Rockwell Automation 6008-SI IBM PC I/O SCNNR 6008-SI - Manual (Page 15)

Chapter 2
I/O Scanner Concepts
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Thus you can be certain that the I/O image tables are refreshed once per
scan list. Partial refreshes take place more frequently if your program
executes a lot of block transfers or management requests: whenever the
global ram is transferred the I/O image tables are refreshed as far as the
scanner has got in the scan list since the last interrupt. Your program can
also force a partial refresh: see the update function in chapter 6, Discrete
I/O.
The scanner has three modes of operation: program, test, and run modes.
Discrete inputs are read in all three modes.
In program mode no discrete outputs are sent to the adapters, and the
adapters are instructed to hold all discrete outputs reset (zero). The
scanner holds any block transfer requests in its queue without servicing
them.
In test mode the adapters are still instructed to hold discrete outputs
reset, but the scanner sends discrete information to them. Block
transfers can proceed in test mode, but their outputs will be held reset.
In run mode discrete output information is sent to the adapters, and the
adapters are permitted to update the output modules. Block transfers
may be performed.
When your program first starts up scanner operation, the scanner is in
program mode. Your program must issue a command to change to run
mode.
The scanner has a 2048–byte area of RAM that we call a global RAM,
shared by both the IBM PC and the scanner. This is not a true global
RAM, because the host and the scanner can’t access it at the same time.
Instead, the host and the scanner exchange control of the global RAM by
means of interrupts. When the scanner interrupts the host, the scanner is
turning over control of the global RAM to the host. When the host
interrupts the scanner, either the host has the global RAM and wants to
return it or the host doesn’t have the global RAM but wants access to it.
Access to the global RAM is ultimately controlled by hardware and by
scanner firmware. For the host side, we supply an interrupt handler or
interrupt service routine that is automatically invoked whenever the
scanner interrupts the host. Our interrupt service routine copies
information as needed between your program’s data area and the global
RAM.
Operating Modes
Global RAM