Paradise 205486 REV F Trailer Packet, Timing issues, Frame Check, 4:Trailer Sub-Packet, Data

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.

HEADER (4 bytes)

DATA

(6-32 bytes)

TRAILER

(1 byte)

Frame Check

Checksum (1 byte)

Figure 10-4: Trailer Sub-Packet

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.

Operations Manual, HPA2, Compact Outdoor SSPA 205486 REV F 91