Rockwell Automation 1747-ASB Remote I/O Adapter - Manual (Page 18)

Publication 1747-UM006B-EN-P - June 2003
1-4 Overview
Scanner I/O Image Division
The scanner allows each adapter to use a fixed amount (user defined)
of the scanner's input and output image. Part of the processor's image
is used by local I/O, the other portion is used by the scanner for
remote I/O. For a PLC-5, logical rack 0 is dedicated for local I/O.
The scanner's remote I/O image is divided into logical racks and
further divided into logical groups. A full logical rack consists of eight
input and eight output image words. A logical group consists of one
input and one output word in a logical rack. Each logical group is
assigned a number from 0 to 7. The number of racks available for
remote I/O depends on the scanner you are using.
The scanner image also contains the image of each adapter on the
RIO link. The adapter is assigned a portion of the scanner image,
which is referred to as the adapter image.
Crossing Logical Rack Boundaries
Adapter image size is expressed in an even number of groups. For
example, the 1747-ASB module image can be any size between 2
logical groups and 32 logical groups (4 logical racks), in 2 logical
group increments.
If the adapter's image size is greater than 8 logical groups, the image
crosses logical rack boundaries. If an adapter's image size is less than
8 logical groups, it too can cross a logical rack boundary depending
upon the starting logical group number. The significance of crossing
logical rack boundaries is discussed in the next section.
Local I/O
Remote I/O
(Scanner
Image)
Processor I/O
Image
Scanner I/O
Image
Adapter
Image
Logical Rack 1
Logical Rack 2
Logical Rack 3
Logical Rack 0
Logical Group 0
Logical Group 7