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Old March 3rd, 2005, 08:00 PM   #1
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Question Real-time clock???

What exactly is meant by a real-time clock when talking about plc's? Do only some plc's have this feature or do all of them. In what applications would you use one??? How do you program the feature?
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Old March 3rd, 2005, 08:17 PM   #2
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I would say about 3/4th of the PLC models now have "real-time" clocks. (If it has a clock, it is almost invariably a real-time clock. I have never seen one with an unreal-time clock). The clock will have some bits that are available to the programmer, such as a 1-second, 1-minute, 0.1 second, and so on. foir example, the ALlen-=Bradley SLC 500 brand has a 16-bit "free-running" clock at Status File location S:4. Any of the S:4 bits can be used by a programmer on a rung of his program.
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Old March 3rd, 2005, 08:42 PM   #3
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In the harware configuration (Siemens Simatic S7) u can assign a clock memory byte. We use the 6th bit of that byte for pulsing signals, a lamp that must blink, pulse, or how do u name it in english.

The real time clock, I always think of the clock in de CPU module that keeps running, even when there is no power supply.


c ya
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Old March 3rd, 2005, 09:21 PM   #4
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A 'real time clock' is a chip, or a small section of a larger chip, insidee the PLC's CPU. It always recieves power even if main power to the PLC rack is off. Using a very accurate crystal oscillator it can keep very accurate date and time, usually to within seconds a month. It is probably called 'real time' because it's time keeping is much more accurate than a ladder program's timers in the long run. It is very similar to the clock chip in your coumputer which keeps time even if it is turned off and unplugged.

The date/time it keeps is available by way of registers. It's setting can be changed from the programming PC and possibly by commands available in the PLC's language. Some programmers may use these to keep a copy of the date/time when some event happens (time stamping). Others may initiate events at a particular date or time. I don't think many of the 'real time clocks' account for 'daylight savings time' and you will see some questions which are asked on how to handle this. Some of the others have pointed out how very accurate pulses produced in addition to the date and time can be used.
Controlling outputs is the PLC's way of getting its inputs to change.
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Old March 4th, 2005, 03:29 AM   #5

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when you look in s7 you can change the date and time of the real time clock with pressing the next key:

alt, L, y

there you see the date and time at that the PLC has at that moment and you can change it there then you can use in ob1
the ob1 date_time to put the time in a db and you can use the different bytes for programming a flashing light or something like that
this is an other way for programming that pulse without changing the hardware config
It is impossible to make anything foolproof because fools are so ingenious
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Old March 4th, 2005, 04:15 AM   #6
Ken M

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What exactly is, and what isn't, "real time" ?

That is entirely determined by your requirements. If the only thing you need to do is keep track of which day we're on, then a calendar is 'real-time'. Sundials, hour-glasses, egg-timers can all be considered 'real-time' if they meet the needs of the process. I've actually heard some people say that a real-time clock needs to go to millisecond resolution. Well, if you're a physicist measuring particle decay, that's probably about 5 orders of magnitude too coarse.

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