IBM System/370 Guide

ROTATIONAL POSITION SENSING AND MULTIPLE REQUESTING
RPS is a standard feature of the 2835 control unit, as is the other
new capability called multiple requesting, which allows up to eight
channel programs to be active concurrently on each of the two 2305
modules that can be attached to the control unit. In other words,
a 2305 module can be viewed as eight logical devices, although
physically it is only one device.
As described previously, rotational position sensing and block
multiplexing permit a direct access device to disconnect during SET
SECTOR
operations. These facilities, used in conjunction with the
multiple requesting feature, permit concurrent operations to take place
on each 2305 module in a facility. Thus, the effective data rate of
a module can be increased substantially.
The multiple requesting capability is implemented by associating
eight logical device addresses (0-7) with a 2305 module. Each logical
device address also is assigned to a specific subchannel of the block
multiplexer channel and a specific register (0-7) in the 2835 control
unit. When a channel program is initiated, it is associated with an
available logical device address (by the I/O supervisor). (Logical
device addresses are not permanently assigned to specific tracks or
data sets in a module.) When the SET SECTOR command is issued, the
specified sector number is stored in the register in the control unit
that is assigned to the logical device address being used for its
channel program. Then the control unit disconnects from the channel.
At this point, another channel program with a SET SECTOR command
can be accepted by the channel and control unit (assuming neither is
busy). This channel program will be initiated using another available
logical device address and its assigned control unit register. This
process can be repeated for up to eight SET SECTOR commands, so that
eight channel programs can be executing concurrently per 2305 module.
Whenever the control unit is not executing a command or is not
otherwise busy, it monitors the rotational position counter in the
module that is being incremented each sector time period. When the
sector number in the counter of a module compares equal with the sector
number stored in one of the registers in the control unit and the
channel is free, the control unit reconnects and resumes execution
of the suspended channel program associated with the logical device
address assigned to the control unit register that compared equal (see
Figure 20.15.3).
It should be noted that one 2305 module requires eight logical
device addresses, each of which requires one subchannel of the 64
available with a 2880 Block Multiplexer Channel. Since each 2835
control unit can have two modules, one 2305 facility uses 16 device
addresses and 16 nonshared subchannels of a 2880 Block Multiplexer
Channel. In addition, the 2305 Model 1 requires the presence of the
Two-Byte Interface feature on the 2880 channel to which it is attached.
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IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English