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The 255's are simply signals to the output cpu telling it that this is the start of the stream of bytes.
If the o/p cpu were not to receive them (maybe it missed them on powerup), it woud sit there listening for 255,255. Once received (the next time the Stamp sends an output stream), the bits would be pigeonholed properly as bit 'n' from the start, bit 'n+1' and so on.
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