DR + GPS
STARTER KIT REFERENCE MANUAL
Lassen DR + GPS Reference Manual 49
Whenever an M-byte in the data fields or the checksum field is equal to one of
the control characters 0x80-0x87, it generates two S-bytes as follows: the M-byte
generates the S-byte pair [0x80, M-byte & 0x7F].
Pre-Parsing: HCC Unstuffing After Reception Pre-parsing (assembly of the M-bytes) occurs as S-bytes are received. HIPPO
pre-parsing begins with the appearance of the SOM S-byte and ends with the
appearance of the EOM S-byte.
Whenever the S-byte is SOM, a new message structure opens
with room for 132 M-bytes. The first M-byte of a message is
always SOM.
Whenever the S-byte HCC appears, it does not generate a new
M-byte. Rather, it generates a signal to OR the following S-byte
with 0x80 to create the next M-byte. Otherwise, the M-byte is the
same as the S-byte.
If the S-byte is EOM, the message structure is closed. The last M-
byte of a message is always EOM.
The last M-byte before the EOM is the checksum. It is computed
so that the sum of all M-bytes, including the SOM, the EOM, and
the checksum, is zero.
After pre-parsing is complete, the message packet is ready to be parsed into
structures according to the rules in Sections 3 and 4. The parser code and
subcodes are the second and third M-bytes, directly after the SOM. The data will
start on the fourth (non-indexed data) or fifth (indexed data) M-byte.
Possible pre-parser errors include:
Two SOMs appear without an EOM in between.
HCC occurs in the first two bytes (parser code and subcode).
The byte following HCC is not equal to the 7 LSB’s of a HIPPO
control character.
Control characters appear between message (after EOM but
before the next SOM).
No EOM appears in the first 134 M-bytes.