Switching between Serial Differential Inputs

Question: - How long does it take for HOTLInk II receiver to switch between the serial differential inputs from IN1(+/-) to IN2(+/-)  - How long does it take for HOTLInk II receiver to switch between the serial differential inputs from IN1(+/-) to IN2(+/-)  - IN1(+/-) is coming from an external link. OUT2(+/-) loop back's to IN2(+/-). Two channels have the same freq. Before the external link is established, we select IN2. We have plenty of time for the receiver to lock in IN2. When the external link on IN1 becomes OK, we switch the receiver to IN1. How long does it take for the clock recovery to lock in? Does it still take 376K UI?

 

Answer:

In the above scenario in which you are switching the HOTLink II serial differential inputs from Input 2 [IN2(+/-)]to Input 1 [IN1(+/-)],  assuming that the data on both IN1(+/-) and IN2(+/-) are within the 1500 ppm frequency offset as described in the HOTLink II datasheet, the phase-locked loop (PLL) will acquire and lock to the new data stream within a few character times. The exact time required involves statistical probabilities related to phase, frequency, and jitter, and cannot be exactly predicted. If the PLL frequency has been moved to its offset limits, it may take more than 60-70 characters before the PLL locks to the good data. If the incoming datastream on the other input is within the ppm offset so that the PLL can track within its frequency offset limits, the PLL will reacquire lock in a few characters  after a good data stream reappears.

The larger problem facing a system protocol that allows switching of serial data streams is character synchronization (byte-framing). After the data-stream has been switched, it must be reframed. This requires that a K28.5 (or two K28.5s within five characters if multi-byte framing is enabled) must be received. The time that elapses before this happens depends on the system protocol and the timing of the data input switch. Correct data may not come out of the HOTLink II Receiver for hundreds of character times due to a lack of framing, regardless of speed of phase/ data acquisition.

Essentially, you will be able to switch from IN2(+/-) to IN1(+/-) within anywhere from 10 character cycles to 70 character cycles as explained above. The essential criteria for the switching time to be within anywhere from 10 character cycles to 70 character cycles and not 376K UI is to ensure that the switching time between IN2(+/-) to IN1(+/-) is instantaineous with good data streams coming on both the inputs at the time of switching the serial differential inputs.If this criteria is not met then the switching time is going to be 376K UI. You will have to transmit framing characters at the beginning of the data stream on IN1 so that the receiver can frame to the proper character boundaries.