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Function Blocks
HOA Hand/Off/Auto Switch Function Block
172
HC900 Hybrid Control Designer Function Block Reference Guide
Revision 11
2/07
HOA Hand/Off/Auto Switch Function Block
Description
The
HOA
label stands for
Hand/Off/Auto Switch.
This block is part of the
Auxiliary
category.
Function
The Hand – Off – Auto (HOA) switch function block permits state change requests from a Local Operator
Interface or a Remote source. The block states are: BYPASS (external manual operation of a device),
HAND (manual operation from an operator interface), AUTO (default – requests are operated
automatically), or OFF (relay to be switched to Bypass, Hand, or Auto)
The HOA switch is also used with the Device Control (DC) function block to comprise a Pump Control
algorithm which is used to manipulate the state of a controlled device (pump).
Each configuration is limited to a maximum of 16 HOA function blocks.
Forcing of outputs is NOT permitted within this block.
Input
REQI
= If the current state of the block is AUTO, then REQO output (on/off) equals the REQI input
(on/off).
BYPS
= If ON, the REQO output is forced off and any state change requests are ignored. If OFF, the block
returns to its previous state (Hand, Off, Auto).
Output
HAND
= ON when the block is in the HAND state, else OFF. Device is in manual operation from an
operator interface; prevents automatic operation; this state forces the REQO output ON.
AUTO
= On when block is in AUTO state, else OFF. Requests are operated automatically.
REQO
= This is ON when in the HAND state, or when in the AUTO state and the REQI input signal is
ON. OFF when in the OFF or BYPASS state.
Note. Both HAND and AUTO are OFF in the OFF and BYPASS states.