INTRODUCTION

CSS fault recovery is typically straightforward: clear the jam or replace the tape and reset the machine.  Occasionally, the operator may encounter a situation where pressing the reset button will not reset a fault. 

OPERATION

Failure to Reset can be caused by a number of issues. The PLC checks numerous systems during operation. Those systems can be divided into internal PLC hardware, external machine hardware, software, and machine states. This article addresses a particular case with PLC Hardware errors: backplane communication loss.  

OBSERVATION

The issue typically presents with a customer complaint that the machine is in a non-recoverable fault state.

• The HMI shows "Fault" without a description of the current alarm.
• The Stack light is completely off.
• Reset button is ineffective.
• Power cycling the machine is ineffective.
• HMI is unresponsive.
  • Login is unavailable, therefore...
  •  "Alarm List" is also unavailable.  
• Flashing Red "OK" LED on PLC 

PROCEDURE

First, eliminate the possibility of other simple hardware faults:

• ALL E-stops are disengaged.
• Air pressure is sufficient. 
• Faulted HMI is unresponsive, and login is not possible. 

If these measures are ineffective, proceed to troubleshooting inside the electrical cabinet. 

Follow appropriate LOTO procedure.

Open the electrical cabinet and power on the machine with the disconnect engaged.

1) Verify:

• All the PLC Modules power up.
• The "OK" LED on the PLC is flashing red. 

2) Power OFF the machine 

3) Reseat the End Cap Terminator. 

The end cap terminator is connected to the far-right side of the PLC rack. The end cap serves as the backplane communication terminator, which lets the PLC know that it is the end of the transmission line. 

The thin module should be removed from the rack:

• Locate the latch at the top of the module.
• Release the latch by swinging it sideways and down (clockwise- from 12:00 to 2:00)
• Slide module out by pulling it straight towards you or away from the DIN rail.

Check the end cap for obvious signs of damage or dirty connectors. Clean the module with clean, dry compressed air if necessary. 

• Reinsert the end cap into the PLC Rack
• Fully close the Latch- be sure it snaps flush with the case. 

4) Power ON

5) Use the mode switch to reset the PLC error.

• Swing the cover open below the LED indicators on the front of the PLC. 
• Move the switch to PROG...
• then back to REM or RUN

6) Power Cycle with the mode switch in Run or Remote

CONCLUSION

After the PLC boots, the "OK" LED should be green. It might take several power cycles, but the flashing red LED should be replaced with a solid green indicator to show that the PLC error has been cleared. If the error returns within a short period, the end cap terminator should be replaced.

When the PLC experiences a major non-recoverable error, it enters a faulted state and stops communicating to any of the attached peripherals. This is why the HMI becomes non-responsive and the Alarm List is unavailable. Communication loss, memory failure, program failure, or power surges can all cause major issues, but backplane communication loss is the most common occurrence.  Reseating the backplane transmission line terminator and clearing the fault should restore machine function.