What is the function of a Watch Dog in PLC systems? What is the function of a Watch Dog in PLC systems? I want to know why is need for Watch Dog in PLCs. PLCs as well as Microcontrollers usually use what is called Watch Dog, may you help explaining why is this Watch Dog for in PLC systems? Watchdogs are basically timers used to keep check that the code does not get into a 'bogged down' state. What is the SCAN time of a DCS/PLC? What is the typical value for scan time of DCS and PLC? SCAN time of DCS /PLC defines as execution of one cycle of program. The Six-Phase Comprehensive Project Life Cycle Model Including the Project. Generally when the watchdog fault occurs the processor is faulted. A watch dog is a timer which must be reset regularly before it times out, or it will cause the PLC or microcontroller to fault or reset. In a microcontroller your program would explicitely reset the watch dog. In a PLC, the watch dog is automatically reset during the I/O update (between program scans). The watch dog timer is intended to prevent software bugs from causing the microcontroller or PLC program from hanging up. If you PLC supports loops or jumps you can test this by programming an infinite loop. The PLC should fault and either go into stop mode or perform whatever other fault action. The watchdog is there to inform you if the scan time of the PLC goes over specified limits, i. To ensure that you are correctly controlling your process you need to start the new scan within a maximum allowed time slot. The watchdog timer verifies that the present scan does not overrun that limit, which would mean you are either in an endless loop or you are not dealing with you prograsm adequately. Hope this helps,Daniel Chartier. Well, PLCs are applied in installations where availabilty and cycle times are important. The Watch dog simply monitors the execution time of a certain task and when it exceeds the permitted time it gives a signal. A PLC relies on one central processor to execute the users' program, but also to do the housekeeping (updating the time registers, the I/O image, network ports, etc.). To make sure that it does not take too long between housekeeping scans, it usually has a watchdog. The watchdog is a timer with a preset higher then the maximum scan time that is to be expected. The operation of a PLC is. A PLC works by continually scanning a program. We can think of this scan cycle as. It will store the execution. The Project Life Cycle. The project life cycle consists of four phases. During the Project Execution phase the project team produces the deliverables while the. A programmable logic controller. Manufacturers of programmable logic controllers. Execute Program (or Logic Execution): The PLC executes a program one. Study hardware and software used in PLC. If the watchdog timer times out, it is usually better to halt the execution and make sure that the programmer resolves the situation, because it might result in unwanted behavior of the machine or production line, even in damage or danger to persons nearby. Because the watchdog is an external circuit, it will still respond even if the central processor is tied up in too long program scans. In a practical situation: If the watchdog is triggered, you should normally check your program for eternal loops, or a too high amount of triggered routines (they are usually not taken into account when calculating the watchdog preset). KR,Theo Baarslag. PLC- /CIMspecialist. IAMS Europe bv. The Watch Dog is a timer. After a scan cycle the watch dog must be reset. If not the Watch dog timer reachs the preset and shutdown the PLC. Bescause it means the PLC didn. Best regards. Marcelo Nunes. Watchdog timers are used for a couple of reasons in a PLC: a. If you want to have an upper bounds for your scan time to ensure you don't get stuck in iterative loops for instance. From a Peer comms or SCADA point of view, a PLC may look like it is OK because the comms is executing fine, but if you look at a watchdog timer and this is not incrementing or changing then you can determine that the processor is in fact not executing or stuck in some state. A watch dog is a hardware timer that is . If the timer rolls to zero it will stop the process. This could indicate a hardware or software problem. Typically, when things are functioning properly, at a predetermined interval the processor will reset the timer before it can roll over. The best thing to do is search the internet, there are some good descriptions out there. When I write embedded software I often include a watchdog timer that will restart the execution of the program if the scan does not complete in a certain amount of time. I have little PLC experience but I would assume that it's included for the same reason. If you think about what a watchdog does in real life.. In the PLC and microporcessor world, its just about the same job. The watchdog timer checks to make sure that the scan time of the PLC or processor does not exceed a predetermined time. If it does, a fault is declared and the program is generally halted as this is an indication that something has gone seriously wrong with the normal processing ofinstructions. In some PLCs this time can be adjusted to whatever value you feel is appropriate. Obviously this value will always be greater than the normal scan time of the program. If the watch dog doesn't get reset, the software is not running properly and the watch dog routine will do something about it, like reboot. Made popular by Windows. Regardscww. Your subscription request is being.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
December 2016
Categories |