The parallel channel is one of the oldest interfaces to a mainframe. It was introduced to the mainframe in the 1960s. There might be some still in use today, but newer mainframes no longer support parallel channels.

The parallel channel (also called an S/370 I/O interface) uses two cables called bus and tag. Of course, it is copper media only. A bus cable carries information (one byte in each direction), and a tag cable indicates the meaning of the information on the bus cable. Devices are daisy-chained off of each other to form a string of devices.

The parallel I/O interface is the communication channel path between a physical channel on a mainframe and an I/O control unit (CU). The interface was designed for past mainframes and works compatibly with older and current mainframe architectures.

The maximum data rate of a parallel channel is 4.5 MBps (megabytes per second).