Simple question (I hope)

[hongkongfooey 0]

I am debugging someone elses ladder code to learn how the system is supposed to run. I haven't been doing ladder for very long, and i have a question about something. What is the purpose of having an internal binary bit that doesn't have any way to either latch it(OTL) or energize it (OTE)? I have attached an example from the code I am working on, as an example of what I am talking about. You can see that B15:0/5 has no way to latch or energize it. How does it toggle? (BTW thanks to Ron Beufort for the "how to" attach files) Edited 7 Dec 2004 by hongkongfooey

[mgvol 0]

I have never seen anything like this that actually WORKED; does this logic actually ever go true? Is it possible that someone inadvertantly deleted the OTL for this bit? What does the rung containing the OTU look like? It could be that this is some sort of test bit that a programmer or maintenance people put in for infrequent use, and the rung w/the OTU just clears the bit if they forget to do it manually. But I'm not even sure that would work. I will try it and post my results.

[mgvol 0]

O.K., here's what I did: I created a rung XIC B3:4/0 that turns on coil B3:4/1. In the second rung I create XIC B3:4/1 starting TON T4:4 (30 seconds) with parallel branch T4:4/DN OTU B3:4/0. To test my maintenance bit theory, I right-click on the XIC B3:4/0 and select Toggle Bit to turn it on. When OTE B3:4/1 turns on, the timer starts timing. When the timer is done (/DN), the OTU turns off the bit I toggled (B3:4/0). Check and see if any of this makes sense by looking at the rung in your program w/ the OTU. See attached screen shot. ladder1.doc

[Snerkel 0]

The bit could be being set by an operator interface, SCADA, or another PLC etc Also check for B15:0 it could be being set at word level. The rung with the OTU may give more clues as to its use.

[Guest]

Greetings hongkongfooey, there are several ways that the bit in question could be controlled through the “backdoor” and not show up on a “find all” search ... here are just a few examples ... sorry that I’m short on time ... but post again with any specific questions that you might have ... I might have time to go into more detail tomorrow ... and please be aware that the instructions shown here are just examples ... there are MANY instructions that can control “at the WORD level” or “at the FILE level” ... one helpful way of trying to track this down is to do a less-restrictive “find all” search ... for example: try searching for just B15 rather than for B15:0/5 ... this might help you find the controlling culprit ... basic idea: in Microsoft Word, searching for “cat” will find the word “cat” and the word “catalog” ... but searching for “catalog” will find ONLY the word “catalog” but NOT the word “cat” ... point: the search for “catalog” is more restrictive ... good luck and best regards, Ron PS edit ... it looks like Snerkel had the same ideas ... and he obviously types faster than I do ... I'll post this anyway since I hate to waste the effort ... sorry if it's repetitious ... Edited 7 Dec 2004 by Ron Beaufort

[Sergei Troizky 26]

I often use similar looking switches or plugs for debugging or diagnostics. I control them from my PC when online and no other control is intended.

[hongkongfooey 0]

I can see how this type of XIC could be used as a Maintenance bit or similar situation. However if you look at this line of code(rung0035) you will see that O:2/5 is a real world output that causes an actuator arm to tilt down. The actuator arm is used several times during the run, and this is the only way that the arm can be actuated, afaik. Also 15:0/5 is located in almost all of the movement output rungs so it is integral to the operation of the program. I looked through the bit usage files as Ron suggested, but I did not see any FW usage for bit 15:0/5. Or any other word, or file usage for it. There is a touch screen operator interface however, but shouldn't I be able to see the "energizing" function from the interface panel located in the code? I guess what I am going to have to do is, hook up my laptop to see when and how this bit is "energized". Thanks for the help, Edited 7 Dec 2004 by hongkongfooey

[mgvol 0]

If the input is from a touchscreen "pushbutton", there wouldn't neccessarily be an output coil. If an object on the touchscreen is addressed to this bit, it would just go true when pressed. Look at the tag database for the touchscreen. I'll bet it is in there and that would explain everything (especially if the property of the touch object is set to "latched" or "maintained").

[Snerkel 0]

The touchscreen setting the bit directly makes sense, it looks like the programmer has written the code in a structured way so you will probably find all B15 addresses are from the operator interface. You may well find that pressing the screen button for a certain function causes the button to appear depressed or a different colour etc until that function has completed, hence the OTU. Example Operator presses the green coloured Vice Open button on the screen, this sets B15:0/0 (Latched) Because B15:0/0 is on (and remains on) the screen button changes colour to yellow and the PLC opens the vice. The vice hits the open limit which unlatches B15:0/0 (using OTU) turning the button back to green.