Panasonic KX-T336 System Reference Manual page 1576

Transmission Format
STX
SOH
Checksum
Data Area
Area
SOH = Start of header
STX = Start of text
Address = System data address
(Is system address is "FFFFF h", it would
be software version) (ASCII code)
The number of byte = 1 to 256 (ASCII code)
Data area = System data (ASCII code)
Checksum = Address + The number of bytes +
data
* The complement of the sum of all bytes (ASCII
code)
ETX = End of text
EOT = End of data transfer
Address Area
EXT STX
Address Area
The number of
bytes
16-B-5
Data Area
Checksum
EXT EOT
Area