Basics of plc_programming1
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The document discusses the basics of programmable logic controller (PLC) programming including PLC architecture, memory organization, programming languages, ladder logic instructions, addressing schemes, and programming techniques. Specifically, it covers the processor memory being divided into program and data memory, the ladder logic programming language using relay-type instructions like examine if closed and examine if open, addressing I/O locations by module and bit, and programming concepts such as parallel and nested rungs, internal control relays, and adjustments for different scan patterns.
Basics of plc_programming1
- 1. Basics of PLC Programming Industrial Control Systems Fall 2006 Lecture – PLC Programming Basics MME 486 – Fall 2006 1 of 62
- 2. PLC Architecture Lecture – PLC Programming Basics MME 486 – Fall 2006 2 of 62
- 3. PLC System Lecture – PLC Programming Basics MME 486 – Fall 2006 3 of 62
- 4. Processor Memory Organization The memory of a PLC is organized by types. The memory space can be divided into two broad categories: program and data memory. Advanced ladder logic functions allow controllers to perform calculations, make decisions and do other complex tasks. Timers and counters are examples of ladder logic functions. They are more complex than basic inputs contacts and output coils and rely heavily upon data stored in the memory of the PLC. Lecture – PLC Programming Basics MME 486 – Fall 2006 4 of 62
- 5. Memory Map A memory map can be used to show how memory is organized in a PLC. § Input/output locations § Internal relay and timer/counter locations Data table User program The user program causes the controller to operate in a particular manner Housekeeping memory Used to carry out functions needed to make the processor operate (no access by user) Lecture – PLC Programming Basics MME 486 – Fall 2006 5 of 62
- 6. Program Files The user program will account for most of the memory of a PLC system. • Program files contain the logic controlling machine operation. Program file organization for SLC-500 controller. Lecture – PLC Programming Basics • This logic consists of instructions that are programmed in a ladder logic format. MME 486 – Fall 2006 6 of 62
- 7. Data Files Data file organization for SLC-500 controller. The data file portion of memory stores input and output status, processor status, the status of various bits and numerical data. Lecture – PLC Programming Basics MME 486 – Fall 2006 7 of 62
- 8. Data Files Data files are organized by the type of data they contain Lecture – PLC Programming Basics MME 486 – Fall 2006 8 of 62
- 9. Input Table File Operation Input module Switch Open Binary 0 stored 0 Lecture – PLC Programming Basics MME 486 – Fall 2006 9 of 62
- 10. Input Table File Operation Input module Processor continually reads current input status and updates input image table file Switch Closed Binary 1 stored 1 Lecture – PLC Programming Basics MME 486 – Fall 2006 10 of 62
- 11. Output Table File Operation Output module Output OFF Processor continually activates or deactivates output status according to output image table file status 0 Status 0 Lecture – PLC Programming Basics MME 486 – Fall 2006 11 of 62
- 12. Output Table File Operation Output module Processor continually activates or deactivates output status according to output image table file status Output ON 1 Status 1 Lecture – PLC Programming Basics MME 486 – Fall 2006 12 of 62
- 13. Program Scan During each operating cycle, the processor reads all inputs, takes these values, and energizes or de-energizes the outputs according to the user program. This process is known as a scan. I/O scan – records status data of input devices. Energizes output devices that have their associated status bits set to ON (1) Program scan – instructions are executed sequentially Because the inputs can change at any time, the PLC must carry on this process continuously. Lecture – PLC Programming Basics MME 486 – Fall 2006 13 of 62
- 14. Scan Process The scan time indicates how fast the controller can react to changes in inputs. Scan times vary with computer model and program content, and length. If a controller has to react to an input signal that changes states twice during the scan time, it is is possible that the PLC will never be able to detect this change. Scan time may be a concern in high speed operations Lecture – PLC Programming Basics MME 486 – Fall 2006 14 of 62
- 15. Scan Process Read inputs The scan is a a continuous and sequential process Adjusts outputs Lecture – PLC Programming Basics Run program MME 486 – Fall 2006 15 of 62
- 16. Data Flow Overview Input modules Input data Input image table file Output image table file Examine data Output data Output modules Return results Program Check/compare/examine specific conditions Lecture – PLC Programming Basics MME 486 – Fall 2006 Take some action 16 of 62
- 17. Scan Process Output Module Input Module Input file I:3/6 Input device Output file Output device O:4/7 O:4/7 I:3/6 Program When the input is closed, the input module senses a voltage and an ON condition (1) is entered into the input table bit I:3/6 I:3/6 O:4/7 During the program scan the processor sets instructions I:3/6 and O:4/7 to ON (1) Lecture – PLC Programming Basics MME 486 – Fall 2006 The processor turns light output O:4/7 ON during the next I/O scan 17 of 62
- 18. Scan Patterns Horizontal Scanning Order The processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung. End of ladder In addition to the program itself, the scan time is also dependent on the clock frequency of the processor! Lecture – PLC Programming Basics MME 486 – Fall 2006 18 of 62
- 19. Scan Patterns Vertical Scanning Order The processor examines input and output instructions from the first command, vertically, column by column and page by page. Pages are executed in sequence. End of ladder Misunderstanding the way the PLC scans can cause programming bugs! Lecture – PLC Programming Basics MME 486 – Fall 2006 19 of 62
- 20. PLC Programming Languages The term PLC programming language refers to the method by which the user communicates information to the PLC. The three most common language structures are: ladder diagram language, Ladder diagram language Boolean language, and functional chart. Functional chart Boolean language Lecture – PLC Programming Basics MME 486 – Fall 2006 20 of 62
- 21. Comparing Programming Language PB1 CR1 CR2 SOL Relay Schematic LS1 PB1 CR1 CR2 SOL Equivalent ladder diagram language LS1 Equivalent Boolean language Lecture – PLC Programming Basics MME 486 – Fall 2006 21 of 62
- 22. Relay-Type Instructions The ladder diagram language is basically a symbolic set of instructions used to create the controller program. These ladder instructions symbols are arranged to obtain the desired control logic. Lecture – PLC Programming Basics MME 486 – Fall 2006 22 of 62
- 23. Examine If Closed (XIC) Instruction Symbol Analogous to the normally open relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) CLOSED (XIC) Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output. Has a bit-level address which is examined for an ON condition. The status bit will be either 1 (ON) or 0 (OFF). Lecture – PLC Programming Basics MME 486 – Fall 2006 23 of 62
- 24. Examine If Closed (XIC) Instruction I:012 I:012 04 Lecture – PLC Programming Basics MME 486 – Fall 2006 24 of 62
- 25. Examine If Closed (XIC) Instruction I:012 I:012 04 If the status bit is 0 (OFF), then the instruction is FALSE. Lecture – PLC Programming Basics MME 486 – Fall 2006 25 of 62
- 26. Examine If Closed (XIC) Instruction I:012 I:012 04 If the status bit is 1 (ON), then the instruction is TRUE. Lecture – PLC Programming Basics MME 486 – Fall 2006 26 of 62
- 27. Examine If Open (XIO) Instruction Symbol Analogous to the normally closed relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) OPEN (XIO). Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output. Has a bit-level address which is examined for an OFF condition. The status bit will be either 1 (ON) or 0 (OFF). Lecture – PLC Programming Basics MME 486 – Fall 2006 27 of 62
- 28. Examine If Open (XIO) Instruction I:012 I:012 04 If the status bit is 0 (OFF), then the instruction is TRUE. Lecture – PLC Programming Basics MME 486 – Fall 2006 28 of 62
- 29. Examine If Open (XIO) Instruction I:012 I:012 04 If the status bit is 1 (ON), then the instruction is FALSE. Lecture – PLC Programming Basics MME 486 – Fall 2006 29 of 62
- 30. Output Energize (OTE) Instruction Symbol Analogous to the relay coil. The processor makes this instruction true (analogous to energizing a coil) when there is path of true XIC and XIO instructions in the rung. Typically represents any output that is controlled by some combination of input logic. Can be a connected device or an internal output (internal relay). If any left-to-right path of input conditions is TRUE, the output is energized (turned ON). Lecture – PLC Programming Basics MME 486 – Fall 2006 30 of 62
- 31. Output Energize (OTE) Instruction O:013 I:012 I:012 I:012 11 15 O:013 01 OUTPUT ENERGIZE instruction - TRUE Lecture – PLC Programming Basics MME 486 – Fall 2006 31 of 62
- 32. Output Energize (OTE) Instruction O:013 I:012 I:012 I:012 11 15 O:013 01 OUTPUT ENERGIZE instruction - FALSE Lecture – PLC Programming Basics MME 486 – Fall 2006 32 of 62
- 33. Status Bit Examples Input module A Bit status Button not actuated Output A OFF False A Output ON True Lecture – PLC Programming Basics MME 486 – Fall 2006 33 of 62
- 34. Status Bit Examples Input module A Bit status Button actuated Output A ON True Output A OFF False Lecture – PLC Programming Basics MME 486 – Fall 2006 34 of 62
- 35. Ladder Rung A B C Output instruction Input conditions D A ladder rung consists of a set of input conditions, represented by contact instructions, and an output instruction at the end of the rung, represented by the coil symbol. Lecture – PLC Programming Basics MME 486 – Fall 2006 35 of 62
- 36. Ladder Rung A B C Output instruction Input conditions D For an output to be activated or energized, at least one left-to-right path of contacts most be closed. A complete path is referred to as having logic continuity. When logic exists the rung condition is said to be TRUE. Lecture – PLC Programming Basics MME 486 – Fall 2006 36 of 62
- 37. Rung Continuity Bit in memory Bit in memory 1 1 LS_1 SOL_5 The Examine If Closed instruction is TRUE making the rung TRUE Lecture – PLC Programming Basics MME 486 – Fall 2006 37 of 62
- 38. Rung Continuity Bit in memory Bit in memory 0 0 LS_1 SOL_5 The Examine If Closed instruction is FALSE making the rung False Lecture – PLC Programming Basics MME 486 – Fall 2006 38 of 62
- 39. Allen-Bradley SLC-500 Controller Addressing Output image table file 0 Address output terminal O0:4/6 O:0:4/6 Bit address Address input terminal I1:3/12 Input image table file 1 Energized output I:3/12 Bit address Closed input I1:3 O:0:4 User-programmed rung 12 Lecture – PLC Programming Basics MME 486 – Fall 2006 6 39 of 62
- 40. Structure of A 16-Bit Word Bit OFF ON Word (16 bits) 0 1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 I/O Connection Diagram L1 L2 L1 L2 PB1 SOL 1 O:2/3 I:4/5 PL 1 LS1 O:3/6 I:4/6 Lecture – PLC Programming Basics MME 486 – Fall 2006 R 40 of 62
- 41. Parallel Input Branch Instructions A C B Lecture – PLC Programming Basics Branch instructions are used to create parallel paths of input condition instructions. If at least one of these parallel branches forms a true logic path, the logic is enabled. MME 486 – Fall 2006 41 of 62
- 42. Parallel Output Branching A C D B E On most PLC models, branches can be established at both the input and output portion of the rung. With output branching, you can program parallel outputs on a rung to allow a true logic path to control multiple outputs. Lecture – PLC Programming Basics MME 486 – Fall 2006 42 of 62
- 43. Nested Input and Output Branches Input and output branches can be nested to avoid redundant instructions and to speed up the processor scan time. A nested branch starts or ends within another branch. Lecture – PLC Programming Basics MME 486 – Fall 2006 43 of 62
- 44. Nested Contact Program A B C Y Nested contact D On some PLC models, the programming of a nested branch circuit cannot be done directly. E A B C D Y C Contact instruction C repeated Reprogrammed to obtain the required logic. E Lecture – PLC Programming Basics MME 486 – Fall 2006 44 of 62
- 45. PLC Matrix Limitation Diagram Max series contacts No. outputs per rung and location of the output in the rung Max parallel lines There may be limitations to the number of series contacts instructions, number of parallel lines, and the number of outputs and their location on the rung. Lecture – PLC Programming Basics MME 486 – Fall 2006 45 of 62
- 46. Programming of Vertical Contacts A D Y Original program C E D B A B C A C Y D E B Y = (AD) + (BCD) + (BE) + (ACE) Reprogrammed to obtain the required logic E Lecture – PLC Programming Basics MME 486 – Fall 2006 46 of 62
- 47. Programming for Different Scan Patterns B A D C Y Original program E F Y = (ABC) + (ADE) + (FE) + (FDBC) A B C A D E F D Reprogrammed to obtain the required logic E F Y B C Lecture – PLC Programming Basics MME 486 – Fall 2006 47 of 62
- 48. Internal Control Relay The internal output operates just as any other output that is controlled by programmed logic; however, the output is used strictly for internal purposes. The internal output does not directly control an output device. The advantage of using internal outputs is that there are many situations where an output instruction is required in a program, but no physical connection to a field device is needed. Their use in this type of instance can minimize output card requirements. Lecture – PLC Programming Basics MME 486 – Fall 2006 48 of 62
- 49. Extending the Number of Series Contacts Using an Internal Control Relay Internal relay coil Rung 1 Rung 2 Internal relay contact Lecture – PLC Programming Basics Discrete output (requires one physical connection on the output module) MME 486 – Fall 2006 49 of 62
- 50. Programming The XIC Instruction PB1 PB2 Hardwired Circuit PL PB1 PB2 PL User program providing the same results Note that both pushbuttons are represented by the XIC symbol. This is because the normal state of an input (NO or NC) does not matter! What does matter is that if contacts need to close to energize the output, then the XIC instruction is used. Since both PB1 and PB2 must close to energize the PL, the XIC instruction is used for both. Lecture – PLC Programming Basics MME 486 – Fall 2006 50 of 62
- 51. Programming The XIO Instruction PB1 PB1 CR PL CR1 PL User program providing the same results Hardwired Circuit When the pushbutton is open in the hardwired circuit, relay coil CR is deenergized and contacts CR1 close to switch the PL on. When the pushbutton is closed, relay coil CR is energized and contacts CR1 open to switch the PL off. The pushbutton is represented in the user program by an XIO instruction. This is because the rung must be true when the external pushbutton is open, and false when the pushbutton is closed. Lecture – PLC Programming Basics MME 486 – Fall 2006 51 of 62
- 52. Operation of The XIC and XIO Instructions Summary of status conditions The status of the instruction is: XIC Examine If Closed XIO Examine If Open Logic 0 False True False Logic 1 True False True If the data table bit is: Lecture – PLC Programming Basics MME 486 – Fall 2006 OTE Output Energize 52 of 62
- 53. Operation of The XIC and XIO Instructions State of the output as determined by the changing state of the inputs in the rung Inputs Time XIC XIO OTE 0 0 0 Goes true 1 0 1 False Goes false 1 1 0 False Remains false 0 1 0 XIC XIO t1 (initial) False True t2 True True t3 True False t4 Input instructions XIC XIO Bit status Output OTE False Output instruction OTE Lecture – PLC Programming Basics MME 486 – Fall 2006 53 of 62
- 54. Entering the Ladder Diagram A personal computer is most often used to enter the ladder diagram. The computer is adapted to the particular PLC model using the relevant programmable controller software. Lecture – PLC Programming Basics MME 486 – Fall 2006 54 of 62
- 55. RSLogix Main Screen Different screens, toolbars and windows dialog boxes are used to navigate through the Windows environment Lecture – PLC Programming Basics MME 486 – Fall 2006 55 of 62
- 56. Bit Instructions Tool Bar To place an instruction on a rung, click its icon on the toolbar and simply drag the instruction straight off the toolbar onto the rung of the ladder. Lecture – PLC Programming Basics MME 486 – Fall 2006 56 of 62
- 57. Select Processor Type Screen The programming software needs to know what processor is being used in conjunction with the program. 1747-L40E You simply scroll down the list until you find the processor you are using and select it. Lecture – PLC Programming Basics MME 486 – Fall 2006 57 of 62
- 58. I/O Configuration Screen The I/O screen lets you click or drag-and-drop a module from an all inclusive list to assign it to a slot in your configuration. Lecture – PLC Programming Basics MME 486 – Fall 2006 58 of 62
- 59. Data File Screen Data file screens contain data that is used in conjunction with ladder program instructions. These include: Input Output Timer Counter Integer Bit Lecture – PLC Programming Basics MME 486 – Fall 2006 59 of 62
- 60. Monitoring a Ladder Logic Program Operation of the logic is apparent from the highlighting of rungs of the various instructions on screen, which identifies the logic state in real time and has logic continuity. Highlighted rungs indicate the instruction is true Lecture – PLC Programming Basics MME 486 – Fall 2006 60 of 62
- 61. Modes of Operation A processor has basically two modes of operation: the program mode or some variation of the run mode. Program Mode – may be used to Ø enter a new program Ø edit or update an existing program Ø upload files Ø download files Ø document programs Ø change software configurations 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. Lecture – PLC Programming Basics MME 486 – Fall 2006 61 of 62
- 62. Variations of the Run Mode Run Mode – is used to execute the user program. Input devices are monitored and output devices are energized accordingly. Test Mode – is used to operate, or monitor, the user program without energizing any outputs. Remote Mode – allows the PLC to be remotely changed between program and run mode by a personnel computer connected to the PLC processor. Lecture – PLC Programming Basics MME 486 – Fall 2006 62 of 62