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ASSIGNMENT 2 ANSWERS Question 1 An NO limit switch is to be programmed to control a solenoid. What determines whether an EXAMINE IF CLOSED or EXAMINE IF OPEN contact instruction is used? The normal state of an input (NO or NC) does not matter to the controller. What does matter is that if contacts need to close or to open in order to energize the output. In this case if an NO limit switch is used to control a solenoid, then the EXAMINE IF CLOSED is used in order to energize the output. Question 2 List the five (5) standard PLC languages as defined by the International Standard for Programmable Controllers, and give a brief description of each. 1. Ladder Diagram (LD)- graphically depicts a process with rungs of logic, similar to the relay ladder logic schemes that were replaced by PLCs. 2. Sequential Function Charts (SFC) - flowchart of steps (one or more actions) and transitions (defined condition before passing to the next step). 3. Instruction List (IL) -assembler-type, text based language for building small applications or optimizing complex systems. 4. Function Block Diagram (FBD) -graphical depiction of process flow using simple and complex building blocks, ranging from analog I/O to closed loop control, algorithms, and diagnostics. 5. Structured Text (ST) -a language developed for IEC to provide high-level syntax using “If” and “Then” statements. Question 3 Explain what is meant by a TRUE rung condition and a FALSE rung condition. A true condition exists when the processor finds a continuous path of true input instructions in a rung. A false condition exists when the processor does not find a continuous path of true input instructions in a rung. Question 4 What type of data is stored in each of the following structured data types? a. Control Control structure for array instructions b. Counter Control structure for counter instructions c. Timer Control structure for timer instructions Question 5 Assuming the hardwired circuit drawn in Figure 5-49 is to be implemented using a PLC program, identify a. All input field devices PB1, LS1, PS1, SS1, LS2, LS3, PB2, PB3, LS4 b. All output field devices PL1, SOLA, SOL B, SOL C, PL2 c. All devices that could be programmed using internal relay instructions CR1, CR2, CR3
