Technical Reference Manual - InduSoft

Introduction
Executing and switching modules
IWS is a SCADA system composed of modules that must be executed simultaneously. Based on the multitasking
concept, each runtime module (Viewer, Driver, and so forth) is a thread and the operating system
switches from one thread to other automatically. It is a common misconception that you execute a SCADA
system when you execute a PLC program. A PLC program contains a simple loop:
PLC Program Loop
However, in a SCADA system, there are several modules running simultaneously, and most of them can read
and write data. Because a SCADA system modifies data (tag values) continuously during task execution, the
preceding diagram is not applicable.
IWS only has one process: Studio Manager.exe. When you execute a runtime project, the Studio
Manager.exe process starts the Tags database and all of the runtime modules configured for the project. You
can specify which modules (such as Viewer and Driver) will start during the runtime.
Each process keeps a list of active threads for the operating system. Actually, each process activates and
deactivates each thread during the runtime, according to the algorithm of each process. Also, when you create
a thread you specify a priority value. The operating system continuously scans all currently active threads,
and executes the threads according to their priority value — executing the higher-priority threads first. When
threads with higher-priority values are active, the threads with lower-priority values are not executed at all.
If there is more than one thread with the same priority value, and there are no other threads with higherpriority
values, the operating system keeps switching between the threads with the same priority.
Note: All IWS threads are set to priority 7, which is THREAD_PRIORITY_NORMAL. (Most programs
contain this priority value.)
Real-time program (such as SoftPLCs and Device Drivers) threads are assigned a higher-priority
value (THREAD_PRIORITY_HIGHEST); however, these programs must provide a mechanism to keep
them inactive for some period of time or the threads with normal priority would never be executed.
IWS uses the UNICOMM.DLL library for serial drivers. This library creates a
THREAD_PRIORITY_HIGHEST thread that "sleeps" (remains inactive) until data arrives in the serial
channel. When IWS detects new data in the serial channel, the THREAD_PRIORITY_HIGHEST thread
"wakes up" (becomes active) and transfers the data from the operating system buffer to the thread
buffer, where it can be read by the Driver. This thread is the only highest-priority thread created by
IWS.
If you allowed threads to remain active all the time, the CPU usage would be 100% all the time, which must
be avoided for performance reasons. Every program provides a mechanism to prevent threads from staying
active all the time.
IWS uses the following parameters to prevent threads from staying active continuously:
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