In the context of a PLC (Programmable Logic Controller), DB stands for Data Block(s).
Understanding Data Blocks (DBs) in PLCs
Data Blocks (DBs) serve a crucial function in PLC programming: they are used by your program to save data in the CPU. Think of them as designated memory areas where your program can store various types of information, such as numerical values, status flags, configuration settings, and more.
According to the provided reference, the CPU can have a certain amount of space available for these data blocks. For example, the reference mentions up to 8 KBytes (8192 Bytes) space on the hard disk for saving data. This data is accessible and modifiable by the PLC program during its operation.
Types of Data Blocks
The reference highlights two primary types of Data Blocks, distinguished by how they are accessed and used within your PLC program:
- Global DBs:
- These are Data Blocks where data can be read from or written to by all organizational blocks (OBs), function blocks (FBs), and functions (FCs) within your program.
- They are suitable for storing data that needs to be universally accessible, such as global variables, shared setpoints, or status information used across different parts of the control logic.
- Local Instance DBs:
- These Data Blocks are assigned to a particular FB (Function Block).
- Each time you call an FB, it can have its own associated instance DB to store data specific to that particular call or instance of the FB.
- This is essential for creating reusable FBs that can manage their own internal data independent of other instances or other program parts.
Here's a quick comparison:
| Feature | Global DBs | Local Instance DBs |
|---|---|---|
| Accessibility | Accessible by all OBs, FBs, and FCs | Assigned to and primarily used by a specific FB instance |
| Purpose | Shared data, global variables | Instance-specific data for FBs |
| Association | Not tied to a specific code block instance | Tied to a specific FB instance |
Why Use Data Blocks?
Data Blocks are fundamental for structured and organized PLC programming. They provide a centralized place to store and manage data, separating the data from the control logic (which resides in OBs, FBs, and FCs). This separation makes programs:
- More readable and easier to understand.
- Easier to maintain and troubleshoot.
- More efficient by avoiding redundant data storage.
For example, you might use a Global DB to store machine setpoints like temperature thresholds or speed limits that several functions might need to access. An Instance DB, on the other hand, would be used by a specific motor control FB to store the current speed command, status, and fault information unique to that particular motor instance, even if you use the same FB to control multiple motors.
