Copyright Goodheart-Willcox Co., Inc. 118 Programmable Logic Controllers: Hardware and Programming Summary • Once a relay logic diagram is constructed, addresses are assigned to each input and output device, those devices are connected to PLC ports, and the PLC ladder diagram is created. • Ladder logic diagrams are created in the offline mode. fl • When converting relay logic diagrams to ladder logic diagrams, you must follow the following six rules: • Place a contact in the upper-left corner of the ladder logic diagram. • Place the coil at the right end of the rung. • All contacts must be placed horizontally. • Outputs cannot be connected in series with other outputs. • Program execution fl ow must be from left to right and from top to bottom. fl • Place the rung numbers on the left side of each rung. • When creating a new PLC file, the first fi step is to select the proper PLC fi processor. • PLC diagrams can be evaluated with normally open and normally closed switches and contacts. • The operational scan rate is the time required to execute the PLC ladder diagram once. After the last instruction in the lower-right corner is executed, the process of scanning restarts. • Each PLC operational cycle has three separate scan parts: input scan, program scan, and output scan. • Since scanning takes place from left to right across each rung, and from the top to bottom rungs, specifi inputs or outputs may not be updated in time fic unless an IIM or IOM instruction is inserted in the middle of the program. • A programmable logic controller project consists of the ladder logic diagram fi le and its associated data files. fi fi • The SYS0 program fi le is used by the PLC processor to keep track of its fi internal register settings and the SYS1 program file is reserved. Therefore, fi these two system fi les are not available to the RSLogix 500 software user. fi • The main program fi le (LAD2) fi holds the main PLC ladder logic diagram. • A PLC project may have several subroutine program files in addition to the fi main program file. fi • There are icons for nine data fi les in the fi Data Files branch of the project tree area: Input (I), Output (O), Status (S2), Timer (T4), Counter (C5), Control (R6), Integer (N7), and Bit (B3 and B10). • If PLC latch and unlatch instructions are used in industrial control systems, it is possible to dictate that an output device must turn on when a pushbutton is pressed and that the same output device must turn off when another pushbutton is pressed. • Placing the PLC in run mode allows the PLC to start controlling the input/output devices. • Placing the PLC in the online monitor mode allows the programmer to see the operation of a running PLC control system on the computer screen. • Offl mode is the mode in which the PLC ladder logic diagram is created. fline • Online mode is the mode in which the PLC program is downloaded into the PC.