What does "Isochrone Mode" mean?

I've seen the expression, "Isochrone Mode" ,as a feature for Siemens I/O cards.Does anybody know what's the meaning?

So how hard is it to go to the Siemens tech support site, http://support.automation.siemens.com/, type in the word 'isochrone' in the search field, and get the Isochrone Manual as the first out of 60 hits?

Ken.

Thanks Ken M,

I followed your advice and studied a pdf about "isochrone mode".As I underestood ( I hope I'm right ) in designing fast automation systems in order to prevent irregularity made by different cycle time of system components ( like PLC, ET200, OS and ...), a unique cycle time is defined for whole system. This is done bye using cards with isochrone mode and some tricks in software programming.
Hint me if I've not get the point

If you imagine the time lapse between reading an input signal on a Profibus slave and turning on an output on a Profibus slave, Siemens reckon there are several different, and here's the important point, unconnected cycles to be accounted for.

There's the switching time of the input card, the update of the Profibus slave interface card, the update cycle of the network to get data from the slave to the CPU, the scan time of the software in the CPU, the Profibus cycle time again, the interface module update time again, and the switching time of the output module. What's important here is not the absolute value of these times, but the fact they are variable and unsynchronised.

The idea behind CPUs and modules with 'isochrone' (from the Greek apparently, iso - the same; and chronos - time) capability is that the time cycles are interlocked and predictable. You apparently get a truly deterministic system in terms of I/O response times throughout the entire system.

Regards

Ken.

PS : Now, why couldn't I just have written that the first time, instead of being supercilious (from the Latin apparently, super - great; and cilious - idiot)

And now that you are so kind let me be a little cheeky to ask another question:
where exactly we use Isocrone devices?
in fast control systems like in motion control?
you may reply "in deterministic systems",then I will be very grateful if you give me an example

Quite often people use the term 'deterministic' to mean fast. While this is often true it is not necessarily true. Ken uses the term correctly in his post. Isochrone mode will give you a strictly fixed I/O response time. This is what determinism really is. In doing this it also guarantees that ALL DATA is chronologically syncronized. Chronologically synchronized simply means that all the data the plc sees in one update cycle occurred at exactly the same instant in time. Just keep in mind that something can be deterministic and still not necessarily update quickly.

The Siemens manual linked above gives very good examples on these two points. The example on 1-5, the camshaft lift measurement, is a good example of where chronological synchronization is important. With a camshaft not only is the total height of a cam lobe important but so is the phase angle between the lobes. When measuring a rotating cam you absolutely need the six measurements to be from exactly the same moment in time to get a good feel for phase. So time synchronization is important.
On page 1-6 the fixed I/O response time is the important factor. To correctly measure speed based on change in position the time between measurements needs to either be accurately measured OR it needs to be very consistent. Speed based on displacement is:
(D_n - D_n-1)/(t_n - t_n-1).
Isochrone mode makes sure the time between measurements is consistent (that is, (t_n - t_n-1)is always exactly the same number). This allows the user to use a fixed time in the calculation and know that the time value is the correct time.
This can be just as important with outputs. Assume two actuators are laid out with crossing path such that one must extend before the other. Strict I/O cycle control allows you to extend the actuators in sequence as closely to each other as possible and know that the action will be consistent over time (within mechanical wear limits, of course).

I hope this helps.
Keith

I actually saw a very good demo by Siemens of an Isochrone system. They had a painted disc (black and white quarters) rotating at a constant speed. I've no idea what the speed was, but it was the usual blur. They also had a proximity sensor mounted to detect whenever a certain point on the disc circumference passed the sensor. This pulse from the prox sensor was the input to the Profibus slave module. In the CPU they had the isochrone blocks plus a single line of logic to fire an output every time the input pulse came on. This output went via Profibus to an output module connected to a strobe lamp. And the lamp illuminated the spinning disc.

First they ran the system with the isochrone functions switched off, and the disc position appeared to be all over the place. The variable delay between detecting the disc position, and illuminating the disc was too much. Then they switched in the isochrone functions. All of a sudden, immediately, the disc position appeared to be locked solid. It looked stationary. The delay between detecting the disc position and illuminating it was now to all intents a constant interval. It was a most impressive demo and, as Keith has said, it showed, especially when isochrone was off, just how 'bad' a fast system could be if used in the wrong application. With isochrone switched in, the overall cycle time was certainly slower, but at least you knew what it was and it stayed the same.

regards

Ken.