Abstract: Data is transferred between a channel and a control unit in interlocked mode or in data streaming mode over a conventional parallel bus and a serial link that permits the control unit and the channel to be located farther apart than the length of the parallel bus. The serial link carries frames for the rise of the data transfer tags but not their fall, and these tags on the parallel bus are dropped by two circuits that connect the serial link to the parallel bus. In data streaming mode, one data transfer tag is dropped when the next data transfer tag is received on the serial link. The circuit at the control unit end of the serial link detects a pause in the tags to begin an operation to drop the last data transfer tags and both circuits respond to other conditions to drop the last data transfer tags according to the protocol of the parallel bus.