Positive Signal Edge Bit not working

[JDCROCKETT724]

I am using Portal for the first time and I am simply trying to do a one shot. My code wasn't working so I made a new network with all new tags so that I know they are not used else where. In the PLC simulator I cannot get the test bit to SET. What am I doing wrong here?

 

[kamenges]

Remove the NO instruction from the start of the rung.
Replace the tag on the top side of the positive edge detect instruction with the tag that used to be in the NO instruction you deleted.
Party on.

The positive edge instruction is a fully autonomous instruction. It doesn't need anything upstream of it to work like the AB equivalent would. The "top" tag is the condition you want to monitor for the leading edge, the bottom tag is the oneshot memory bit.

Keith

 

[JDCROCKETT724]

Thanks for the info. That makes sense. I'm a little mentally drained right now; so bare with me. With AB I could put several conditions before the ons. With what you're saying I wouldn't be able to do that without adding another rung just for the positive signal edge. Am I missing something here?

 

[kamenges]

No, you aren't missing anything. If you have multiple conditions that all need to be true to trigger the oneshot then you will need to combine them on a rung prior to the rung with the set instruction and drive an instruction you will use on the positive edge detect instruction.

However, I would challenge you to think through this somewhat carefully. Generally speaking, users really only expect one condition of the multiple conditions to trigger the edge. The others are just permissives to the edge detection. So if you are really just concerned about a bunch of conditions being true at the time one specific condition becomes true you just need to use that one condition in the positive edge detect instruction and use the others as permissives (normally open) in the same rung. If the order of the conditions is random and you truly want to trigger the oneshot when all of them become true regardless of order then you will need to add the extra rung and combine the conditions.

Keith

 

[mk42]

Because Siemens likes providing OPTIONS, there are actually 4 different types of edge detection instructions. I'm speaking in terms of positive edge detection, but everything is repeated for negative/falling edge detection.

--|P|-- is a contact that reads the top tag, stores it in the bottom tag, and evaluates TRUE for one scan on the rising edge.

--(P)-- is a coil that reads the rung status (RLO in Siemens speak), stores it in the bottom tag, and sets the top tag as true for one scan on a rising edge. It does not affect the RLO of the rung, so if you put a coil after it, the coil will be true based on the preceding logic, unaffected by the edge detection.

The P_Trig instruction reads the RLO, stores it in the bottom tag, and the q output evaluates as true for one scan. This affects the RLO, which means you can use it to directly feed your SET instruction.

The R_Trig instruction is an FB, which means that it creates an instance DB (or can be multi-instanced into another FB) instead of being fed a single bit. It basically works the same as the --(P)-- coil, but it stops evaluating if you disable the block at the EN input.

Personally, I rarely use R_Trig or --(P)--, I mostly stick with P_Trig and --|P|--. Mostly personal preference, but the R_Trig instruction takes a lot more screen space.

 

[JDCROCKETT724]

All great info. Thanks for your help. This must be new with portal. With step 7 I remember using the P edge like an AB ons. Once the conditions before are true then it triggers. At least I know how to work it now. Their help files on this instruction were vague at best.

 

[kamenges]

Originally posted by mk42:

The P_Trig instruction reads the RLO, stores it in the bottom tag, and the q output evaluates as true for one scan. This affects the RLO, which means you can use it to directly feed your SET instruction.

I didn't know that. More correctly, I was too lazy to ever look for it. For old AB guys like me, this is the one that "feels" most familiar, although, like I said, the |P| variant probably matches what we usually want to accomplish most closely.

Keith

 

[mk42]

I didn't know that. More correctly, I was too lazy to ever look for it. For old AB guys like me, this is the one that "feels" most familiar, although, like I said, the |P| variant probably matches what we usually want to accomplish most closely.

Keith

If it's any consolation, the edge detection commands are also different between Simatic Manager and Portal, so it confuses experienced Siemens guys as well. Both of the edge detection instructions change, and then 2 more are added. One of the new instructions looks the same as the old instructions, but works differently.

The --(P)-- command in v5.5 works the same as the P_Trig in Portal. They switched it from a coil to an instruction/box, I think to line up with the IEC standard. The NEW --(P)-- instruction is completely different.

The POS command in v5.5 was an instruction/box that worked like the --|P|-- command in Portal. Again, I think it was changed for closer IEC alignment.



I like having options, but I have to look at the help every single time to figure out which edge instruction does what I actually want.