IBM System/370 145 Manual page 45

2. Data transfer operations for concurrently operating devices on a
block multiplexer channel are interleaved on a first-come. first-
served basis as the desired records become available. Thus.
devices are serviced in the order in which their records become
available. not necessarily in the order in which their channel
programs are initiated.
Block Multiplexer UCW Assignment
The number of UCw's required for -the block multiplexer channels
installed on a Model 145 depends upon the number and types of
I/O
devices in the system configuration. When a system is ordered. the
number of UCw's required must be specified so that a UCW address table
is established and enough co~trol storage is reserved for a pool of the
specified UCW's by the customized disk cartridge sent to the
installation. The maximum nUmber of block multiplexer UCW's possible
per system is 512 (assuming all a['e nonshared).
I/O
devices are grouped into three types for
~he
purpose of UCW
assignment as follows:
• Type 1
each device in the g['oup requires a nonshared UCW
• Type 2 - all devices in the g['oup use the same shared UCW
• Type 3 a UCW is not assigne1cl to. any device in the group and the
devices operate in selector mode (there is no. disconnection during
command-chained operations)
A
UCW (or a subchannel) is reflerred to as nenshared if it is
associated and can be used with. only one device. Examples ef Model 145
devices that should be assigned to a nenshared UCW because they have
block multiplexing capability are:
'. 3330-series disk sterage - eDie UCW per drive in the string is
required; a bleck of eight
UC1~·s
is always assigned.
• 2305 facilities - eight UCw·s per 2305 module are required.
• 2540 Card Read Punch unit - Olne UCW fer the reader and ene for the
punch
• 3505 Card Readers and 3525 Calr:d Punches - ene UCW for each reader
and one for each punch
• 1403 Printers attached to. a 2:821 Central Unit - one UCW per printer
• 3211 Printers - one UCW per plr:inter
A shared UCW can be used by a set of devices, ene device at a time.
A shared UCW generally is assigned. to a centre
I
unit that has multiple
devices attached. only ene of whi(!h can be in eperation at a time.
Devices for the Model 145 that must be assigned to. a Type 3 greup
(cannot have
a
UCW assigned and must operate in selector mode) are
magnetic tape units and direct aC4:::ess devices witheut rotatienal
pesition sensing. such as the 2311, 2321, 2303. 2319, and 2314.
The type and number of
I/O
device greups in a given system
configuration and the addresses
t4~
be assigned to the shared block
multiplexer UCW's are established when a Medel 145 is installed. The
custemer engineer inserts this injEormatien into. a plug card for each
channel and during the IMPL proced.ure the UCW address table is
initialized with the data from the plug cards.
A
Guide to the
IBM
System/370 Modt:!l 145 35