IBM 1 Series Manual page 66

Unit-Load Characteristics
This section describes the specifications for a unit load on 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 notrestrict
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 each I/O channel line 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 on the
particular configuration of attachments on the channel and, consequently,
would be generally unpredictable.
Figure 2-24 showstheclassifications of driver/receiver types to I/O
channel signal lines. Figure 2-25 illustrates each driver/receiver type in
general terms and establishes the reference current direction for the
elements of each type. Figure 2-26 showsthe drivers and receivers for
each type. Two types of unit loads are specified: a general unit load and a
TTL unit load. IBM attachments present general unit loads to the
interface.
Processor I/O Channel 2-55