Cheapest way to communicate

[dhouston]

Smoke signals?

[tenaja]

Pics get pretty expensive once they let out a puff or two of smoke...

[Jumper]

There are plenty of 433 Mhz modules you can use for this. Pick the right one and you can have up to 500m.

One way or 2 way communication?

Cable? fiber optics...

Usually the communication part is not the problem, it is to provide power to all units without having to change batteries every day.

/me

[koossa]

Is wireless cheaper than RS485?

[ScaleRobotics]

Depends on what type of cable you run...........

[koossa]

So, If I'm going for a cheap cable then RS485 is cheaper than wireless.
Is there anything cheaper than RS485?

[ScaleRobotics]

From what I have read, RS-422 or RS-485 can handle about 4000 feet on cat-5 network cable, which is pretty cheap. Sounds like even RS-232 will get close to the distance you want, but you would need good cable, and a low com rate, (and very low noise in your environment). So cheapest sounds like RS-485 or RS-422.

[Melanie]

If it's just two PICs, Point-to-Point, proper RS232 will easilly work down 100m of junk Telephone or Burglar Alarm cable. It should have a range of about 1km without Repeaters (using decent screened Cable).

[dhouston]

Pics get pretty expensive once they let out a puff or two of smoke...

But that might be a one-time expense. The OP really hasn't provided sufficient detail. Even Melanie's educated string probably cannot hazard an educated guess. What's the duration of the communications link? Is it one-to-one or one-to-many? What type of messages? What's the terrain? Is the path horizontal or vertical? Is it level or does it span a canyon? How can a wire be routed? Stapled to tree trunks or utility poles, buried in a trench? Must it be taut or can it sag? Etc., etc.,...

[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