PPX-1 Protocol
The following is the format of the
BYTE | SOH | ;Start of header for synchronization |
|
| (ASCII 01H) |
BYTE | LEN H | ;High byte of length |
BYTE | LEN L | ;Low byte of length |
BYTE | HEADER | ;= NOT(LEN L) + |
| CHECK | ;NOT(LEN H), this is an |
|
| ; arithmetic sum |
BYTES | DATA | ;This is where the MCP COMM- |
|
| ;AND is contained (see section 3) |
BYTE | CHECK- | ;16 bit check sum of data |
| SUM H |
|
BYTE | CHECKSUM L | |
Note that the LEN field is the length of the DATA section, not including the CHECKSUM field.
The purpose of the header is to make frame synchronization easier. In asynchronous communications where the data field may have any ASCII value, this is a difficult problem. The HEADER CHECK field is intended to provide a fairly secure way of mak- ing sure that a header has actually been found.
The maximum permissible packet length is 1456 bytes (not including the header and checksum).
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