Paradise Datacom 4 RU Chassis, Руководство по эксплуатации

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86 201627 REV X 4 RU SSPA Chassis Operations Manual
7.2.3 Trailer Packet
7.2.3.1 Frame Check
The trailer component contains only one (1) byte called the Frame Check Sequence,
shown in
Figure 7-7
.
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 Check-
sum” 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=(BYTE[byte_index]) MOD 256
NEXT byte_index
7.2.4 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. Gen-
erally, there will be no spacing between characters in replies generated by unites. 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 re-
quired for the controller to detect a “Line Cleared” condition with half duplex communi-
cations. Maximum controller respond time is 200 ms.
HEADER
(4 bytes)
DATA
(6-32 bytes)
TRAILER
(1 byte)
Frame Check
Checksum (1 byte)
Figure 7-7: Trailer Sub-Packet