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Chapter 10 Advanced Programming, PLC Interfacing, and Troubleshooting 1 Jump Commands • PLCs have instructions that allow the normal sequential execution of the program to be altered if certain conditions exist • Output instructions that perform this function are known as override commands • An example of an override command would be the jump instruction 2 Implementing Jump Instructions • • • • The jump instruction allows the program to skip certain parts of a program, effectively allowing several programs A set of rung conditions precede the jump instruction. When rung conditions are true, the jump will take place The jump will be always be associated with a label, which defines the jump to location Several jump instructions can use the same label as the jump location 3 Jump to Subroutine • Another jump operation is the subroutine operation (JSR) • A JSR label tells the program where to jump to • A return (RET) instruction is used at the end of the subroutine to tell the program to continue executing where it left off 4 Data Manipulation • Data manipulation instructions allow data to be manipulated by shifting bits left or right in a register permitting more efficient programming of the PLC • Shift registers are used for most data manipulation instructions • An example of forward shift registers in an application is demonstrated on the right 5 Programming Implementation of Shift Register Example 6 Shift Register Operations • Bit Shift Left (BSL) will shift data left within a register for each falsetrue rung transition • Bit Shift Right (BSR) will shift data right within a register for each false-true rung transition 7 Sequencers • Before PLCs, the control of an automatic machine was often performed by a drum sequencer • As the drum rotated, pegs and cams determined what outputs and the dwell of the outputs during a cycle 8 PLC Sequencers • • • PLCs can perform sequencer functions on entire words within the PLC The words are transferred from memory to output modules in consecutive order Parameters necessary for sequencer operations include: – File - designated memory location that forms the 16-bit pattern of the outputs during the sequence – Mask - some bits in a word need not be controlled during a sequence. These bits are masked out – Dest - the output destination to which the data in the sequencer is to be transferred to – Control - this parameter indicates the address of where the control bits of the instruction are located – Length - number of steps of the sequencer file starting at position 1 – Position - this parameter shows the actual step in the sequence that’s being performed 9 Architecture of Sequencer Output Instructions 10 Discrete Input/Output Modules • Discreet I/O modules contain the necessary connections and interfacing between field devices and the internal process unit • Many types of I/O modules are available to accommodate different requirements for power, interfacing, and logic 11 Types of Field Devices Connected to I/O Modules 12 Discrete Input Modules • There are two types of field devices that operate in on/off modes: – – • • Relay contacts Solid-state relays PLC produce several types of discrete input modules for connection to field devices Specifications for input modules should take into consideration the following requirements: – – – – Type of current (AC or DC) Voltage level Number of inputs Active-high or Active-low inputs 13 Relay Contact Input Modules 14 Discrete Output Modules • Several types of discrete output modules are available to handle a variety of applications • Requirements for output modules should take into consideration: – – – – – AC or DC current Voltage level Active-high or Active-low outputs Relay or Solid-state relay switching Load capability 15 Types of DC Output Modules Relay output Solid-state active-high Sinking active-low 16 Troubleshooting I/O Interfaces • Three steps are used to test the proper operation of discrete input interfacing: – Input Testing • Observe indicator light on the PLC module, it should turn off and on in response to the push button • If the light doesn’t turn on, a voltmeter should be used to test the signal at the input terminal • In an improper reading is measured, the next step is to disconnect from the terminal of the module and the power source, then perform a continuity test 17 Troubleshooting Discrete Output Modules • The following steps should be used to troubleshoot output modules: – Activate the output terminal of the module – An indicator lamp connected should energize – If the lamp fails to light and the field device fails to energize, the problem may lie within the program 18 Analog Input and Output Modules • These modules are fed analog signals and then convert them into equivalent digital signals for the PLC to work with. • Analog input modules are sometimes used with sensors and transducers to perform process control functions such as PID control • The most common analog signal used is the 4-20 mA level associated with process control, however other ranges are available or selectable 19 Special Purpose Modules • Several types of special purpose modules are available for a variety of applications, including: – – – – – – – Bar Code modules Radio Frequency (RF) modules Vision System modules PID Control modules Fuzzy Logic modules Stepper Motor modules Thermocouple modules 20 Troubleshooting Programmable Controllers • The most effective approach to troubleshooting PLC faults is to use a logical procedure • Typically, six areas of a PLC are likely to be the source of faults: – – – – – – I/O field wiring Incorrect wiring of the field devices Input module CPU or power supply Programming error Output module 21