Operations Manual: HPAC-100/125/150/200/250-RM SSPA
3-19
3 Operation of Stand-Alone Unit
Frame Check Sequence: The checksum field is a 1 byte field that provides a
parity check during packet transmission. This value is computed as a function of
the contents of the packet length, destination address, source address, com-
mand field, and data field. In general, the sender formats a message frame,
calculates the checksum, appends it to the frame, and then transmits the packet.
Upon receipt, the destination node re-calculates the check sequence and com-
pares it to the check sequence embedded in the frame. If the check sequences
are the same then the data was transferred without errors, otherwise, an error
has occurred and some form of recovery must take place.
Checksums are generated by summing the value of each byte in the packet
while ignoring any carry bits. A simple algorithm follows:
chksum = 0
FOR (byte_index = 0) TO (byte_index = ( packet_length - 1 ))
chksum = ( BYTE [ byte_index ] ) MOD 256
NEXT byte_index
Commands
Packets are categorized by the function they serve, commands, queries, or responses.
All essentially have the same format. The M&C system sends commands and queries
to the SSPA system while the SSPA system sends response messages to the M&C.
Only one command can be sent at a time and the M&C system should wait until a valid
response is returned from the SSPA system before sending another. Commands and
queries can be directed to the SSPA system controller or to an individual SSPA unit.
Steering a command in this way is accomplished by selecting the appropriate unit ID.
Table 3-22 contains a list of the valid commands the M&C can issue. The SSPA re-
sponds with an ACK to indicate that the command was received and implemented or a
NAK to indicate that the command was not implemented.
Table 3-21: Valid M&C commands.
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Summary of Contents for HPAC-100-RM
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