158 HC900 Process & Safety Controller User and Installation Manual Revision 6
21 April 2017
Execution sequence
The type of control functions executed during a scan is determined by the system configuration.
Controller configurations contain a series of algorithms in the form of function blocks that get
executed in a fixed sequence. The first 100 function blocks are pre-assigned by the system to
handle communication tasks, alarm processing, system monitoring functions, etc. and cannot be
changed by the user. Starting with function block number 101, the user may select the type of
function to be executed.
The sequence of function block execution is initially determined by the sequence in which the function
blocks are placed on the graphic diagram in HC Designer.
Final desired sequence must be set by the user to achieve proper and optimum performance.
CAUTION
Incorrect execution sequences can contribute to delays in processing outputs and/or improper
or unexpected operation.
The HC900 controller samples all inputs before the start of a controller scan.
Each input being used in the configuration must be assigned to a function block. The sequence order
of the function block determines when in time the actual value will be updated. It is important that
algorithms that need updated input values for their calculations have the inputs execute first in the
sequence.
Except for Time Proportioning Output (TPO), Three-Position-Step-Control (TPSC) and Position
Proportional Output (PPO) function block types that update their physical output values while the
function blocks are being executed, all physical outputs are updated at the end of a scan.
Controller Modes
The HC900 Controller includes three operating modes. The purpose of each mode is described
immediately below, and salient characteristics of each are described in Table 22. The functions of the
Mode Switch are described in Table 23, and the procedures that the controller performs in transitions
between modes are described in Table 24.
PROGRAM Mode
In the PROGRAM Mode, active control processing is suspended. This mode is used for safe execution of
utility functions such as configuration download and calibration of analog inputs and outputs.
All outputs are Off.
RUN Mode
The Run Mode is used for normal operation of the controller; that is, for running the control configuration
that was previously downloaded. Configuration download and other utility functions can be performed in
this mode when the physical mode switch is placed in the Run/Program position. See the Designer
Software User Guide for precautions, restrictions, and procedures. Functions placed on the Safety
worksheet may not be forced or written to in this operational mode when the physical mode switch is
placed in the RUN position. The mode switch must be placed in the RUN position. for runtime on Safety
applications. The physical mode switch while in the RUN position prevents unintentional configuration
changes and is the recommended operational position when modifications to the configuration are not
anticipated.
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