The Device Control (DC) function block provides setpoint control for multistate discrete devices, such as motors, pumps, and block valves. The block compares the requested state (setpoint) to the actual state reported from the device and, after allowing time for the device to change state, detects alarm limits on any error. The basic functionality is augmented by an assortment of interlocks and device control options to customize the block's operation for your application.
The Device Control function block supports mode control, setpoint tracking, simulation, and alarm limit detection. You can select options that specify the control strategy used in the block.
The setpoint requests the device to go to one of two or three supported states: Passive, Active 1 and Active 2 (optional). The Passive state is the power failure (safe) state, such as OFF or CLOSED. An Active state usually requires energy (or allows energy to flow), such as OPEN, RUN, FORWARD, or REVERSE. You configure one or two Active states (Active 1 and Active 2) to match the device you want to control. You select the set of state names that applies to the device, such as STOP/FORWARD/REVERSE or OFF/LOW/HIGH.
The Device Control block uses as many as eight discrete I/O channels to command a device to the requested setpoint state and to read back its confirmation contacts. Discrete I/O is associated with the Passive and Active states by means of a mask for each state that allows each bit to be defined as True (1), False (0), or not used. You can configure four bits as outputs to the device and four bits as the contacts that confirm the device state. The confirm contacts must be maintained because the block is designed to alarm on loss of confirmation.
CAS_IN_D is the discrete value and status of a setpoint from another block used when the block is in Cascade mode.
SHUTDOWN_D is the Emergency Stop discrete value and status input that forces and holds the device in the Passive state.
PERMISSIVE_D is the optional discrete input value and status that must be True when the Permissive device option is enabled to allow the device to be commanded to an Active state.
TRK_IN_D is the discrete input value and status that forces the block mode to Local Override and causes the output to follow the field value (FV_D).
SIMULATE_IN_D is the optional discrete input value and status that is used to simulate the field value.
INTERLOCK_D is the optional discrete input value and status that must be True when the Interlock device option is enabled for the device to remain in the Active state.
OUT_D is the discrete output value and status that reflects the current commanded device state.
The following figure shows a simplified view of the internal components of the Device Control function block:
