Oh, yes. It's badly named. This function actually turns the bytes (which ARE bytes) from the binary packet frame into "cavac-encoded" half-bytes for serial transmission.

So, $highbyte (which should actually be $highnibble or something) can't go over 255).

The reason i'm using this encoding is because it's dead simple to turn it back into real bytes, while allowing proper framing for transmitting over serial.