Cheapest way to communicate

[koossa]

What's the duration of the communications link?

it is a 24/7 communication link, every second.

Is it one-to-one or one-to-many?

one-to-many

What type of messages?

short, status monitoring

What's the terrain?

Horizontal, maximum of 1 meter difference in elevation

Stapled to tree trunks or utility poles, buried in a trench?

Buried in a trench

Must it be taut or can it sag?

Can be sag

Thank you!!

[Luckyborg]

Probably the cheapest solution would be a 20 ma current loop circuit. 1 transistor (PN2222A), 1 opto-coupler (we use a 4n26) and a few resistors and you are set. I think I remember something about that being good for around 2000 feet, especially if you can go with a lower baud rate. Having said that, I would probably still go with RS422/485 just because it is more of a standard these days.

David

[dhouston]

one-to-many

If you have to have a two-way communication with more than one node, I think RS-485 is the best choice. Two way wireless gets complcated and expensive rather quickly. Also, most jurisdictions have limts on how often a wireless node can transmit. B&B Electronics has an excellent whitepaper on RS-485.

• http://www.bb-elec.com/tech_articles...f_contents.asp

[dhouston]

Here's another helpful article on RS-485.

• http://www.embeddedsys.com/subpages/..._art_RS485.pdf

Also, even if buried in a trench, you will need to protect against lightning by opto-isolating the nodes. See the ISOLATED LINK section of the Axelson article. You can use direct burial CAT5 cable to avoid the need for conduit.

If you can provide more detail about the number of nodes and the nature of the messages we might be able to suggest less costly methods. It might be less costly to use WiFi which can be done for under $60 per node. It's inherently isolated and you can use serial communications.

• http://www.ewiznet.com/goods_detail.php?goodsIdx=135

[dhouston]

One caveat on the WiFi module. Its range is specified as up to 200m. That is in a straight, unobstructed line. YMWV

[koossa]

Hi Dave

Thank you very much for the info and links!!!

[imjames407]

i think Walkie talkie is the best option.