The PDP-11 computer adopts a more simple approach than the general bus presented in the previous page. Here, the memory and I/O busses are combined into one bus (called the Q-BUS in the case of the PDP-11).
The physical lines (the wires) in the bus differ with respect to their function. Some form a continuous line from one end of the bus to the other. Others simply form point-to-point connections from one I/O controller (device) to the next.
The arrangement of line X means that all devices have access to all signals on that line.
The arrangement of line Y means that each device is capable of deciding whether it wishes to pass a bus signal down the bus to the next device. It could decide to "grab" the signal on the line and not pass it along, or to simply pass it unaltered to the next device.
