___ | ___ | | | IRDY# x \___/ x \___ ___ | | | | TRDY# x x \___ ___ | | | | DEVSEL# \___ ___ | | | | FRAME# \___ _ _ _ _ _ |_ _ |_ |_ |_ CLK _/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ 0 1 2 3 4 5 6 7 8 9 This continues the address cycle illustrated above, assuming a single address cycle with medium DEVSEL, so the target responds in time for clock 3. However, at that time, neither side is ready to transfer data. For clock 4, the initiator is ready, but the target is not. On clock 5, both are ready, and a data transfer takes place (as indicated by the vertical lines). For clock 6, the target is ready to transfer, but the initiator is not. On clock 7, the initiator becomes ready, and data is transferred. For clocks 8 and 9, both sides remain ready to transfer data, and data is transferred at the maximum possible rate (32 bits per clock cycle). In case of a read, clock 2 is reserved for turning around the AD bus, so the target is not permitted to drive data on the bus even if it is capable of fast DEVSEL. Fast DEVSEL# on reads A target that supports fast DEVSEL could in theory begin responding to a read the cycle after the address is presented. This cycle is, however, reserved for AD bus turnaround. Thus, a target may not drive the AD bus (and thus may not assert TRDY#) on the second cycle of a transaction. Note that most targets will not be this fast and will not need any special logic to enforce this condition. Ending transactions Either side may request that a burst end after the current data phase. Simple PCI devices that do not support multi-word bursts will always request this immediately. Even devices that do support bursts will have some limit on the maximum length they can support, such as the end of their addressable memory. Initiator burst termination The initiator can mark any data phase as the final one in a transaction by deasserting FRAME# at the same time as it asserts IRDY#. The cycle after the target asserts TRDY#, the final data transfer is complete, both sides deassert their respective RDY# signals, and the bus is idle again.