Proxima ASA 7911, 7910 manual PPX-1 Protocol

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).

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