IBM Series 1 User Manual page 78

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Load Characteristics. The information in this
sUbsection specifies a unit load for the channel.
A device adapter or attachment represents a single drop
on the I/O channel. It can represent one or more logical
devices, each with a device address. For a device adapter
to attach to the I/O channel and not restrict the I/O
configuration because of improper loading, it must present a
unit load to the I/O channel. A unit load means that every
specification on each of the drivers and receivers to the
I/O channel has been met or exceeded. A violation of any of
the driver/receiver specifications constitutes a violation
of the unit load requirement. Proper operation of the
channel under these circumstances would then be dependent
upon the particular configuration of attachments on the
channel and consequently would be generally unpredictable.
Pigure 2-24 assigns a classification of driver/receiver
type to I/O channel signal lines. Figure 2-25 depicts and
describes each driver/receiver type in general terms and
establishes the reference current direction for the elements
of the driver/receiver type. Figure 2-26 then specifies the
driver and receivers for each type. Two types of unit loads
are specified 1} a general unit load and 2} a TTL unit load.
IBM attachments present general unit loads to the interface.
Processor I/O Channel 2-63