The PRBS test modes bypass the JESD204C transport layer and link layer and are therefore neither scrambled nor encoded. These test modes produce pseudo-random bit streams that comply with the ITU-T O.150 specification. These bit streams are used with lab test equipment or logic devices that can self-synchronize to the bit pattern. The initial phase of the pattern is not defined since the receiver self synchronizes.
The sequences are defined by a recursive equation. For example, Equation 12 defines the PRBS7 sequence.
Table 8-37 lists equations and sequence lengths for the available PRBS test modes where ⊕ is the XOR operation and y[n] represents bit n in the PRBS sequence. The initial phase of the pattern is unique for each lane.
|PRBS TEST MODE||SEQUENCE||SEQUENCE LENGTH (bits)|
|PRBS7||y[n] = y[n – 6]⊕y[n – 7]||127|
|PRBS9||y[n] = y[n – 5]⊕y[n – 9]||511|
|PRBS15||y[n] = y[n – 14]⊕y[n – 15]||32,767|
|PRBS23||y[n] = y[n – 18]⊕y[n – 23]||8,388,607|
|PRBS31||y[n] = y[n – 28]⊕y[n – 31]||2,147,483,647|