Siemens SIMATIC NET PROFIBUS Servisní příručka stáhnout pdf (Strana 209)

FAQ (Frequently Asked Questions)

DP Base Programming Interface for CP 5613/CP 5614

C79000-G8976-C108-01

209

What are the disadvantages of simply polling in the user program?

If your user program does not restrict itself, the entire available CPU performance

of the of the PC will be used to query the process image unnecessarily often. If the

program restricts itself too much, on the other hand, the data will no longer be up to

date. We recommend that you restrict polling using a time control suitable for your

system.

What are the advantages of hardware events ?

Your user program can react to changes quickly. Since it waits, it does not take up

unnecessary CPU time on the PC.

What are the disadvantages of hardware events?

If your user program monitors a large number of active slaves using hardware

events, the overheads for fetching events is high because semaphores must

constantly be incremented and run through. This increases PC-CPU load. If you

therefore have a lot of slaves or very active slaves, we recommend that you use

hardware events for only a few slaves and poll the others periodically.

What are the advantages of programming with multiple threads?

With multiple threads, you achieve a clearer structure in your user program. This is

particularly useful when you want to run different processes at the same time, for

example, periodic polling combined with hardware events on the one hand and

asynchronous DPC1 jobs on the other. These unrelated tasks with their different

dynamic characteristics can be implemented in two threads.

What are the disadvantages of programming with multiple threads?

The threads of an application, for example, must not interfere with each other when

accessing global variables. Overwriting data and deadlocks must be avoided. It is

particularly important to avoid multiple threads accessing the process image of the

CP 5613 or CP 5614 at the same time otherwise they will overwrite the each

other's control register for consistent reading and writing of slave data. This could,

for example, jeopardize the consistency of the slave data currently being read by a

thread.

1 2 ... 204 205 206 207 208 209 210 211 212 213 214 ... 237 238

Žádné komentáře

Siemens SIMATIC NET PROFIBUS Servisní příručka Strana 209