Rockwell Automation 6008-SI IBM PC I/O SCNNR 6008-SI - Manual (Page 18)

Chapter 2
I/O Scanner Concepts
2-10
Suppose that your program crashes, either because of logic errors or
because of operator intervention. Or suppose that through logic errors
your program gets into an infinite loop. In these cases the program is no
longer sending meaningful information to the scanner through the interrupt
handler. There is no way for the interrupt service routine (ISR) to
recognize all possible host program failures reliably, so instead a “host
watchdog’’ scheme has been implemented.
In essence, your program must take a particular action every so often (by
default, every second). If the ISR recognizes that the required action has
not been taken recently enough, the ISR infers that your program has failed
and simply stops talking to the scanner. The scanner in turn recognizes this
as a host failure and goes off the link within 50 ms; all the adapters go
inactive and output terminals go to last state or reset as determined by
switches you set on the chassis.
We’ll tell you about the necessary programming steps in chapter 5, Startup,
Status, and Shutdown.
If the host computer doesn’t respond to an interrupt from the scanner
within 100 ms or less, the scanner assumes that the host hardware and
BIOS is no longer active. In this case the scanner goes off the link, and 50
ms later the adapters set the output modules in last state or reset according
to your switch settings. The scanner then goes into its power–up sequence,
waiting for new startup commands from the host.
This scanner watchdog feature lets you end one program run and start
another without cycling host power. Even if your program locks up the
host computer, if you are able to do a soft reset (
Ctrl–Alt–Del
) the scanner
is ready and waiting for your program. More importantly, if your program
fails or is interrupted, even by a reboot of the computer, all discrete outputs
are in last state or reset, according to the switches you set on the adapters.
Host Watchdog
Scanner Watchdog