Why am I getting a steady red OK led on my HI 1756WS?

We recently experienced three instances where our HI 1756WS module had a steady red OK led. We reset it by removing it from the PLC chassis and plugging it back in. Do you think my HI 1756WS is faulty, or could this problem be caused by something else?

After resetting the module in this manner, why do we need to do a calibration to get it to weigh correctly?

Based on the symptoms your provided, it sounds like your HI 1756WS module is experiencing a voltage spike or static discharge "hit", causing it to shut down communication between itself and the PLC. It can also be caused by a voltage spike from the load cell cable being routed too close to a higher voltage cable (over 50 volts) that can transmit noise to the load cell line.

Please check the grounding on your load cells. Verify the cable shield is connected to a single good ground point at the controller. If your facility is susceptible to static build-up you should install a ground strap bypassing each load cell from the live load side to ground.

When the module takes a "hit" like this, it can corrupt the memory of the HI 1756WS module. This can cause an interruption in operations like what you are experiencing. If your HI 1756WS module loses calibration after a "hit", it would indicate that either the Save command is not being run after doing the calibration, or the memory is so corrupted it cannot recover by itself.

If the save command is not being performed after a calibration, if you cycle power on the module or pull it from the chassis and plug it back in, the calibration would go away. If this is the case, then doing a save command may allow quicker recovery after a "hit".

If the save command is being performed and your 1756WS module loses calibration after a "hit", then the prevention of the "hit" would be the only way to simplify this issue. Installing a ground strap that bypasses your load cell should direct any static build-up to ground.

NOTE: It is possible that the module took a hit at one time and the corruption is still in the module. If this is the condition, the corrupted memory may lay dormant until a certain condition arises and then it corrupts the operation. In this case you will need to do a "Set Default Parameters" command and clear the memory of the module, then re-calibrate your system (don't forget the save command).