Paradise Datacom RA 5785, Руководство по эксплуатации

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Operations Manual, HPA3, Mini Compact Outdoor SSPA 208143 REV - 65
8.1.3 Trailer Packet
The trailer component contains only one byte called the Frame Check Sequence. This field
provides a checksum during packet transmission. See
Figure 8-5 .
8.1.3.1 Frame Check
This value is computed as a function of the content of the destination address, source ad-
dress 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 ignor-
ing 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
8.1.4 Timing issues
There is no maximum specification on the inter-character spacing in messages. Bytes in mes-
sages 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 8-5: Trailer Sub-Packet