B875-111 Analog Card w/2 DT500-A111 Devices

[CoastalControl]

Hello friends,

The opportunity I am faced with is as follows.

B875-111 Wired for analog 4-20, Dip switches set correctly.
Wire a DT500-A111 (4-20) to Channel 1
Wired another DT500-A111 (4-20) to Channel 6

Both have the same power supply @ 24V.

Problem (Opportunity): When either of the DT500-A111 are connected, it functions perfectly. Set up fine and reads fine. All measurements are exact.
When both are connected at the same time, the counts change considerably and it appears as if they are at the same count they would be if it were averaging each others current input.

I would appreciate any help on this issue.

Thank you,

Pat

 

[CoastalControl]

UPDATE

It appears that the power supply is the culprit. When I put the "SICK" DT500-A111 scanners on different power supplies, the count come through correctly on each scanner.

Why? I do not know.... When both scanners use the same 24v power supply, the counts are close to the same. When using different 24v power supplies, they operate correctly.

 

[danw]

I assume you have wired each input as a differential input (the diagram on the right in the lower image), for example, device 1 across terminals 3 and 4 for Input 1 with terminal 2 tied to terminal 3 and device 2 across terminals 7 & 8 with 6 jumpered to 7?

The specs are quite reasonable for isolation. The effect you're seeing says that the currents merge at some point in their circuits. By any chance is one side of the power supply grounded, or is the power supply floating?

 

[CoastalControl]

Both power supplies are grounded on the 120V side. This is the ground used throughout the system.

I agree with you that:
The effect you're seeing says that the currents merge at some point in their circuits.

I am trying to isolate this. I sure do appreciate your input.

Yes, as you have shown in the diagram is how my devices are wired. The card is able to
accept 8 analog inputs and these are split into two halves. I have one device on the top half and the other on the bottom. I am using 30020 as one input and 30025 as the other.

Any other suggestions would be helpful at this point.

Thank you!

 

[danw]

1) Is the negative (return) of the DC power supply grounded?

2a) Does the cable have a shield?

2b) A shield has two ends, field end and I/O card end.

Is the shield floating (ungrounded)?
Is the shield grounded at either end? Which end?
Or is the shield grounded at both ends?

 

[CoastalControl]

I'm sorry for being away... my other work.

1) Is the negative (return) of the DC power supply grounded?
Answer: The power supplies are powered with 120VAC. 3 wires Black,White,Green
I only have this ground (green). The 24VDC out from the power supply has + and -
Automation Direct PSM24-180S 180W
I have checked ground to neutral with my meter and discovered .5V between them.
I am in the process of running a separate ground.

2a) Does the cable have a shield?
Answer: NO The DT-500 5-pin M12 connector/cable is NOT shielded? I don't know why.
I have one that is shielded coming in soon.

2b) A shield has two ends, field end and I/O card end.
Answer: I understand that only one end is grounded. For noise....

Is the shield floating (ungrounded)?
Is the shield grounded at either end? Which end?
Or is the shield grounded at both ends?

Answer: No shields. huh.. do ya think this could be the culprit?

Probably. I am in the process of trying to have as little potential as possible on my grounding.

Any suggestions? Another ground rod? Isolated PS?

Thank you again for your help!

Pat

 

[danw]

I can never de-crypt Sick's wiring puzzle, there's never a legend. I haven't a clue which terminal is which for the DT500. How did you wire it?

 

[CoastalControl]

B875-111 card using Channel 1 and 2
the first channel is wired this way.

L+ ----is 24VDC + (Brown) to positive terminal on Power Supply
M ----is 24VDC - (Blue) to negative terminal on Power Supply
Qa ----is current IP+ (White) Terminal # (2 and 3) they are jumped for 0000-9999 counts
MQa --is current IP - (Black) Terminal # (4)

The MF is a trigger of some sorts. I am not using this one.

Like I said, the cables from SICK do not have shields.

I did get one of the channels to be constant. I do not know how. I just touched terminal (5) with a ground jumper from the PS and walla , it became solid. The other one tied to a different PS still floats around. I then just changed cables from the scanners. Swapped cables and the floating also followed after swapping. It is a mystery.

Thanks for the help! I will keep trying. I had two completely different PS's so I have ordered another Automation Direct PSM24-180S. We will see what happens when it arrives on Thursday.

Thanks again,

Pat

 

[CoastalControl]

Hello,

I have resolved the issue.

The SICK scanners use the "DC COMMON" electronically internally tied together.

When two scanners are connected to a B875-111 Analog Input Module, there "I-" are tied together when you use "differential mode". This caused the plc to received the signals coming from each scanner to be "averaged" or share the same input signal.

When I put the card into "Single Ended" mode, and tied both I- from each sensor to terminal 5 ( scanner 1 tied to Terminal 2 and scanner 2 tied to Terminal 3), I had no problem with using the same power supply and the signals were perfect.

I wast to thank "danw" for his help and want to add that this site has helped me through many unanswered issues since I have been a member.

Thank you!

Pat