Rockwell Automation 6008-SI IBM PC I/O SCNNR 6008-SI - Manual (Page 16)

Chapter 2
I/O Scanner Concepts
2-8
Because our interrupt handler takes care of all details of the global RAM,
you don’t have to be concerned with the bits and bytes. You should know
that the global RAM contains two kinds of information:
the I/O image tables are comprised of an output image image table and
an input image table. Every time control of the global RAM is
transferred, the interrupt routine copies new inputs to your program’s
data area from the global RAM and new outputs from your program’s
data area to the global RAM.
a mailbox area is where the host can send commands to the scanner and
the scanner sends back confirmations and data. A list of commands is
given later in this chapter.
Here is the path followed by a discrete input bit:
An external device causes an input of a discrete input module to turn
“on.”
When next asked by the adapter, the input module reports the new input
information. The adapter updates its internal input image table by
setting the bit corresponding to the particular input point.
When next scanned by the scanner, the adapter reports the new input
information. The scanner updates the input image table in the global
RAM by setting the bit corresponding to the particular input point.
The host interrupt handler reads the input image information in the
global RAM and copies it to a duplicate input image table available to
your program. Your program now knows that an input on a particular
input module is “on.”
The path of an output bit is essentially the reverse of the input path:
Your program sets a bit in its output image table. Your program knows
that this bit maps to an output on a particular output module.
The scanner interrupts the host, the interrupt handler copies your
program’s output image for that rack to the global RAM.
When the scanner next scans the adapter controlling the particular
output module, it tells the adapter to update its output image table with
the new information.
The adapter tells the discrete output module to update its outputs with
the new information.
Data Paths