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3. Check Value.
Each B Plus packet contains a Check Value to allow detection
of transmission errors. The B Plus Protocol supports two
Check Value methods as follows:
a) Standard B Protocol Checksum.
The Standard Checksum is calculated as follows:
o The checksum is initialized to zero.
o For each byte that is to be checksummed:
- The old checksum is rotated left 1, with the
old bit 7 becoming bit 0 in the rotated value.
- The new byte is added to the rotated checksum.
- If addition of the new byte causes a carry,
then one is added to the new value.
b) XMODEM-Style CRC-16.
B Plus implementations may optionally specify that the
defacto standard XMODEM CRC-16 be used as a check value.
The only departure is that the CRC-16 value is initialized
to 0xffff instead of zero.
Sample C code for calculating these Check Values is presented in
Section "H. Check Value Calculation."
6
4. B Plus Packet Types.
The following Packet Types are the minimum required to
support the B Plus Protocol:
o `+': Transport Parameters
o `T': File Transfer
o `N': Data
o `F': Failure
a) Packet Type `+': Transport Parameters.
This is the B Plus Transport Parameters Packet. It
describes an entity`s Transport capabilities. The Transport
Parameters Packet is always sent with all characters in the
ranges 0x00 thru 0x1f and 0x80 thru 0x9f quoted, and using
the Standard Checksum.
The Body of the <+> packet is a series of bytes as follows:
WS WR BS CM DQ TL Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 DR UR FI
where:
WS = Window Send. 0x00 implies that a single packet at
a time can be sent, 0x01 two packets. The default
is 0x00.
WR = Window Receive. 0x00 implies that the entity
cannot receive other than a single packet at a
time (that is, requests the other entity to use
WS = 0x00); 0x01 implies two packets. The
default is 0x00.