> If you are careful with the data you send via RS232 so that it has
> approximately 50% 0's and 1's then you should be able to use a low-cost
OOK
> (on-off-keying) radio module to do the job.
>
> An example of such a module is TX-433 and the matching RX-433 receiver
from
> Ramsey Electronics. Total cost about $40.00 including shipping.
>
> You can get your data to be half 0's and half 1's by sending each data bit
> as a pair of actual bits:
>
> Send a 0 bit as 01 and a 1 bit as 10.
>
> Thus, you'll have to send 2 bytes for each byte of real data.
>
> You'll need to keep your baud rate low. I would suggest 300 for maximum
> reliability.
>
> I'd also add some sort of checksum or CRC.
>
> For extra credit, use an FEC (forward error correcting) code.
>
> Bob Ammerman
> RAm Systems
> (contract development of high performance, high function, low-level
> software)
>


As an addendum to Bob's message, I would like to suggest that you look up
Manchester encoding.  Manchester (or is it Manchester II?) encoding is what
Ethernet uses for its on-wire symbology.  If you look at the Power Spectral
Density of Manchester NRZ, you'll note that it doesn't have any DC
component.  This is critical for proper operation at the receiver when that
receiver is, effectively, AC coupled to your data output.  RF Monolithics
has an excellent How-To on it at:

http://www.rfm.com/corp/appdata/AN43.pdf

titled "Unique Considerations for Data Radio UARTS".  Although their
examples are, naturally, aimed at supporting their own equipment, the well
written concepts apply nearly universally to OOK RF.

-d

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