Fanuc Robot Group IO Problem

Hi all. This is a Fanuc Robotics related question. I could really use some help with this unusual problem. I have a Fanuc M-16i robot (w/ an RJ-3 controller). I also have a Micrologix 1400 used in conjunction with the controls. I’m trying to use the Group IO function on the robot for the first time. I want to use only 5 bits. It all seemed fairly simple to configure. The problem I’m having is that when I turn on a certain set of bits in this group, I’ll get an INCORRECT integer number. I hope this explains the problem I’m encountering:

I configured GI [1] = Rack 1, Slot 2, Start Point 1, Num Pts 5 (remember I only want to use five bits as a group. I assigned 5 (five) Micrologix output bits to each Group input point for GI[1]. I verified my group signals this way:

1) When I turn on ONLY Bit 1, GI[1]= 1 (Correct!)
2) When I turn on ONLY Bit 2, GI[1]= 2 (Correct!)
3) When I turn on ONLY Bit 3, GI[1]= 4 (Correct!)
4) When I turn on ONLY Bit 4, GI[1]= 8 (Correct!)
5) When I turn on ONLY Bit 5, GI[1]= 16 (Correct!)
6) When I turn on Bits 1 and 2, GI[1]= 3 (Correct!)
7) When I turn on Bits 1, 2 and 3, GI[1]= 7 (Correct!)
8) And as I keep checking the bit sequence like this, all seems to be going accurately….EXCEPT....when I start using Bits 3, 4 and 5 together, the problem arises.

When I turn on Bits 3 and 4, GI[1]= 28 (INCORRECT!) – This should be 12.

As I’m discovering, turning on bits 3, 4 and 5 individually will give me the correct values. But when I turn on any two of these bits (3,4,5) I’m always getting a value of 28. Basically, any combination of two inputs between IN3,IN4 and IN5 is giving me 28. This is wrong.

Example:
Bits 3 and 4 turned on is giving me 28 (should be 12)
Bits 4 and 5 turned on is giving me 28 (should be 24)
Bits 3 and 5 turned on is giving me 28 (should be 20)

By the way, all bits ON is giving me 31 (Which is correct!)

I don't get it! I’m stumped as to why this is happening. I hope my explanation makes sense. And I hope it’s something as just overlooking some kind of configuring thing I may have missed. Can anyone that understands my problem help me.
I truly appreciate it!
Thanks a bunch!

You might want to post this question at www.robot-forum.com as well.

Is the same input space mapped into DI or DO bits? If so, check that they match what you are seeing in input group #1.

Have you physically measured the inputs to see that there isn't a short?

Any background tasks running that step on GI[1]?

Any UI or UO bits assigned to the same rack/slot?

Problem solved!!!

The issue was with how I wired the outputs on my Micrologix 1400 (or should I say, how I didn't wire them).

My ML 1400 controller has groups of outputs that are isolated relay outputs and one group of outputs that are FET outputs. I didn't catch this when I wired all my outputs.

The last 3 bits of my robot Group Inputs (bits 3,4 and 5) are wired to these FET outputs. I needed to wire a 24vdc (-) to the COM terminal for this group to function properly. I don't have this wire on that terminal. So, basically, I had a voltage leakage without this COM terminal being wired.

So I added a 24vdc (-) on the COM terminal, tried my sequencing test again and....PRESTO!

All five of my bit signals are now returning the correct integers on my robot group input (GI[1]. Whew!

I appreciate all the help guys.
Thanks!

Good catch by going back and checking the basics. I see a lot of people run into this sort of thing and they will dwell on the robot and start believing in "magic" and start trying to program around the issue. Always good to step back and check yourself, we all make mistakes like that especially when its crunch time.