Creating More Than One Logical Rack Device - Allen-Bradley 1747-SN User Manual

Crossing Logical Rack Boundaries - Example 1
Scanner Input or Output Image
Bit Number (Decimal) 15 8
Group 0
Group 1
Group 2
Logical
Group 3
Rack 0 Group 4
Group 5
Group 6
Group 7
Group 0
Group 1
Group 2
Logical
Group 3
Rack 1
Group 4
Group 5
Group 6
Group 7
Adapter image is 12 logical groups in size and
crosses a logical rack boundary due to its size.
adapter is configured to
start at Logical Rack 0,
Logical Group 0, and uses
14 words of I/O image.
Note that two RIO discrete
transfers are required for
the scanner to update the
adapter image containing
Scanner Configuration and Programming
7 0
Adapter
Image

Creating More than One Logical Rack Device

RIO discrete transfers occur on a logical device basis, not on an
adapter basis. A logical device is any portion of a logical rack that is
assigned to a single adapter.
When the scanner image assigned to an adapter is more than one
logical device, the scanner sees the single physical device as multiple
logical devices on the RIO link. The scanner communicates with
each logical device independently, even if the logical devices are all
assigned to one adapter. If a physical device image is more than one
logical device, the following is true:
The scanner does not update all of the adapter image at the same
time. The number of logical devices determines the number of
RIO discrete transfers that are needed to update the entire adapter
image.
The adapter may receive different communication commands for
each logical device. In this case, the adapter decides which
command it responds to.
Bit Number (Decimal)
In this example the
Logical
Rack 0
Logical
two logical devices.
Rack 1
Crossing Logical Rack Boundaries - Example 2
Scanner Input or Output Image
Bit Number (Decimal) 15
Group 0
Group 1
Group 2
Logical
Group 3
Rack 0 Group 4
Group 5
Group 6
Group 7
Group 0
Group 1
Group 2
Logical
Group 3
Rack 1
Group 4
Group 5
Group 6
Group 7
Adapter image is 6 logical groups in size and crosses a logical
rack boundary due to its starting logical group number.
Scanner Input or Output Image
15 8 7
Group 0
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Group 0
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Publication 1747 6.6 - July 1996
4–11
8 7 0
Adapter
Image
0
Logical
Device
Adapter
Image
Logical
Device