Rockwell Automation 6008-SI IBM PC I/O SCNNR 6008-SI - Manual (Page 14)

Chapter 2
I/O Scanner Concepts
2-6
A slot is a position in an I/O chassis for one I/O module. In one–slot or
single–slot addressing, an I/O group represents a single slot. In two–slot or
double–slot addressing, an I/O group represents two slots.
The scanner runs asynchronously in relation to the host. When either one
wants to get the other’s attention, it must issue a hardware interrupt.
Information is passed through a global RAM. Both the host and the
scanner have the ability to postpone servicing an interrupt if in the middle
of another interrupt–driven task.
The scanner maintains a scan list, which is a list of adapters to be serviced
by the scanner. A given adapter may appear once, several times, or not at
all in the scan list. (The scan list is empty until you perform
AUTOCONFIGURE at which point AUTOCONFIGURE puts every
adapter in the list once.) The scan list starts as simply the list of adapters,
each occurring once, but your program can issue a scanner command to
alter the scan list.)
An exchange (or an adapter scan) is the scanner’s interchange of
information with one adapter. During an exchange, the scanner may
receive data or status information from the adapter or send data or
commands to the adapter, or both. Both block transfers and discrete I/O
transfers can be done during the same exchange if the adapter’s chassis
contains both kinds of I/O modules.
After servicing each adapter, the scanner looks at its command queue to
see if any commands are waiting. If so, and if the current operating mode
allows, the scanner executes one command. If the scanner has a
confirmation of this command or of a previously executed command, it
puts the confirmation in the global RAM and interrupts the host.
The scan list is circular: each time the scanner reaches the end of the scan
list it starts again at the beginning. An I/O scan (sometimes called just a
scan) is one complete cycle by the scanner through the scan list, from any
point to the same point.
Every time the host interrupts the scanner, the scanner puts a marker in the
scan list at the point of the adapter most recently scanned. If the scanner
works its way through the scan list to the same point without receiving an
interrupt from the host, the scanner interrupts the host. (If the scan list
contains no adapters, the scanner waits for 5 ms before interrupting the
host.)
What the Scanner Does