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Goals and Symptoms
This resolution has been written to inform any automation designer using PLCs(or other controllers) of a potential problem when using inputs in parallel particularly on the same module. It can be applied to any kind of PLC input module. The potential risk, in this case, depends mainly on the design of the input interface module.
Facts and Changes
The input could be part of a PLC or part of a process interface.
· If an input module has multiple inputs channels with common return voltage.
· If the application has inputs connected in parallel (one contact drives more than one input). The inputs could be in different module or part of a process interface
· If the inputs do not have direct serial diode
· If we have disconnection of the return voltage on the common return voltage
· If we have contact active on the module having the defective connection on return voltage
The module having the defective grounding could supply the inputs that are connected in parallel with its input.
Normal condition (application logic)
The gear control device will be active if both input E1 and E2 are ON. In this case, the DIT product will monitor the gear device state.
If one of the input is OFF the gear device will be inactive.
Defect on return voltage (0V)
The E1 and E2 contact are OFF and the gear device should be OFF.
Defect: the return voltage connection of the DIT1 product is defective in open condition.
Then, if one or more contact is ON among the other input of the DIT1, a current can flow back through E1 and E2 of the DIT1 and supply the E1 and E2 input of the Gear control Device.
Depending on the impedance of the system, the Gear control Device could become active.
Remark: the logic application state E1 and E2 of the DIT1 remain stable and are detected as OFF state by the PLC application.
Analyzing Premium schematic.
On TSX57 Premium range, we have 2 inputs modules offer :
· PRODUCTS with possible parallel connection (one contact and 2 inputs or more), in this case the schematic integrates a serial diode
· PRODUCTS where parallel connection is not allowed (one contact with one input only), in this case, the schematic does not integrate a serial diode.)
For both offers, the end user documentation is consistent and precise.
Analyzing the TSX37 Micro range, oldest product and STB product range
We do not have information in the user documentation that provides information on parallel connection of input module.
Causes and Fixes
What could be our recommendation?
To avoid this kind of potential problem, try to use inputs of different modules. In case of use of the same module, use a module offering by design possible parallel connections (if the catalog is explicit enough with this subject or make a test) or add external diodes.
· secure grounding with redundant wiring
This solution consists in creating a loop on the existing wiring system. It prevents basic open wiring defect in the process.
It does not prevent an open connection in the module. We will not recommend this solution.
· Split the input in between 2 DIT PRODUCTS
If a defect occurs case 1, E1 could be ON, but E2 would remain OFF.
If a defect occurs case 3, E2 could be ON, but E1 would remain OFF.
If a defect occurs case 2, E1 and E2 could be ON at the same time
This solution prevents an open connection inside the product.
This recommendation is not proof depending of the ground wiring system.
We recommend adding a ground loop on the ground distribution. A specific care will be taken by the end user on the loop distribution and potential defect analysis on the ground connection.
· Implement a block diode
It consists in adding a serial diode with the input used in parallel interface. The diode is in direct connection with the input.
The global reverse leakage current of the diodes must be compare to the max OFF state current of the gear device.
This solution prevents any current going back to the other input.
This solution prevents an open defect on the ground connection.
This solution is the preferred solution.