Data synch and detect with rf

[Guest]

Hello -
I would like to transmit sensor data (multi byte) by rf at infrequent intervals. I have a Linx Rx/Tx pair that connect directly to the Rx and Tx micros with one wire: serial data in and serial data out. Just for fun, I tried them using RS-232 protocol with some "header" bytes so the Rx micro could tell the next data packets coming were real data. I know: crude, no CRC, unbalanced, etc.. - not the route for a robust link. And the Rx module has "carrier detect" capability, so it is in standby (low power) mode until the presence of data is detected. Obviously, you don't want to use a protocol where "off" or "0" is high all the time.

What is the best way to signify and confirm the start of data transmission on the Rx side? The Rx module wakes up in about 50 microseconds after it receives a burst of signal from the Tx side. The digital serial out stream picked up on the Rx side is nearly identical to the transmitted signal as long as the sent pulse widths are above a few hundred microseconds. Given this, what is a good "preamble" to use? Do people normally send a square wave and count pulses to signify valid data is coming? Any ideas here please?

Secondly, assuming we accept that valid data is coming in serially to the Rx micro, What is the best way to decode the data? Start a clock and "sample" the center of each bit as in RS-232? Manchester encode and count "going high" and "going low" pulse edges?

There are encoder/decoder chips that may do some of this, but the ones I know about (Holtek) consume lots of pins or extra interface parts, like shift registers.

Also note that this is not a homework project, not for a customer, or a commercial product. I am just trying to learn about this stuff and reading this forum has already taught me quite a bit, although at times its over my head. I don't have an electronics background, but have been reading as much as possible lately and asking lots of dumb questions.

Thank you very much.

Will

[SherpaDoug]

Sending a string of "uuuuu" is an easy near square wave, though it is hard for a UART to sync to. Maybe a string of hex 0F would be better. We have a senior engineer that always uses hex BABEFACE, but I don't know why.
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