Teledyne LeCroy Line Code and Symbolic Decoders - Manual (Page 8)

Line Code and Symbol Decoders
6 922663 Rev A
D ECODE D IALOG
These settings refine the Basic dialog bit-level decoding so that some transitions are skipped, and
subsequent bits are grouped into words that can be interpreted LSB first or MSB first.
Data Mode - This selection drives the level of decoding. The default is Bits, and we recommend that you
use this setting initially as you continue to tune the bit-level decoding. It can also be set to Words.
Decoding into Bits
After selecting Data Mode Bits, configure:
First Trans. Used (FTO)- Manchester and NRZ schemes may utilize a preamble, a synchronization
sequence, or a voluntary violation. The FTO begins decoding after this period/event, when the real Data
payload starts, avoiding the intricacies of dedicated protocols in the initial segment of the packet. The
setting specifies on which transition the bit-slicing algorithm should start and can range from 0 to 400 in
steps of 1. In many cases this value is likely to be set to 0. However, there are cases when the value
needs to be non-zero:
a. when the initial transitions carry no information at all and must be skipped;
b. when the first transitions are at another rate then the main bitrate of the signal;
c. for performance reasons. When decoding long Bursts, it is possible that the initial segment of
the Burst does not carry pertinent information and its processing could be skipped, making
the decode relevant.
Bit Stretch Tol(erance) - The Manchester bit-slicer hops from midbit to midbit. However, due to
hardware or signal propagation issues, the midbits might not be perfectly equidistant. In this case, Bit
Stretch Tolerance can be manually increased to attempt to decode jittery signals. Conversely, it can be
decreased until the decoding starts showing anomalies (e.g., 2 or 3 instead of 1 or 0 in the Data) to
assess the stability of the midbit distribution. Continue to tune Bit Rate and Bit Stretch Tolerance until
Bit Stretch Tolerance is less than 5 % without changing the output of the decoder observed in the table.
Decoding into Words
Select the Data Mode Words. In this mode, all of the fields on the Decode dialog are activated. These
settings apply to both Manchester and NRZ Decoders.
In addition to the above settings, configure:
Viewing - How to view the decode data, in Binary, Hexadecimal (Hex), or ASCII format.
Bit Order - Whether to convert the word with the Most Significant Bit (MSB) first or the Least
Significant Bit (LSB) bit first.
Sync Bits - At which bit the packetizing should start. The algorithm will start at “Sync Bits” and group bits
into the three fields “PrePad”, “Data Bits” and “PostPad”. Then it will restart with the “PrePad” of the
next sequence. There can be 0 to 100 Sync Bits.
PrePad - The “PrePad Bits” are used to group information preceding the Data Bits. There might be 0 to
32 PrePad Bits. PrePad bits might be used to group Address bits, Preambles, Sub address, etc.
Data Bits - The number of bits grouped together to form a single word. The “Bits per Words” can take
values from 1 to 32 in steps of 1.
PostPad - The “PostPad Bits” are used to group information following the Data Bits. There can be 0 to 32
PostPad Bits. Post Pad bit might be used to represent a CRC, a checksum, a Value or any other protocol
construct.