The basic steps in flow control counter initialization

1. The transmitter in Device 1 initializes its Posted Request (CMD) counter to 0 at reset (all transmit counters reset = 0). It then waits for the receiver on the other side to update this counter with the starting buffer depth available (this will be the maximum depth the receiver supports).

2. Device 2 loads its receiver Posted Request counter = 5 (its maximum).

3. Device 2 then sends two NOP packets which carry this buffer availability information: the first NOP has a 11b (3) in the Post CMD field (Byte 1, bits 0,1 above), and the second NOP has a 10b (2) in this field. Total = 5.

4. Upon receipt of these two NOPs, the Device 1 has updated its transmit counter, first by three then again by two. It now has 5 "credits" available for sending Posted Request packets — representing five separate Posted Requests which may be initiated.

5. Having sent the NOPs, the Device 2 RCV counter is now at 0, and will remain that way until additional packets are received, processed, and move out of the buffer, thereby creating new entries.

Note that this process will be repeated for each of the six required flow control buffers; it will also be done for the six isochronous flow control buffers if they are supported. In the NOP packet format (see above), six transmit registers can be updated at once using the six fields provided. The Isoc bit (Byte 2, bit 5) would be set if the NOP update was to be applied to the isochronous flow control buffer set.