PLC QUIRKS i.e. actions, connections...

I do not know why but I thought an AB timer would accumulate time beyond the preset value, got any idea why I thought that?

I just hooked up an old L30 with a flip flop (flasher) program in it and naturally the timers are going to preset then stop accumulating and setting the DN bit.

Sometimes I just have to connect to different plc's to verify what I think is correct, dealing with so many different brands and models I get confused, get confused much easier these days too. I do not work on anything on a daily basis though.

Tomorrow will be a connect to everything I have and look at different things, having a problem with a 90-30 anyway.

As mentioned in another thread there can be things that can bite you in the arse if your not aware or forget about them. One example is AD (Koyo) has memory ranges that are retentive by default, this can cause issues.

Anyway, if you think of any of those "quirks" of the plc's you deal with then please tell about them. This could also be things about communication connections or wiring, anything that could bite ya in the arse per se.

Example: Never connect an SLC fixed 500, 5/01, 02, with an Ethernet cable to your Ethernet port. The ports are DH485 and use an RJ45 connector i.e. NOT ETHERNET

An A/B counter will keep counting past the preset. That is probably what you remembered.

Thanks Okie, I did remember that, not sure why I thought a timer would but I am still RTFM.

In the process I got this from the PLC 5 Enhanced manual:
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1785-um012_-en-p.pdf

1. ​

1. ​

1. false logic executes faster than true logic​
   
   [*]
   ​
   ​
   different instructions execute at different rates​
2. different input states cause different sections of logic to be executed​
3. interrupt programs affect program scan times​
4. editing programs while online affects housekeeping times​

The 2 highlighted I am not sure I was specifically aware of so it is good to RTFM in full.

The round DH+ converter that plugs into SLC's and PLC5's looks darn close to the connector on a micrologix. It will fit but it won't work.

If after trying the above your micrologix doesn't work, you need to reload the firmware and cross your fingers.

Won't forget that one for awhile.

allscott said:

The round DH+ converter that plugs into SLC's and PLC5's looks darn close to the connector on a micrologix. It will fit but it won't work.

If after trying the above your micrologix doesn't work, you need to reload the firmware and cross your fingers.

Won't forget that one for awhile.

Click to expand...

I was so used to using ML1200's that when I opened up a cabinet and tried to connect to the processor, I started trying to fit my cable into the front there. Turns out it was an ML1100, and it's connection was on the side.

I am not sure what I was trying to plug it into, but what ever it was that is on the front of those ML1100's, it faulted the processor and caused it to lose it's stored program... oops.

What is there on the FRONT of a Micrologix 1100 that you could possibly try to plug into?

See that rectangular pin connector on the lower left hand side of the front?

I wasn't looking and tried to plug in to that, and was moving it back and forth trying to figure out why it wasn't finding it's home.

I didn't realize it was a ML1100, just thought it was a ML1200 with 24 I/O.

Tharon said:

See that rectangular pin connector on the lower left hand side of the front?

I wasn't looking and tried to plug in to that, and was moving it back and forth trying to figure out why it wasn't finding it's home.

I didn't realize it was a ML1100, just thought it was a ML1200 with 24 I/O.

Click to expand...

See pic: trying to plug in an external memory module

That explains why shorting some of those pins together wiped the memory and faulted the plc

rsdoran said:

I do not know why but I thought an AB timer would accumulate time beyond the preset value, got any idea why I thought that?

Click to expand...

If the timer's time base is 0.01 (10 msec) and program scan time is greater than 10 msec, then you're likely to see the accumulator overshoot the preset before setting the 'done' bit and stopping.

An AD timer will accumulate past its preset. I thought it was odd the first time I saw it do that after working on a few AB programs.

Brian

rsdoran said:

Anyway, if you think of any of those "quirks" of the plc's you deal with then please tell about them. This could also be things about communication connections or wiring, anything that could bite ya in the arse per se.

Click to expand...

Okay, here's one...

Never 'assume' that labels line up with terminals. Look at the labeling on this power supply from AD...



At first glance, it appears that DC+ is on terminals 2 and 3 (counting from the left). Just look at it... The + labels line up almost perfectly with those terminals. Now that you've finished connecting your DC+ wire, you figure that the next 2 terminals (4 and 5) would be the DC- terminals. Not true! DC+ is on terminals 1 and 2. DC- is on terminals 3 and 4, and terminal 5 is the "DC-OK" terminal, which just so happens to output POSITIVE 24VDC!...

This label mismatch has fooled me more than once...



-Eric

P.S. IMHO, the DC- terminals should really be labeled "COM"...

Re: changing minus labeling on power supply to "common": Since the DC output is isolated from frame ground you could, theoretically, make either the minus or positive terminal your reference (common) or even leave the voltage floating. Their labeling is probably the most clear given these possibilities.

a few quirks from the "PLC Boot Camp" ...

back when I used to be the “PLC Answer Man” for an Allen-Bradley distributor, I had several customers phone up for help with these issues:








Gotcha #1 ... I’ve had several technicians assure me that “Yes, I’m POSITIVE it’s in the RUN mode - because the RUN LED is glowing solid green!” ... notice that on an Ethernet-type processor there IS an LED that lights up green PRECISELY below the “RUN” legend for the keyswitch ... what many people miss is that this particular LED is the “Status” indicator for the processor’s Ethernet channel ... it has absolutely NOTHING to do with the RUN mode ... the LED that the technician SHOULD be looking at is marked “PROC” - for “Processing” ... turns out that the customer’s processor was actually in the “Remote Program” mode - and that’s why nothing was running ... side trip: notice that the picture in the official book calls this LED the “RUN/FAULT” indicator - and not the “PROC/FAULT” indicator ... now why didn’t they just print that on the front of the processor? ...



Gotcha #2 ... there are two EXTREMELY similar legends right next to the keyswitch that give some people problems ... one is the “PROC” legend which stands for “Processing” ... the other is the “PROG” legend which stands for “Program” ... I’ve had several people tell me over the phone that their processor was definitely in the “PROGRAM” mode - because the “PROGRAM” LED was lit up solid green ... source for confusion: the only difference between the two legends is that little “-” thingy that distinguishes the letter “G” from the letter “C” ... people with less-than-perfect eyesight have been known to miss that critical distinction ... the most confusing thing about this is that the two terms “PROCessing” and “PROGram” have EXACTLY opposite meanings ... one means that the processor is “going” - the other means that it ain’t going - and yet both legends are right there on the front of the processor - about one inch apart ...



Gotcha #3 ... a customer called in and wanted me to tell him how to find the forces in a PLC-5 processor ... I was in the middle of the explanation when he said something like: “Yeah, when I came in this morning and saw that FORCE LED flickering green like that, I knew that someone on night shift must have left a force in the system.” ... “Hold everything!” I said ... “You’re telling me that the FORCE LED is flickering GREEN? ... that won’t happen on an Allen-Bradley PLC-5 ... the FORCE LED only lights AMBER - not green.” ... he got a little annoyed and said “Well, I’m LOOKING right at the %!$*& thing!” ... turns out that his PLC was mounted near the bottom of the enclosure - and the technician was standing up while talking to me on the phone ... what he was seeing was the COMM LED for Channel 0 flickering green ... he had his laptop hooked up and communicating through the Channel 0 port ... moral of the story: you need to be on an eye-level with the PLC-5 when you examine the LEDs ... there’s enough space between the cover on the front of the processor and the LEDs behind it to allow viewing an LED through the WRONG little hole in the mask ...