Honeywell HC900 User And Installation Manual page 181


Lead Controller handles communications with Modbus RTU slave devices

Lead and Reserve exchange system status data to determine conditions for failover.

I/O Scanners relay system status data between each Controller to determine conditions for failover
Execution time
HC900 Controllers are designed to execute control functions within fixed scan cycles for analog data types
and logic data types. In redundant controllers, the default scan time is 500ms for analog data types and
53ms legacy/ 25 mS for new controllers, for logic data types.
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.
Lead/Reserve controller synchronization

Lead Controller automatically synchronizes the Reserve with the configuration database

During download of a configuration from a Host to the Lead

During process operation to bring a Reserve Controller from the Unavailable state to the On-Line
state

Lead Controller automatically synchronizes the Reserve with run-time data during each function block
execution cycle

Both the Lead and Reserve Controllers execute the function blocks in the control strategy, but only the
Lead Controller writes the physical outputs to the I/O Racks. See

Figure 88.
Revision 6
21 April 2017
HC900 Process & Safety Controller User and Installation Manual 167