Chapter 1
Understanding I/O Scanner Concepts
1-7
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. Thus you can be certain that the I/O image tables
are refreshed once per cycle through the scan list.
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.
No link activity occurs until you send an
Autoconfigure
or
Set Scan List
command. Then, when your program begins scanner operation, the scanner
is in Program mode. Your program must issue a command to change to
Run mode.
Dual-port RAM gives you almost 100% access to the interface at all times,
therefore your entire operation runs more quickly. The only time you will
be denied access is when the host and the KTx try to access the same
memory location at exactly the same time.
DATA
ADDRESS
DATA
ADDRESS
Host
processor
Dualport
RAM
2K memory
KT
x
processor
KT
x
card
Operating Modes
Dualport RAM
