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Operations Manual, HPA2, Compact Outdoor SSPA 205486 REV F 91
10.3 Trailer Packet
10.3.1 Frame Check
The trailer component contains only one (1) byte called the Frame Check Sequence, shown
in Figure 10-7.
This field provides a checksum during packet transmission. 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
10.4 Timing issues
There is no maximum specification on the inter-character spacing in messages. Bytes in
messages to amplifier units may be spaced as far apart as you wish. The amplifier will
respond as soon as it has collected enough bytes to determine the message. Generally, there
will be no spacing between characters in replies generated by units. The maximum length of
the packet sent to the amplifier node should not exceed 64 bytes, including checksum and
frame sync bytes. Inter-message spacing, must be provided for good data transmission. The
minimum spacing should be 100 ms. This time is required for the controller to detect a “Line
Cleared” condition with half duplex communications. Maximum controller respond time is
200 ms.
HEADER
(4 bytes)
DATA
(6-32 bytes)
TRAILER
(1 byte)
Frame Check
Checksum (1 byte)
Figure 10-4: Trailer Sub-Packet