Understanding Primary Blocks

Primary blocks receive data from an I/O driver or OPC server and generate alarms based upon this information. Primary blocks are usually associated with one or more pieces of process hardware. For example, a pump, a tank, a temperature sensor, a photo cell, a limit switch are all process hardware with which you might associate a primary block.

Most primary blocks, listed in the following table, also include a scan time. The scan time controls when SAC scans the blocks in the database. Refer to the section Scan Times to learn more about them.

Standard Primary Blocks
The block...	Function:
Analog Alarm (AA)	Provides read/write access to analog data and lets you set and acknowledge alarms.
Analog Input (AI)	Provides read/write access to analog data and lets you set alarm limits.
Analog Output (AO)	Sends analog data to an I/O driver or OPC server when the upstream block, an operator, a Program block, a script, or an Easy Database Access (EDA) program supplies a value.
Analog Register (AR)	Provides read/write access to analog data only when a Data link connected to the block appears on an operator display.
Boolean (BL)	Performs boolean calculations based upon up to eight inputs.
Digital Alarm (DA)	Provides read/write access to digital data and lets you set and acknowledge alarms.
Digital Input (DI)	Provides read/write access to digital data and lets you set alarm limits.
Digital Output (DO)	Sends digital data to an I/O driver or OPC server when the upstream block, an operator, a Program block, a script, or an Easy Database Access (EDA) program supplies a value.
Digital Register (DR)	Provides read/write access to digital data only when a Data link connected to the block appears on an operator display.
Multistate Digital Input (MDI)	Retrieves digital data for up to 3 inputs from an I/O driver or OPC server, combines the inputs into one raw value, and lets you set alarm limits.
Text (TX)	Lets you read and write a device's text information.

See Also