A PCI-PCI bridge is composed of two physically different controllers, i.e.,
a primary PCI serial transfer controller arranged at a PC, and a secondary
PCI serial transfer controller arranged at a docking station. In data
transfer between these controllers, preliminary transmission of block data
having a block ID assigned thereto and a return of ACK having the block ID
assigned thereto are performed asynchronously. The transmission party can
determine which data item is the last the receiving party has received by
employing a block ID assigned to NAK. When NAK is received from the
receiving party, re-transmission from a block waiting for NAK is started.
In this manner, data required to transmit a bus transaction between buses
can be transferred accurately and speedily between two controllers.
