"When the memory controller wants to read data from a memory module, it sends a DQS (data strobe) signal to the target module. It takes some time before the target memory module actually fires up the DQS data strobe in return to send the data.
During that time, it is possible for signal noise to register on the memory controller's DQS receiver and misinterpreted as actual data, resulting in data corruption. To prevent such an occurrence, the memory controller does not immediately enable its DQS (data strobe) receiver. Instead, it waits for a prescribed period of time before activating the DQS receiver for the memory module's actual DQS signal.
This prescribed period of time is known as the Read Preamble..."
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