Probably channels. In an IBM mainframe, each I/O device is connected on its own channel, which is actually a separate computer that handles the transfer to/from

Probably channels. In an IBM mainframe, each I/O device is connected on its own channel, which is actually a separate computer that handles the transfer to/from the main CPU. This has been the case going back to the System/360, which is why mainframes are legendary for their transaction throughput and reliability. There's probably a lot of redundancy in the I/O hardware, as they have to be rock solid and hot swappable while the system is running.