On Tue, 18 Aug 1998 12:38:07 -0600 Justin Crooks <jcrooks@WYOMING.COM>
writes:
>Welcome to the world of RF.  An easy way to ignore noise is send a
>difficult-to-duplicate prefix character before each data character.

For this and other reasons, synchronous data is usually used for RF
transmission.  A fairly long (such as 32 bits) "preamble" sequence can be
used to be quite certain of starting reception at the right time.

I assume you are using simple AM transmitters with the start bit being RF
on and the idle condition RF off.  The regenerative receivers commonly
used with these transmitters cannot handle much DC component in the data
signal.  Asynchronous data has a large DC component.  Usually Manchester
or similar coding is used to ensure that there is no long run of one
polarity of data.  If you must persist with asynchronous, it can work,
but not as well as a more optimal method.  One way around the DC problem
with asynchronous would be to use only some bits in each character as
valid data, and set the extra ones so the signal has low DC content.

Here are some thoughts on implementing an asynchronous system.  First
consider that the receiver may be in the middle of a character (or worse,
it thinks it is reciving a valid packet) that started from noise when
your data arrives.  Thus there's a need to send some "don't care" data
before starting with real data.  It would be good to send a couple of all
zero characters first to charge up the coupling capacitor in the
receiver.  Then send a character of all ones (only the start bit is zero)
to let the receiver synchronize so it will start on the start bit of the
next character.  Now it should be safe to send data.  You probably want
to start each data with a specific sequence of characters to
differentiate it from noise (remember the receiver may not receive the 00
00 7F sequence properly, so don't require it to).  Ideally the
synchronizing character(s) should be ones that would never occur in valid
data (One possibility would be giving them intentionally bad parity).

It's probably best to always send the same number of data characters.  If
you let the number vary, if the receiver misunderstands the number of
characters to be transmitted, it may still be expecting more data from
the last packet when the next one arrives.  It may also be good to use
parity on each character and cancel reception immediately if a character
with bad parity (which includes the "don't care" data before the start of
a new packet) is received.  The packet would not be useful anyway.  Of
course, include a checksum or CRC in the data.  This is just a final
check for any errors that occurred during transmission and not the
primary determination of whether the transmitter is really on the air or
not.  If you waited to receive a whole packet before deciding that it is
bad, there would be a lot of lost data due to false starts.


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