PCI Express System Architecture

ACK/NAK Protocol Summary

Refer to Figure 5-3 on page 212 and the following subsections for a summary of the elements of the Data Link Layer.

Transmitter Side

Non-Error Case (ACK DLLP Management)

• Unless blocked by the Data Link Layer, the Transaction Layer passes down the Header, Data, and Digest information for each TLP to be sent.

• Each TLP is assigned a 12-bit Sequence Number using current NEXT_TRANSMIT_SEQ count.

• A check is made to see if the acceptance of new TLPs from the Transaction Layer should be blocked. The transmitter performs a modulo 4096 subtraction of the ACKD_SEQ count from the NEXT_TRANSMIT_SEQ count to see if the result is >= 2048d. If it is, further TLPs are blocked until incoming ACK/NAK DLLPs render the equation untrue.

• The NEXT_TRANSMIT_SEQ ...