Why is 0x7E commonly used as Start Delimiter in Communication Protocol?

Question

i am looking to implement a serial communication between 2 devices using just a simple communication.

It seem that protocol such as those used in HDLC and xBee uses 0x7E as the start delimiter.

Is there any particular reason or i can use any character as an alternative.

Answer 1

The reason is related to framing of data.

• They needed a flag that will mark a beginning and an end of transmission.
• They wanted to be able to replicate that flag few times if needed - in example if you send
  01111110 01111110 01111110 - it is very easy to identify this set compared to other sets even if some bits didn't made it to the other side, so it is uniquely much less error prone.
  (0x7E=01111110)

Extended Example:

machine 1 sent: 01111110 01111110 01111110
some options for error in receive:
machine 2 recv: 011110011111001111110 - notice that you can identify 3 frames very easy.
machine 2 recv: 11111001111110011111 - again you can identify 3 frames quite easy.
machine 2 recv: 01111110111111001111110 - once more 3 frames can be identified quite easy.

Now let's see what happens if we choose different sequences as flags.

Example 1:

machine 1 sent: 01011010 01011010 01011010 (here flag chosen is 01011010)
some options for error in receive:
machine 2 recv: 010010100101101001011010 - you cannot identify if those are even frames..

Example2:

machine 1 sent: 00011000 00011000 00011000 (here flag chosen is 00011000)
some options for error in receive:
machine 2 recv: 00110000001100000011000 - this one is ok... we can easy see 3 frames.. but..
machine 2 recv: 0010000001100000011000 - this is harder.. could be 3 frames but can't be sure..

You can read more in Wikipedia information about HDLC (check the frames subject)

One more thing.. SO Folks I know this has a flame potential :)
Please keep in mind I didn't choose this arbiter sequence.. I only explain the reason for this particular sequence as choice.