Input and Output Control

The complete process control system program has inputs and outputs corresponding to the process equipment. These I/O points should be treated differently within the control system code. The inputs should appear wherever they are needed throughout the code. The outputs, however, are located in the appropriate unit or connection section of the program, and demand a more formal structure.

Because a particular output may be referenced in more than one phase block within a unit or connection, a conflict could arise if the direct output address is used in multiple places of the program. Therefore, we strongly suggest that you place all outputs, no matter how many times you use them, in the global output section. Any phase block requiring the output would energize an intermediate discrete memory location that would then be placed in the appropriate location of the output section. Thus, an output used by more than one phase block would contain multiple discrete locations connected in parallel, each of which would independently turn on the particular output. Also, all manual device operation logic exists in this section. The reasons behind placing all outputs in the global output section are consistency and ease of troubleshooting. Finally, for safety reasons, alarms that disable one or many outputs should exist in the global output code and not scattered throughout the phase logic. This ensures that the outputs are disengaged.