histogram

[MIKE31 0]

HI what is the purpose of the histogram in RSLOGIX 500 Thanks

[gravitar 3]

Well.. It seems to me NOT MUCH! It lets you plot/log a timing diagram for *ONE* bit of data. Now, any meaningful timing diagram I've seen has several bits of data compared against each other.. But in RSI jargon, that is a TREND. And to plot a trend, you need to buy an additional piece of software called (you guessed it) RSTrend. I've actually got an old copy of RSTrend, but I can't seem to get it to install. I think it is TOO old!

[Guest]

Greetings MIKE31, well ... actually the histogram is pretty useful in some troubleshooting situations - as long as you recognize its limitations ... as with any tool, improper use can make a mess of things ... the histogram basically “writes a history” of a sixteen bit word ... you can see an example in the figure below ... here the histogram is recording the on/off status of (some of) the bits in S:42 (if you could scroll down you’d see the rest) ... S:42 is the “seconds” value of the real-time clock in my SLC-5/04 ... this example isn’t likely to be very handy - unless you just need a refresher in how to count in binary from 0 to 59 ... but it shouldn’t be too hard to think of examples where it might be useful to know “which limit switch came on first?” or maybe “how long was output X on before output Y went off?” ... some other features that I’ve highlighted show the decimal value of the seconds - and the time since the value changed - and notice that you can have this information written to your computer’s hard disk - so that you can come back in the morning and see what went on last night ... and by setting the time base entry to a faster setting, you MIGHT (see below) be able to get somewhat better resolution on what the histogram records ... so that’s the good news ... now for the bad ... the histogram in RSLogix500 has a significant drawback ... specifically, all of the data which the SLC’s histogram records must come through the communication channel in order to reach the programming terminal ... this doesn’t sound like a big deal - but if you’re used to working with the histogram on the larger and more powerful PLC-5 processors, it can become quite a headache ... here’s the secret handshake ... with the PLC-5 and RSLogix5 the histogram is actually monitored by the PROCESSOR itself ... the data is automatically stored in a buffer inside the processor until a timeslot opens up and allows the processor to communicate with the programming terminal ... then when that communications event takes place, the processor gives the terminal (and thereby the histogram) a “chunk” of information detailing how the monitored word changed since the last communications took place ... in other words, the PLC processor stores a series of (possibly) several off/on changes - and then says “here, histogram - put these on your chart” ... it’s different with the SLC-500 processors and RSLogix500 ... here the processor does NOT monitor or store any information for the histogram ... specifically, all of the data must be monitored by the programming terminal ... and if the programming communications channel isn’t quite as fast as you’d like for it to be, then the histogram won’t see or display all of the on/off data that you’re expecting it to ... oops! ... the worst case scenario is that you might grow up using the much more reliable histogram on the PLC-5 platform - and not realize that its little brother on the SLC-500 platform isn’t nearly as trustworthy ... I’ve personally been called out (twice - but at different plants) to help troubleshoot equipment problems where if the in-plant technicians had only known about this “dark side” of the RSLogix500 histogram, they’d have gotten the equipment going again without any outside assistance ... unfortunately they had been basing all of their troubleshooting on the ERRONEOUS assumption that the histogram was telling them “the truth - the whole truth - and nothing but the truth” about how the machinery was acting ... so they missed the fact that certain switches were coming on or going off or things along those lines - but these events were NOT being recorded by the histogram ... as soon as I turned the histogram off completely and started actually confirming the true status of the suspected field devices, then troubleshooting the system began to fall right into place ... many unkind words were said about the histogram that day - none of which I’ll repeat here ... now on to other things ... gravitar said: now I’ll be the first to admit that the terms Rockwell uses can often be confusing ... but whenever I hear the word “trend” I usually think of graphing analog values - things like temperatures and flow rates and levels, etc. ... and the good news is that most (but not all) versions of RSLogix500 have this ability built in ... check near the bottom of your “project tree” at the left side of your screen and see if you have a “Trend” folder ... if it’s not grayed-out (or missing altogether) then right-click it and start a “new” trend ... play around with it and post again if you have any questions ... and yes, it IS possible to graph the on and off status of a bit on a “trend” ... but like I said, in most cases it’s used for graphing analog signals ... the biggest difference (besides the price) between the RSLogix500 built-in Trend feature and the non-free RSTrend is this: the built-in version can’t record the incoming data on the computer’s hard disk - but RSTrend can ... so if you want to be able to look back and see what the oven’s temperature was last Tuesday afternoon, then you’ll need to spend some money for RSTrend ... but if all you need to do is watch the temperature rise and fall in real time (as in tuning a PID loop) then maybe the built-in trend feature will meet your needs ... note to gravitar: RSTrend requires an activation key - just like RSLogix and most other Rockwell products ... maybe that’s why your copy won’t work ????? ... Edited 11 Aug 2005 by Ron Beaufort

[gravitar 3]

Well I hadn't used the histogram function in RSLogix 500 until about a month ago. I knew there was a reason why I never used it, but I had forgotten :) I guess the lack of accuracy was the reason. I'll bet this would be of minimal concern over ethernet, but we're still in the dark ages with DH485. (shared with a panelview usually, as well) Anyway, another guy in the shop said "can't you show how those signals change over time??" and I decided to give it a try. Now I'm not the sharpest tool in the shed, but I'm pretty sure it was only letting me graph one signal vs. time. Thinking I *MUST* be missing something, I went to the online help file. sure enough, under 'Create a histogram', it says "A histogram logs the data contained in a single address over a period of time. A trend logs the data for more than one address (typically related) over a period of time." So when you search the help file for 'Trend', it says "The following feature is only available if you have the RSTrendX Active X control resident on your computer." Actually, I don't even think I have "RSTrendX", what I've got is just "RSTrend". All I know is it is very old.. It still wants to install it to an "ICOM" folder :) And I never get as far as being able to load activation, it just plain won't install. I think it is looking for a directory path that doesn't exist. What I'd like to be able to do is plot data table values from a variety of sources against each other. Like for example, I'd like to know how B3/11 corresponds to I:3/0 and N7:10/4. Now I suppose I could move all those bits to a single word and just display the individual bits of the word vs. time, but I'd prefer not to have to add any rungs to get what I want. But I'm not even there yet.. I'm still stuck trying to figure out how to get the histogram to display more than one bit!

[Guest]

Greetings gravitar, you mentioned that the "book" says: let’s concentrate on the built-in Trend feature in RSLogix500 for a second ... did you check for the “Trend” folder in the RSLogix500 project tree? ... is it there? ... is it grayed-out? ... now for RSTrend ... I have an older copy of RSTrend running on a machine at home ... if you have the activation for yours, then I’ll be glad to see how mine is set up ... maybe yours can be salvaged ... I’m pretty sure that mine is installed in an “ICOM” folder - and it worked just fine the last time I tried it ... what version do you have? ...