Talk about the function of optimizing block access in S7-1200/1500 series PLC

Siemens S7-1200/1500 series PLC supports optimized block access. It is usually understood that data blocks (DB) have this function.

In fact, in addition to data blocks, organization blocks, function blocks and functions also have this function.

In the TIA Portal environment, when you create these four types of program organization units, their properties are all checked by default with "optimized block access".

In this article, let's talk about this function in detail.

1. Data Block (DB)

Create a new data block in the TIA Portal environment. In its properties, you can see that "Optimized block access" is checked, as shown in the following figure:

The optimized data blocks have the following characteristics:

① The variables in the data block can only be accessed in a symbolic manner (absolute address access is not supported). For example, if the name of an optimized data block is "optDB" and there is a start variable inside it, the access syntax is:

 "optDB".start

Note: The double quotes around optDB are automatically added, indicating that this is a global variable; if the pound sign (#) is added, it means that it is a local variable;

To illustrate the difference in storage strategy between standard data blocks and optimized data blocks, let's take an example. Suppose variables are stored in the following order:

start: Boolean;

speed: real number type;

stop: Boolean;

For standard data blocks, variables are stored using absolute addresses (in bytes). The first Boolean variable occupies one byte (B0, although it actually only uses one bit).

The second variable is a real number and is stored starting from an even address, so its starting address is B2, occupying 4 bytes (B2~B5); the third variable is a Boolean type and occupies one byte (B6).

It can be seen that under this storage strategy, these three variables occupy 7 bytes of storage space.

Of course, if the order is adjusted and the two Boolean variables are put together, one byte of space will be saved, but this requires the programmer to be familiar with its storage strategy.

In the case of multiple data types, manual optimization of storage is often laborious and difficult to achieve the desired effect.

The optimized data block helps us solve this problem. It will automatically store variables of the same type together to maximize the use of storage space. The comparison between the two is shown in the figure below:

② The optimized data block can set the retention attribute for the variables in it separately, while the standard data block either retains all or cancels all retention;

③For S7-1200, the optimized data blocks adopt a storage priority strategy (because its data space is relatively small); for S7-1500, the optimized data blocks adopt an access priority strategy (because its data space is relatively large, in order to improve the access rate);

2. Organization block (OB)

Optimized the startup information.

After optimization, the organization block has only a few startup information. For example, after optimization, the Startup organization block has only two startup information, as shown in the following figure:

3. Function Block (FB)

① Optimize its background data block, similar to the optimization of data blocks;

② Complex variables can be addressed through symbols;

4. Function (FC)

①Optimize the use of temporary variables and initialize them before each block call;

② Complex variables can be addressed through symbols;