STEP; SNXT

[Guest student]

hi Enclosed a programmed traffic light . The red light doesnt come. I surmise the HR area wasnt allocated right, even though i have downloaded the HR area from the data view editor to the PLC. Has a 1 to be set anywhere in the binary data view editor (eg 100000000000000) ? Thanking you in anticipation! traffic.doc

[Jay Anthony 118]

Is that all there is to your ladder? Is there any END function? Are there any other steps? Why are you SNXT to 0000.01? Your step sequence is using HR0 as the Step word. If you use 253.13 (Always On) you will never leave step HR0000.00. If you have more ladder, please zip the CXP file and post it.

[Guest student]

hello Jay Well, i tried instead always_on a normal input, but i couldnt leave the 1st SNXT.I dont think always_on is the problem here ( i wanted a cyclic step programe).I rather think the problem lies with the HR area since i cant switch to the next step. Attached is word.doc and the syswin ladder. NB: i dont know what to do without you *hugs* Edited 16 Jul 2003 by student

[Guest student]

here we go Traffic.SWP

[Jay Anthony 118]

OK. You were definitely on the right track. You just got bitten by the HR bug. HR relays are battery backed and retain their status on power down. Apparently in your debug stage, you left an HR bit on all the time and there was no way to turn it off including power down. I have put a MOV statement at the top of your ladder which should take care of getting two HR bits on at the same time. I also used the first scan flag to start your sequence on power up. Other than that, you understand the concept of Step/Snxt. A traffic light (with apologies to PLCs.net) is an excellent use for Step sequences. I will post the code in the following post.

[Jay Anthony 118]

Heres the ladder code: TRAFFIC1.SWP

[Guest student]

hi there yes it works. maybe it works too good now i cant stop the sequence @ the PLC.It runs and runs.... there must be a reset switch ?

[Jay Anthony 118]

How about a enable switch in the first Step? Use Input 0.00.