resistors at each end of the bus, typically on a terminator card. Type 2 lines are selectively propagated by each card to the next slot – if the card wants to keep the request grant it will assert the SACK line and not propagate the request to the next slot. If a slot is empty, it is necessary to install a "grant continuity card" in the slot to propagate the four type 2 signals to the next card. Type 3 signals are generated by the power supply and have only a single sender. They warn the devices on the bus when the power is about to fail, so those devices can execute an orderly shutdown, and disable operations to prevent spurious writes. The two control lines (C0 and C1) allowed the selection of four different data transfer cycles: DATI (Data In, a read) DATIP (Data In/Pause, the first portion of a Read-Modify-Write operation. A DATO or DATOB operation completes this.) DATO (Data Out, a word write) DATOB (Data Out/Byte, a byte write) During an interrupt cycle, a fifth style of transfer was automatically invoked to convey an interrupt vector from the interrupting device to the interrupt-fielding processor. References Computer buses DEC hardware PDP-11