Abstract: A peripheral subsystem includes a peripheral control unit coupled to a plurality of peripheral devices having different read/write speeds. In response to a command from a central processing unit, the peripheral control unit selects one of the devices designated in the command, and polls the device to obtain an indication of the read/write speed of that device. From this speed indication and a density designation included in the command, the peripheral control unit produces a read/write pulse constant which defines the rate at which data is to be read from or applied to the selected peripheral device. Data is then applied to or read from the selected device at the rate defined by the constant.