QUESTION:
What is the effect of different data-block programming (opening and accesses) on the cycle time?

ANSWER:  
The execution times depend on the respective CPU.

Example of a shorter cycle time (no symbolic representation):

AUF DB5
U DBX 0.0
U DBX 0.1
= DBX 0.2

 

		CPU412	CPU414	CPU416	CPU417
AUF DB5	Open 1x DB	0.6µs	0.3µs	0.24µs	0.3µs
U DBX0.0
	1x DB access	0.3µs	0.1µs	0.08µs	0.1µs
U DBX0.1	1x DB access	0.3µs	0.1µs	0.08µs	0.1µs
= DBX0.2	1x DB access	0.4µs	0.2µs	0.16µs	0.2µs
Total		1.6µs	0.7µs	0.56µs	0.7µs

Table 1: Comparative cycle times (no symbolic representation)

Example of a longer cycle time (with symbolic representation):
U "Data block".Value_1  //DB5.DBX0.0
U "Data block".Value_2  //DB5.DBX0.1
= "Data block".Value_4  //DB5.DBX0.2
Even when the same data block is accessed here again and again, the block is always opened anew each time. For example, with a CPU414 it is about 0.3µs per command line.

 

		CPU412	CPU414	CPU416	CPU417
U DBy.DBX0.0	Open 1x DB 1x DB access	0.6µs 0.3µs	0.3µs 0.1µs	0.24µs 0.08µs	0.3µs 0.1µs
U DB5.DBX0.1	Open 1x DB 1x DB access	0.6µs 0.3µs	0.3µs 0.1µs	0.24µs 0.08µs	0.3µs 0.1µs
= DBy.DBX0.2	Open 1x DB 1x DB access	0.6µs 0.4µs	0.3µs 0.2µs	0.24µs 0.16µs	0.3µs 0.2µs
Total		2.8µs	1.3µs	1.04µs	1.3µs

Table 2: Comparative cycle times (without symbolic representation)

Note:
When comparing absolute and symbolic programming, however, the following must be borne in mind:

• In the case of small CPUs, the greater memory requirement during symbolic programming has an effect.

• For reasons of clarity, symbolic programming is preferable.

• Data structures can only be addressed symbolically.

• Symbolic programming is preferable because it is easier to change (e.g. 100 digits change from U E 4.5 -> U E 4.6 compared to a centralized system of changes in the Symbol Editor). Whichever is the best choice depends on the circumstances!

For further information on execution times for instructions in STEP 7 for S7-300 and S7-400 CPUs, please refer to the relevant operations lists on the Internet:

Keywords:
Cycle time, Access time, Execution time