Editing IBM System/360 (section)

===={{anchor|BMuxChan}}Block multiplexer channel====
IBM first introduced a new type of I/O channel on the Model 85 and Model 195, the [[IBM 2880|2880]] block multiplexer channel, and then made them standard on the [[System/370]]. This channel allowed a device to suspend a channel program, pending the completion of an I/O operation and thus to free the channel for use by another device. A block multiplexer channel can support either standard 1.5&nbsp;MB/s connections or, with the 2-byte interface feature, 3&nbsp;MB/s; the latter use one tag cable and two bus cables. On the [[IBM System/370|S/370]] there is an option for a 3.0 MB/s data streaming<ref name=OEMI>{{cite book
 | title     = IBM System/360 and System/370 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information 
 | section   = Data-Streaming Feature
 | pages     = 3-4&ndash;3-7
 | edition   = Tenth
 | date      = February 1988
 | publisher = IBM
 | url       = http://bitsavers.org/pdf/ibm/370/channel/GA22-6974-9_360_370_IO_Interface_Channel_to_Control_Unit_OEM_Information_Feb88.pdf
 | mode      = cs2}}</ref> channel with one bus cable and one tag cable.

The initial use for this was the 2305 fixed-head disk, which has 8 "exposures" (alias addresses) and rotational position sensing (RPS).

Block multiplexer channels can operate as a selector channel to allow compatible attachment of legacy subsystems.<ref>{{cite book
 | title       = System/370 Principles of Operation
 | id          = GA22-7000-4
 | url         = http://www.bitsavers.org/pdf/ibm/370/princOps/GA22-7000-4_370_Principles_Of_Operation_Sep75.pdf
 | publisher   = IBM
 | date        = September 1975
 | page        = 189
 | access-date = December 30, 2015}}</ref>