Commissioning is the key work to check whether the PLC control system can meet the control requirements and is an objective and comprehensive evaluation of the system performance. The system must be put into use after strict commissioning of the whole system function until it meets the requirements and is confirmed by the relevant user representatives, supervisors and design signatures before it is put into use. The commissioning personnel should have received special training on the system and be familiar with the composition of the control system, the use and operation of hardware and software.

The commissioning staff should contact the relevant designers in time for any problems found during commissioning, and make changes only after the designers have agreed to do so, and make detailed records of the changes, and back up the modified software. And the debugging modification part of the good documentation and archiving. The debugging content mainly includes input and output functions, control logic functions, communication functions, processor performance testing, etc.

Input and output circuit debugging
1, the analog input (AI) circuit debugging. To carefully check the address assignment of the I0 module; check whether the circuit power supply (internal or external power supply) is consistent with the field instrument; use the signal generator to add a signal to each channel at the field end, usually taking 0, 50% or 100% three points to check. For AI circuits with alarm and interlock values, the alarm and interlock values (such as high alarm, low alarm and interlock points as well as accuracy) should be checked to confirm the correctness of the alarm and interlock status.

2, analog output (AO) circuit commissioning. According to the requirements of the circuit control, manual output (i.e. set directly in the control system) can be used to check the actuator (e.g. valve opening, etc.), usually also taking 0, 50 % or 100 % of the three points to check; at the same time, through closed-loop control, check whether the output meets the relevant requirements. For AO circuits with alarm and interlock values, the alarm and interlock values (e.g. high and low alarm and interlock points and accuracy) are also checked to confirm the correctness of the alarm and interlock status.

3, switch input (DI) circuit commissioning. Short or disconnect the corresponding field end, check the change of the light-emitting diode of the corresponding channel address of the switching input module, and check the change of the channel on and off.

4, switch output (DO) circuit debugging. The output points can be checked by the forced function provided by the PLC system. By forcing, check the change of the light-emitting diode of the corresponding channel address of the switching output module, and check the change of the channel on and off at the same time.
Note for circuit commissioning
1、For the switching input and output loops, attention should be paid to the principle of consistency of the state; the principle of positive logic is usually adopted, i.e. when the input and output are charged, the state is “ON” and the data value is “1″; conversely, when the input and output are de-energized, the state is “OFF” and the data value is “1″. When the input and output are de-energised, the state is “OFF” and the data value is “0″. This makes it easy to understand and maintain.

2, the switch input and output modules with large loads should be isolated from the site through relays; that is, the site contact should not be directly connected to the input and output modules as far as possible.

3, when using the forced function provided by the PLC, it should be noted that after the test is completed, the state should be restored; at the same time, too many points should not be forced to operate to avoid damage to the module.
Control logic function debugging
The control logic is to be debugged in conjunction with the design and process representatives and project management. The processor’s test function is used to set the input conditions and check that the output state changes correctly according to the processor logic to confirm the control logic function of the system. For all interlocking loops, the process conditions of the interlock should be simulated, the correctness of the interlocking action should be carefully checked and the commissioning records and countersignature should be made.

This is the most complex, technically demanding and difficult part of the commissioning process. Especially in the case of patented technology applications, special software, etc., the correctness of the control should be carefully checked and a certain margin of operation should be left, while ensuring the normal operation of the process operation and the safety, reliability and flexibility of the system.
Processor performance testing
Processor performance tests are carried out in accordance with the system specification to ensure that the system functions as described and is stable and reliable, including checks on system communications, battery backup and other special modules. Redundancy testing must be carried out on systems with redundant configurations. This means a thorough check of the redundant parts of the design, including power supply redundancy, processor redundancy, I0 redundancy and communication redundancy.

(1) Power supply redundancy. If one of the power supplies is cut off, the system should continue to operate normally without disturbance; the power supply that has been cut off can be restored to normal after power is added.

(2) Processor redundancy. If the power supply of the main processor is cut off or the operation switch of the main processor is switched, the hot standby processor should be able to become the main processor automatically, the system should run normally and the output should be free of disturbance; the processor that has been powered off can be restored to normal and in standby state after power is added.

(3) I0 redundancy. Select input and output points that are mutually redundant and have corresponding addresses, with the input module applying the same input signal and the output module connected to a status indication instrument. The redundant input and output modules are switched on and off separately (or hot-swapped, if allowed) to check that their status remains unchanged.

(4) Communication redundancy. The system can be checked for proper communication and operation by disconnecting one of the communication modules or disconnecting a network; after resetting, the corresponding module status should automatically return to normal.

The redundancy test should be carried out on all modules with redundant design according to the design requirements. In addition, the check of system functions includes system self-test, file finding, file compilation and down-loading, maintenance information, backup and other functions. For more complex PLC systems, the system function check also includes logic diagram configuration, loop configuration and special I0 functions.