communications systems and equipment. topic
Posted by naveen on 18 January, 2009 - 11:28 am
Dear sir,
there is an rs485 link which is not working at the distance of 300 meters. I have tested it for 100 meters successfully, but more than that it is not working. I have tried the termination 120 ohms on both ends of the link. I have used 24AWG UTP cable for this. Is that practically possible, rs485 works for 1200 meters. The baud rate i have used is 9600 bps

please suggest
Naveen


Posted by Roy Matson on 19 January, 2009 - 10:46 am
You should have no problem at that length, there must be something wrong with one of your devices. I suggest you measure the resistance between the pins of each device, and between pins and ground perhaps you have 2 termination resistors in parallel or the common mode voltage level is outside the operating limits. Check also that both end devices are solidly grounded.

Hope this helps
Roy


Posted by Timothy Niemczyk on 21 January, 2009 - 2:05 pm
More information needed:

Is the cable twisted pair and shielded?
Is the cable located near high voltage wiring?

Most of the issues I experienced are from theses common problems, i.e. straight pair cable or not shielded. It is possible to have RS485 working at 1200 meters.


Posted by Mehul on 22 January, 2009 - 12:40 am
RS485 must work, we are using for more than 1000 meters for more than 4 years with similar baud rate. We are using telephone twisted cable pair for the same purpose. The cable resistance also < 200 ohm.

Check for the following:
1) Use twisted pair or you can make 02 nos of pair parallel for reducing resistance.
2) If other pairs are free, ground it. This will reduce noise pickup.
3) Check your driver for the proper functioning (Whether properly programmed, dip switch setting if any, configuration of device).

Give your feedback for the same.

Regards

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