Essential Steps for PLC Debugging for Electrical Engineers

Input and Output Loop Debugging

Carefully check the address allocation of the I0 module; check whether the loop power supply method (internal or external) is consistent with the field instruments; use a signal generator to add signals to each channel at the field end, usually checking three points: 0, 50%, or 100%. For AI loops with alarm and interlock values, check the alarm interlock values (such as high alarm, low alarm, interlock points, and accuracy) to confirm the correctness of the relevant alarm and interlock status.

According to the requirements of loop control, check the actuators (such as valve openings, etc.) using manual output (i.e., directly set in the control system), usually checking three points: 0, 50%, or 100%; at the same time, check whether the output meets the relevant requirements through closed-loop control. For AO loops with alarm and interlock values, check the alarm interlock values (such as high alarm, low alarm, interlock points, and accuracy) to confirm the correctness of the relevant alarm and interlock status.

Short-circuit or disconnect at the corresponding field end to check the changes of the LED corresponding to the channel address of the digital input module, while checking the changes of the channel’s on and off states.

Check the output points using the forced functions provided by the PLC system. By forcing, check the changes of the LED corresponding to the channel address of the digital output module, while checking the changes of the channel’s on and off states.

Loop Debugging Precautions

The positive logic principle is generally used, which means that when the input and output are powered, it is in the “ON” state, and the data value is “1”; conversely, when the input and output are powered off, it is in the “OFF” state, and the data value is “0”. This makes it easier to understand and maintain.

This means that field contacts should not be directly connected to input and output modules.

At the same time, do not force too many points at once to avoid damaging the module.

Control Logic Function Debugging

Control logic function debugging must be completed in conjunction with design, process representatives, and project management personnel. Use the processor’s testing functions to set input conditions, and check whether the output state changes according to the processor logic to confirm the control logic function of the system. For all interlock loops, simulate the interlock process conditions, carefully check the correctness of interlock actions, and keep detailed debugging records and confirmations.

Inspection work is the process of acceptance for the design control program software, and it is the most complex, technically demanding, and difficult task in the debugging process. Especially in cases involving patented technologies and specialized software, it is even more critical to check the correctness of the control, ensuring a certain operating margin while guaranteeing the normal operation of process operations, as well as the safety, reliability, and flexibility of the system.

Processor Performance Testing

Processor performance testing must be conducted according to the requirements of the system manual, ensuring that the system has the functions described in the manual and is stable and reliable, including checks on system communication, backup batteries, and other special modules. Systems with redundant configurations must undergo redundancy testing, which involves comprehensive checks of redundant design parts, including power redundancy, processor redundancy, I0 redundancy, and communication redundancy.

Cut off one power supply, and the system should continue to operate normally without disturbance; the powered-off power supply should return to normal after being powered on.

Cut off the main processor power or switch the main processor’s operation switch; the hot standby processor should automatically become the main processor, and the system should run normally without disturbance; the powered-off processor should return to normal and be in standby status after being powered on.

Select input and output points that are mutually redundant and correspond to addresses, apply the same input signal to the input module, and connect the output module to a status indication instrument. Turn on and off (or hot swap, if permitted) the redundant input and output modules separately to check if their states can remain unchanged.

Redundancy checks must be performed on all modules with redundant designs according to design requirements. Additionally, checks on system functions include system self-checks, file searches, file compilations and downloads, maintenance information, backups, and other functions. For more complex PLC systems, the system function checks also include logic diagram configurations, loop configurations, and special I0 functions.