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stuffed
02-03-2005, 11:48 AM
Sorry if this is off subject but thought all you switched on types out there might be able to assist with someone out of his depth on this one!
Is there a way to transmit RS485 data about 30 meters (i.e. across a street) by wireless?
Many thanks

Graham L
02-03-2005, 03:20 PM
Umm. ;)

There are a number of ways you can get information across a street. Unfortunately, the logical, easy, and cheap way (a pair of copper wires) is out of reach.

If you want to use the multidrop (multiple receivers) or the full 10 Mbps rate, I'd forget about RS485 and use Ethernet and Wifi. ;)

You can get (relatively cheap) UHF modules which will take a serial stream and transmit it. And matching modules which will receive the UHF and produce a serial stream. A simple directional antenna at each end will give reasonable security, and a bit of height will avoid the problem of a bus stopped in the beam. But you won't get a high bitrate --- maybe 9600. I don't know. The modules are lowpower (100 mW or so) and "licence free". That's a reasonably easy to implement technique.

You could build an infrared setup without too much work. If you send bursts of 40kHz carrier through a bunch of IR diodes (perhaps using a parabolic "mirror" to tighten up the beam) you could get 30 m to a TV remote control detector device. (the 40 kHz carrier is to avoid problems with ambient light ... and the demodulation is built into the receiver chip). Again your bit rate wouldn't be all that high. The receivers are rated for about 5m range, but that's with (usually) one IR diode sending. A bit of total power, and some directional beaming should get the range up markedly.

A microwave link would be expensive. :(

Of course we have suddenly not not got a RS485 link. ;)