@AWI
Thanks for your efforts and persistence 
Yes, before the packet will be sent, the MyTransport485 will check if there any bytes arrived recently from the serial port. The collision itself can occur during period of the first byte of the packet. (I wish to have time to draw a time diagram for this.)
I think I forgot to mention one more change to the original library:
//The number of SOH to start a message
//some device like Raspberry was missing the first SOH
//Increase or decrease the number to your needs
#define ICSC_SOH_START_COUNT 3
Could you please try to modify this constant ?
It may affect amount of lost packets.
You may also try to increase speed up to 250000 baud.
I have one more idea about collision avoidance:
(In this case we need to make a small modification to the original RS485 module connection, /RE input should be connected to the ground to make receiver always active - then we would be able to receive own messages sent)
We can try to implement the following algorithm:
(Send a packet procedure):
-- check if there any bytes received recently - continue if we have a silence on the bus (done at this moment)
-- send a first SOH (start of header byte)
-- check if it is received by our serial and it's equal SOH.
(so we know at this moment we have no collision on the bus for sure)
-- send next SOH bytes and all packet bytes.
It's like we are sending bytes to the line and reading them at the same time to make sure we had no collision at the same time.
This way we can even resend our packet if collision was detected during packet transmission.
Also we can implement one master and slaves nodes poling mode. But this will not be easy to make in the mySensors protocol architecture.
Let me know what you think about, you probably should have some other ideas too.
