This is similar to PCI Retry Protocol, however in this case part of the data is already transferred and the transaction stops midway and thus disconnected.

Procedure

1. Master initiates a transaction

   • Example: North Bridge (master) starts a burst read from the Ethernet device (target).

2. Target claims the cycle and transfers some data

   • Unlike Retry, at least one doubleword is transferred successfully.

3. Target can’t continue

   • Example: Ethernet device runs out of buffer data.
   • It has two choices:
     • Insert wait states → if it just needs a few cycles to get more data.
     • Disconnect (STOP#) → if it needs more than 8 clocks.

4. How disconnect works

   • The target asserts STOP# during the data phase.
   • This tells the master: “End this burst now — you got some data, but I can’t continue.”
   • The master ends the cycle early, keeping track of the address where it left off.

5. Master after disconnect

   • Waits at least 2 clocks.
   • Re-arbitrates for the PCI bus.
   • When granted, resumes the transaction from the disconnected address (not from the beginning).

6. Bus efficiency

   • Disconnect prevents the PCI bus from being tied up with long wait states.
   • While the master waits, the arbiter can let other masters use the bus.