On Thu, 3 Sep 1998, Ken wrote:

> My system uses two fully-featured transmitters, either can be master or
> slave.  Whoever is designated as master has to hold a switch down to turn of
> his radio transmitter, and let it up to turn off the other transmitter.  The
> protocol is complex, and is a frame-based serial protocol.

If you are SURE about this then borrow a DSO and perform some of what is
sometimes called 'hacking'. There are only 2 kinds of protocols in wide
use for this kind of application:

1. Dumb: All state data is encoded in a large packet, and sent. Packets
are the same size, always. May be SPI or RS232 on TTL levels. Pay
attention to the number of bits in ALL packets, some 'far-sighted' designs
use the 9th bit available with some UARTs for extensions (such as
slave/master addressing).

2. Smart: A full state frame is transmitted once a second or so, meanwhile
nothing if no change in state, else a code with the item that has changed
vs. the previous data. This is less likely to use the 9th bit and may be a
bit-banging protocol with less than 8 bits per word for some messages.

3. You may still have been fooled by the PWM data travelling in both
directions to allow either Tx to become a master. To check this, examine
the cable. It should not be a 1:1 straight through cable if this is true
and the Txs are identical (some pins may be reversed for the double PWM
system), at least 2 in fact, probably 4.

hope this helps,

Peter