Program Mode The program mode is used to enter a new program, edit or update an existing program, upload files, download files, document (print out) programs, or change any software configuration file in the program. When the PLC is switched into the program mode, all outputs from the PLC are forced off regardless of their rung logic status, and the ladder I/O scan sequence is halted. Run Mode The run mode is used to execute the user program. Input devices are monitored and output devices are energized accordingly. After all instructions have been entered in a new program or all changes made to an existing program, the processor is put in the run mode. Test Mode The test mode is used to operate or monitor the user program without energizing any outputs. The processor still reads inputs, executes the ladder program, and updates the output status table files, but without energizing the output circuits. This feature is often used after developing or editing a program to test the program execution before allowing the PLC to operate real-world outputs. Variations of the test mode can include the single-step test mode, which directs the processor to execute a selected single rung or group of rungs; the single-scan test mode, which executes a single processor operating scan or cycle; and the continuous scan test mode, which directs the processor to continuously run the program for checking or troubleshooting. Remote Mode Some processors have a three-position switch to change the processor operating mode. In the Run position, all logic is solved and the I/O is enabled. In the Program position, all logic solving is stopped and the I/O is disabled. The Remote position allows the PLC to be remotely changed between program and run mode by a personal computer connected to the PLC processor. The remote mode may be beneficial when the controller is in a location that is not easily accessible. 5.11 Connecting with Analog Devices Electrical devices and signals can be divided into two categories: analog and digital. Digital devices operate using discrete ON or OFF signals that have only two possible values. Analog signals can take any shape and represent an infinite number of possible values, as illustrated in Figure 5-53. Analog circuits are usually much more susceptible to noise (small, undesired variations in voltage). Small changes in the voltage level of an analog signal Highlighted rungs indicate the instruction is true. Figure 5-51 Monitoring a ladder logic program. for PLC control. Not only is it reasonably intuitive, especially for users with relay experience, but it is also particularly effective in an online mode when the PLC is actually performing control. Operation of the logic is apparent from the highlighting of rungs of the various instructions on-screen, which identifies the logic state of contacts in real time (Figure 5-51) and which rungs have logic continuity. For most PLC systems, each Examine If Closed and Examine If Open contact, each output, and each branch Start/End instruction requires one word of user memory. You can refer to the SLC 500 Controller Properties to see the number of instruction words used and the number left as the program is being developed. 5.10 Modes of Operation A processor has basically two modes of operation: the program mode and some variation of the run mode. The number of different operating modes and the method of accessing them varies with the manufacturer. Figure 5-52 shows a typical three-position keyswitch used to select different processor modes of operation. Some common operating modes are explained in the following paragraphs. RUN REM PROG Figure 5-52 Three-position keyswitch used to select different processor modes of operation. Basics of PLC Programming Chapter 5 93
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