Editing IBM System/370 (section)

==I/O evolutions==

===I/O evolution from original S/360 to S/370===
{{main article|IBM System/360#Channels}}

The block multiplexer channel, previously available only on the 360/85 and 360/195, was a standard part of the architecture. For compatibility it could operate as a selector channel.{{sfn|S370|loc=[http://bitsavers.org/pdf/ibm/370/princOps/GA22-7000-10_370_Principles_of_Operation_Sep87.pdf#page=356 Programming Note]|p=13-5}} Block multiplexer channels were available in single byte (1.5 MB/s) and double byte (3.0 MB/s) versions.

===I/O evolution since original S/370===

As part of the DAT announcement, IBM upgraded channels to have Indirect Data Address Lists (IDALs). a form of I/O MMU.

Data streaming channels had a speed of 3.0 MB/s over a single byte interface, later upgraded to 4.5 MB/s.

Channel set switching allowed one processor in a multiprocessor configuration to take over the I/O workload from the other processor if it failed or was taken offline for maintenance.	 
	 
System/370-XA introduced a channel subsystem that performed I/O queuing previously done by the operating system.

The System/390 introduced the [[ESCON]] channel, an [[optical fiber]], [[half-duplex]], [[Serial communications|serial]] channel with a maximum distance of 43 kilometers. Originally operating at 10 Mbyte/s, it was subsequently increased to 17 Mbyte/s.

Subsequently, [[FICON]] became the standard IBM mainframe channel; FIbre CONnection (FICON) is the IBM proprietary name for the [[ANSI]] FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for [[Fibre Channel]] (FC) protocol used to map both IBM's antecedent (either [[ESCON]] or parallel Bus and Tag) channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure at data rates up to 16 Gigabits/sec at distances up to 100&nbsp;km. [[Fibre Channel Protocol]] (FCP) allows attaching SCSI devices using the same infrastructure as FICON.