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The Sequence is incremented by one for each successfully |
transmitted packet. It wraps from 0x39 back to 0x30. (Note |
that 0x30 is the ASCII character 0 and 0x39 is 9.) |
The Sequence, Type, all Body data, and <ETX> are included in |
the Check Value. Note that <Body> and <Check Value> are |
Quoted, and that the Check Value is computed on the actual |
data bytes rather than their quoted replacements (the |
quoting <DLE> is not included). Hence, if the character |
0x13 is to be sent, it is sent as <DLE> <S> but the value |
0x13 is included in the Check Value. |
If Standard Checksum is used, <Check Value> consists of a |
quoted byte. |
If the XMODEM CRC-16 is used, <Check Value) is transmitted |
as two quoted bytes with the most significant byte appearing |
first. By performing the CRC calculation on the XMODEM |
CRC-16 value, the result will be 0x0000 if no errors |
occured. |
A sample B Plus packet, sent using the Standard Checksum, is |
as follows: |
<DLE> B 7 T D A S . C <ETX> 0x2A |
0x10 0x42 0x37 0x54 0x44 0x41 0x53 0x2E 0x43 0x03 0x2A |
where |
Lead-in = <DLE> B |
Sequence = 7 |
Type = T |
Body = DAS.C |
Trailer = <ETX> 0x2A |
If the packet was sent using the XMODEM CRC-16 option, the |
Check Value would be 0x57 0xFF instead of 0x2A. |
4 |
2. Quoting of characters. |
The B Plus Protocol quotes certain ASCII control characters. |
The default set of quoted characters is: |
o <ETX> 0x03 |
o <ENQ> 0x05 |
o <DLE> 0x10 |
o <DC1> 0x11 |
o <DC3> 0x13 |
o <NAK> 0x15 |
The quoting mechanism uses the <DLE> character to prefix the |
quoted character. The quoted character with 0x40 added |
follows the <DLE>. For example, <ETX> is quoted as: |
<DLE> C |
0x10 0x43 |
B Plus also provides a means for the Initiator and Responder |
to establish a different set of quoted characters. This set |
may consist of any or all characters in the ranges 0x00 thru |
0x1f and 0x80 thru 0x9f. |
Quoted characters in the 0x80 thru 0x9f range are sent as: |
<DLE> ((char and 0x1f) + 0x60) |
Hence, 0x93 is quoted as: |
<DLE> s |
0x10 0x73 |
When receiving protocol data, the program should be capable |
of decoding any quoted character, regardless of the set of |
quoted characters in use. This must be accomplished as |
follows: |
Get a character. |
If the character is <DLE> |
then |
Get next character. |
If character is less than 0x60 |
then |
character = character and 0x1f |
else |
character = (character and 0x1f) + 0x80 |
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