Agilent Technologies J-BERT N4903B, User Manual, Page: 69 / 572

How the Serial BERT Generates Memory-Based Patterns
The basic concept of how the Serial BERT generates memory-based patterns is
relatively simple: The Serial BERT has 32Mbits of internal RAM memory used to
store data patterns. The RAM is organized in 64k of 512-bit blocks.
During test run, the pattern generator reads this memory bit by bit and generates
the corresponding voltage at the output port (1=high, 0=low). The error detector
compares the incoming signal voltage (=data bits) with the bits in the memory and
counts the errors.
The Serial BERT has actually three RAM memory locations where patterns are
stored: in the Pattern Editor memory, in the pattern generator, and in the error
detector. When you modify patterns with the pattern editor, the patterns in the
pattern generator and error detector are not automatically updated.
For this, you need to write the pattern to the pattern generator and/or the error
detector. There the pattern is saved in the RAM of the respective module.
Why the Serial BERT Repeats Memory-Based Patterns
To generate output signals, the Serial BERT can send only complete 512-bit blocks
of data. To maintain predictability of the output signal, it is necessary to send
complete patterns. This applies to user patterns, alternating patterns, memory-
based PRBN patterns, mark density patterns, and zero substitution patterns.
Therefore, when the Serial BERT loads a pattern into its internal memory, it repeats
the pattern as necessary until the pattern reaches a block boundary. For example,
if a 1023-bit pattern is loaded, this is repeated in the RAM 512 times, and thus
occupies 1023 blocks. Or, if a 192-bit pattern is loaded, this is repeated 8 times,
and occupies 3 blocks, as indicated in the following figure. This applies to all
memory-based patterns available to the Serial BERT.
During test run, the Serial BERT always sends out all blocks of a pattern. In the
case of the 192-bit pattern mentioned above, the pattern is repeated 8 times when
sent out (in effect, making it a 1536-bit pattern).
The same behavior of repeating a pattern to fill up complete blocks also applies to
alternating patterns. In this case, each half of the pattern is loaded into a separate
location in the RAM. When an alternating pattern is generated, first all blocks of
pattern A are sent, then all blocks of pattern B are sent, and so on.
Setting up Patterns
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Agilent J-BERT N4903B High-Performance Serial BERT 69
How the Serial BERT Handles
Alternating Patterns