Question regarding I/O module positive/negative rail voltage

maceng

Member
Join Date
Jul 2015
Location
Toronto
Posts
3
Hello,

I am a new member and have a question regarding old PLC systems. We are still using Modicon 484 PLC at work. The system uses discrete I/O modules with +/- 48VDC control voltage. Both voltage rails are individually fused. While performing load verification, a maintenance personnel pulled the negative rail causing the PLC to send a false signal causing inadvertent actuation of an end device. It is important to note that when pulling the positive rail fuse by itself, the system did not respond the same way.

What is the significance of the negative rail? why did the system behave abnormally when the negative rail fuse was pulled?


Thank you very much.
 
It's difficult to tell without an equivalent schematic of the output circuit of the module. If you have a link to the module documentation it would help.

The description of +/- 48 volt control voltage is different from my usual experience. For DC outputs we generally use 24VDC sourcing output modules. The low side of our 24 VDC supply is attached to the - common. The high side of the supply is routed through safety contacts (for motion causing devices) then to the + common. In some cases removing the - common causes the output circuitry to pull high even if not turned on. We never fuse the - common connection.

This may not help at all and a better evaluation would depend on an analysis of the circuit. You could also directly ask the people at Modicon.
 
Thanks for the reply Bernie!

I tried to reach out to Modicon tech support but the system is old and no longer supported. We replicated the fault by removing the negative fuse; the P190 computer was connected this time so we were able to see the input module pulling high! The schematic of the module (B553) is found on page 11 of the attachement:

http://v1.graybar.com/automation/ga_manuals/Hardware/484/484 500 Series IO Devices.pdf

It would be highly appreciated if you could have a look and help me understand what is happening with the circuit when disconnecting the negative line (fuse not shown in the schematic).


Regards.
 
It dos make sense that it would act the way you described, it would depend on the internal wiring of the module pull up or pull down resistors connected to the + or – rail are common on both inputs and outputs.
If you can program the unit I would monitor the rail voltage and shut down the outputs it a problem is found that way should you blow a fuse while running the outputs will just turn off.
On large system I normally monitor the module power supply and at least post an alarm so the operator knows that the inputs are unreliable and to get it fixed
 

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