Explain the procedure of building a test harness for verifying PLC code.

Building a test harness for verifying PLC code is a process of creating a simulated environment that allows you to systematically test the PLC program's functionality and ensure it behaves as expected under various conditions. A test harness is a set of tools, data, and procedures used to test a software component or system. For PLC code verification, the test harness simulates the inputs and outputs of the PLC, allowing you to test the code without connecting it to the actual physical equipment. The procedure typically involves several steps. First, identify the key inputs and outputs of the PLC program that need to be tested. Inputs can include sensor signals, operator commands, and network data. Outputs can include actuator commands, alarm signals, and data transmissions. Next, create a simulation environment that mimics the behavior of the external devices connected to the PLC. This can be done using software simulation tools, hardware emulators, or a combination of both. The simulation environment should allow you to define the values of the PLC's inputs and observe the resulting values of its outputs. Then, develop test cases that cover all the important scenarios and edge cases that the PLC program is expected to handle. Each test case should specify a set of input values and the expected output values. Run the test cases and compare the actual output values to the expected output values. If there are any discrepancies, it indicates that there is a bug in the PLC code. Then, document the test results. The test harness can find bugs.