Paradise RA 5785 User Manual download pdf (Page 65)

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=(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

1 2 ... 60 61 62 63 64 65 66 67 68 69 70 ... 81 82

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Paradise RA 5785 User Manual Page 65