Specifications

Operations Manual, Redundant System Controllers 209351 REV B 79
7.2.3 Trailer Packet
The trailer component contains only one byte called the Frame Check Sequence. This
field provides a checksum during packet transmission. See Figure 7-7.
7.2.3.1 Frame Check Sequence
This value is computed as a function of the content of the destination address, source
address and all Command Data Substructure bytes. In general, the sender formats a
message frame, calculates the check sequence, appends it to the frame, then
transmits the packet. Upon receipt, the destination node recalculates the check
sequence and compares it to the check sequence embedded in the frame. If the check
sequences are the same, the data was transmitted without error. Otherwise an error
has occurred and some form of recovery should take place. In this case the amplifier
will return a packet with the “Bad Checksum” error code set. Checksums are generated
by summing the value of each byte in the packet while ignoring any carry bits. A simple
algorithm is given as:
Chksum=0
FOR byte_index=0 TO byte_index=packet_len-1
Chksum=(chksum+BYTE[byte_index]) MOD 256
NEXT byte_index
HEADER
(4 bytes)
DATA
(6-32 bytes)
TRAILER
(1 byte)
Frame Check
Checksum (1 byte)
Figure 7-7: Trailer Sub-Packet