Figure 3: Timing Diagram (for the new protocol)
Figure 3 shows the timing diagram from the new acknowledgement protocol. As before, a cell must be presented to the fabric 8 clock cycles after the `frame start' pulse (as shown between lines A and B in figure 3). The receiving device will see the cell some number of clock cycles later (B to C on the figure) - the number being dependent on the depth of the fabric (see document 25: Delay Through the Switch Fabric). The receiver then has 32 clock cycles to decide whether or not it is going to acknowledge the cell (C to E) after which it must retain the state of AckIn until the next `frame start' pulse (E to G) when it must set AckIn high (indicating acknowledgement - as in the period A to C).
The transmitting device should wait 16 clock cycles after it presents the cell to the fabric (B to D) and the look at the AckOut signal coming out of the fabric. If at any point after this and before the end of the cell is put into the fabric (D to F), this signal goes low then the cell has been NACKed otherwise is has been ACKed.
It is valid to hold the AckIn line low after the `frame start' pulse provide it isn't pulled high before the next `frame start' signal. This indicates that any cell presented to the receiving device will be NACKed.
It is also valid to hold the AckIn line high between `frame start' pulses, indicating that any cell presented will be ACKed.