I would check the instrument manual to see if there is a limit on the maximum number of bytes that it will send in one packet. Still try getting it to send 255 bytes and see if that works. If it does, then try 256 bytes to see if the limit is there (a one-byte counter for number of bytes sent). If so, then you may have to get the instrument to send data in multiple packets and put the data back together in your program.
Rob
