Calculating The Code Rate; Framing - Advantech AMT-73 Installation And Operation Manual

In some cases, where the uncoded block size does not exactly line up on a byte-boundary, a
number of padding bits are inserted after the CRC. They are set to zero.
Once the CRC is inserted, the uncoded data is then always scrambled. This does not add or
remove data from the stream, merely exclusive-ORs it with a pseudo-random binary sequence that
can be removed in the decoder.

Framing

9.2.2
In order that the decoder can detect the start of an encoded data block, it is necessary to insert
framing marks around (and within) the encoded data. It is insufficient to simply mark the start of an
encoded block, given the large block sizes. Therefore, additional framing marks are inserted into the
encoded data at regular intervals to aid in this process. The start of frame marks is distinguished by
being the inverted form of the sync mark. The encoded 2D block looks thus:
In this case, there are three evenly spaced sync marks per block, the first of which is inverted to
indicate the start of the frame.
For all codes, the sync mark is 8-bits long (with value 0xE2), with 128 bytes of encoded data
between them. The number of marks varies with the block size: for 16384 bit codes, the block
contains 16 marks (1 inverted); for the 4096-bit code, there are only four marks (1 inverted as
before).

9.3 Calculating the Code Rate

The code rate is the ratio of raw input data size (without CRC etc.) over the total encoded and
framed data size.
The total encoded data size in bits E is dependent on the block size and the
=
framing:
E TPCBlockSi
bits
For 16k blocks, the basic block size is 16384 bits; to which must be added 16 bytes (128 bits) of
sync mark, making a total of 16512 bits.
For 4k blocks, the basic block size is 4096 bits with 4 bytes (32 bits) of sync mark, making a total of
4128 bits.
The unencoded data size is harder to calculate. First, the uncoded block size is calculated from the
product of uncoded elements of the code. Then, deduct 32 bits for the CRC. It is then necessary to
determine if any padding is required, since the raw data must be in bytes.
AMT-70/AMT-73/AMT-75 Installation and Operation
ISM
Uncoded Data
FSM
CRC
FSM
+
ze ( NumberOfSy ncMarks
×
) 8
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