In 360 a channel could connect to a number of control units that each connected to a number of devices. Early on, just one IO could run on a channel; so all oth

In 360 a channel could connect to a number of control units that each connected to a number of devices. Early on, just one IO could run on a channel; so all other devices were locked out until the IO was performed (several details omitted). Early disk IO held the channel until the desired record came under the head. That was after the arm was positioned. 370 brought in rotational position sensing allowing other disk IO to take place. Later on, tapes also accommodated record positioning. Forward Space File came with 360. Remember though that these days tapes are often archived in a virtual tape library physically on disk, but processed with tape channel commands. Back in 360,control units that could handle simultaneous IOs appeared. They were connected to two channels.