Proxima ASA 7911 - Manual (Page 79)

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PPX-1 Protocol
The following is the format of the PPX-1 packet:
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).