IBM System/370 Manual page 151

channel contains the hardware required to control its I/O operations
(channel registers, local storage, control functions, buffers, etc.).
A channel interferes with the Model 165 CPU if the CPU accesses a
logical memory that is busy because of a channel operation. Contention
between channels and the CPU for processor storage is reduced
drastically by the use of high-speed buffer storage, which eliminates
CPU to processor storage fetches for approximately 95~ of the fetches
required.
The standard instruction set also includes a new I/O instruction
called HALT DEVICE. This instruction is specifically designed to stop
an I/O operation on a particular device on a multiplexer channel without
interfering with other I/O operations in progress on the channel.
HALT DEVICE should always be used instead of HALT I/O to stop an I/O
operation on a 2880 Block Multiplexer channel.
The 2870 Multiplexer can control concurrent execution of 192 slow-
speed to medium-speed devices, one with each of its 192 subchannels.
Depending on the channel priority assigned, the 2870 can support a
data rate of up to 110 KB. The maximum aggregate byte data rate of
the 2870 Multiplexer subchannels is reduced
by
the inclusion of one
or more selector subcbannels, each of which can have up to 16 I/O
devices attached. Each of the first three selector subchannels included
can operate at a 180 KB rate, can handle one burst operation at a time,
and reduces the aggregate byte rate of the multiplexer interface by
10 KB to 25 KB depending on the priority of the 2870 and the total
I/O configuration. The fourth selector subchannel can operate at a
rate of 100 KB and further reduces the maximum aggregate rate of the
multiplexer subchannels by 14 KB. If two 2870 Multiplexer Channels
are installed, the second 2870 can have only two selector subchannels.
The 2860 and 2880 channels permit attachment of a wide variety of
high-speed I/O devices to the Model 165. The 2860 Selector Channel
handles data rates of up to 1.3 megabytes, while the 2880 Block
Multiplexer can handle a 1.5-megabyte rate, which permits attachment
of the 2305 Model 2 facility. The Two-Byte Interface optional feature
can be installed on a 2880 to double its data transfer rate capability
to 3.0 megabytes so that the 2305 Modell facility can be attached.
The Model 165 user has more flexibility than the Model 65 user when
configuring channels. On a Model 165, channel address and channel
priority are not related, as they are on the Model 65. Any channel
address (0 through 11) can have any channel priority (1-12) assigned.
Thus, the Model 165 user can assign channel priority by channel type
in order to achieve the desired aggregate channel rate and insure that
the highest speed devices are assigned the highest channel priorities.
The maximum speed of an I/O configuration possible on a Model 165
with minimal overrun exposure is a function of the I/O devices used,
the channel types installed, the channel priority assignments, and
the types of channel programs operating concurrently at any particular
instant. Examples of configurations that will operate on the Model
165 follow.
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