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I1 - INPUT - Stores the state of the PLC inputs. Note that many times where the word is always 0, it will be omitted and stated as O:0/0.Īnalog outputs on slot 1 O:1.0 and O:1.1. 0/0 through 0/15 are the first 16 outputs of the slot, 1/0 through 1/15 are outputs 16-31 of the slot, etc.Įxamples of outputs used on your trainer:ĭiscrete outputs on the base PLC O:0.0/0 through O:0.0/5. If this is an analog module then 0 is the first channel, 1 is the second channel, etc.ī is the bit and is used for discrete outputs in combination with the slot. We'll go more into detail of what a word is in the N7 explanation below but for discrete outputs, 0 is outputs 0-15 of the slot, 1 is outputs 16-32 of the slot. 0 being the PLC base's built in outputs and 1 being the first expansion module, 2 being the second expansion module, etc. O0 - OUTPUT - Stores the state of the PLC outputs. In general I don't like the "pulser" systems because with an analog system you get some additional "diagnostic" type data that allows you the opportunity to diagnose potential problems.The data files are a way of organizing and optimizing the data storage in a PLC.
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You can use a high speed counter (though this isn't really high speed) to completely eliminate the problem, but otherwise realize that pulse length is going to be an issue ahead of time and plan accordingly to deal with this. Often these devices only pulse for a very short period of time and frequently if the PLC scan time is long (or remote IO is in use), the PLC might miss the pulses. Some (Micrologix) support "fast" inputs and/or PII's which allow you to capture fast pulses. As to the comment about using "pulsing" type outputs.these are tricky with a PLC. Best to simply make the system more robust by making it ignore values below a certain level. No amount of playing around with calibration and zeroing is going to last forever as temperatures, drift, and so forth will eventually invalidate your perfect calibration for zero.
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Most of the time it's best to simply set a threshold where you know that the flow is definitely never "that low" without being zero and stop totalizing (or use a zero) whenever that condition exists. Paulĭon't forget that if your system spends any amount of time at all at "zero" flow because the output is rarely perfectly zero (it's either going to be very close or perhaps even negative), totalizing under these conditions will result in an erroneous value because of the accumulation of the error over time. But if you're already within 1 or 2% accurate over a 24 hour period, that's probably nearly as good as you can get. I would suggest double checking your analog scaling with a precise meter, and verify the scaling numbers are right as suggested by J.B. AND, I see you are ignoring flow signals when (either?) pump current is less than 0.5 (amps?) This seems fishy.I know you don't want to accumulate noise error when the flow is known to be stopped, but you're chopping off the ends of every ramp of the signal with that. When I construct a timer like that for interval accuracy, I normally set the preset to the max of 32767 and put in a comment to explain why its done that way. You might be confident it never times out, but if it does, you will lose any accumulation that occurs. In your program, where does F8:19 come from? Also, since you are using T4:30 as your interval timer, you might want to set its preset a little higher than you have it. Let the meter do its thing and use a MSG to read the total. We went the extra mile with the STI for a minor improvement and to satisfy the boss that we'd done everything possible to get as close as possible without putting in a new meter capable of communicating digitally with the PLC. In practice this is only slightly better than your timer logic. We have a system doing the same thing you appear to be doing, and it's always off by about 1 to 5 thousand gallons.not bad considering the daily total is over 400,000.To do this, the logic that totalizes is in an STI routine so that it totalizes at a definte frequency. With that said, you can still get close, and should be able to get within a couple percent. I don't care how fast you scan the input and how precisely you scale the signals, there will be at least some tiny error.and that tiny error gets added to the total 10 time a second for 24 hours.So don't expect a perfect match. If you're using a PLC analog input from the flowmeter analog output which represents the flow rate at any given time, there will always be a discrepancy. Being off by 2 thousand gallons a day might be pretty darn good if you're using a half million gallons a day.