Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicCONFIGURING THE PLC SYSTEMPLC configuration should be considered during flowcharting and logicsequencing. The PLC’s configuration defines which I/O modules will beused with which types of I/O signals, as well as where the modules will belocated in the local or remote rack enclosures. The modules’ locationsdetermine the I/O addresses that will be used in the control program.During system configuration, the user should consider the following:possible future expansions; special I/O modules, such as fast-response orwire fault inputs; and the placement of interfaces within a rack (all AC I/Otogether, all DC and low-level analog I/O together, etc.). Consideration ofthese details, along with system configuration documentation, will resultin a better system design.REAL AND INTERNAL I/O ASSIGNMENTThe assignment of inputs and outputs is one of the most important proceduresthat occurs during the programming organization and implementationstages. The I/O assignment table documents and organizes what has beendone thus far. It indicates which PLC inputs are connected to which inputdevices and which PLC outputs drive which output devices. The assignmentof internals, including timers, counters, and MCRs, also takes place here.These assignments are the actual contact and coil representations that areused in the ladder diagram program. In applications where electromechanicalrelay diagrams are available (e.g., modernization of a machine or process),identification of real I/O can be done by circling the devices and thenassigning them I/O addresses (see Example 1).Table 2 shows an I/O address assignment table for real inputs and outputs,while Table 3 shows an I/O address assignment table for internals. Theseassignments can be extracted from the logic gate diagrams or ladder symbolsModuleTypeRackI/O AddressGroupTerminalDescriptionInput0 0 0 LS1—Position0 0 1 LS2—Detect0 0 2 Sel Switch—Select 10 0 3 PB1—StartOutput0 0 4 SOL10 0 5 PL10 0 6 PL20 0 7 Motor M1Output0 1 0 SOL20 1 1 PL3Table 2. I/O address assignment table for real inputs and outputs.Industrial Text & Video Company www.industrialtext.com 1-800-752-839810
Introduction to PLC Programming and Implementation—from relay logic to PLC logicDeviceInternalDescriptionCR71010CR7 replacementTDR10T200ON-delay timer 12 secCR101011CR10 replacementCR141012CR14 replacement— 1013Setup interlockTable 3. I/O address assignment table for internal outputs.that were used to describe the logic sequences. They can also come from thecircled elements on an electromechanical diagram. The numbers used forthe I/O addresses depend on the PLC model used. These addresses can berepresented in octal, decimal, or hexadecimal. The description section of thetable specifies the field devices that correspond to each address.The table of address assignments should closely follow the input/outputconnection diagram (see Figure 6). Although industry standards for I/Orepresentations vary among users, inputs and outputs are typically representedby squares and diamonds, respectively. The I/O connection diagramforms part of the documentation package.L1 L2L1 L2LS1InputsProgram000 Coding 004OutputsSOL1LS2001 005PL1RFigure 6. Partial connection diagram for the I/O address assignment in Table 2.During the I/O assignment, the user should group associated inputs andoutputs. This grouping will allow the monitoring and manipulation of agroup of I/O simultaneously. For instance, if 16 motors will be startedsequentially, they should be grouped together, so that monitoring the I/Oregisters associated with the 16 grouped I/O points will reveal the motors’starting sequence. Due to the modularity of an I/O system, all the inputs andall the outputs should be assigned at the same time. This practice will preventthe assignment of an input address to an output module and vice versa.EXAMPLE 1For the circuit shown in Figure 7, (a) identify the real inputs and outputsby circling each, (b) assign the I/O addresses, (c) assign the internaladdresses (if required), and (d) draw the I/O connection diagram.Industrial Text & Video Company www.industrialtext.com 1-800-752-839811
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