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Number Systems
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4. Bits, Bytes, and Words
Each binary piece of data is a bit. Eight bits make up one byte. Two bytes, or 16 bits, make up one word.
5. Logic 0, Logic 1
While PLCs are capable of sensing and generating analog values, internally, programmable controllers use signals that are on or off. These on and off conditions correspond to the binary values 1 and 0. For example, a binary 0, also called logic 0, can be used to indicate that a switch is off and a binary 1 (logic 1) can be used to indicate that a switch is on.
6. BCD (Binary-Coded Decimal)
While it is necessary for PLCs to use binary values, humans often need to see values represented in decimal. As a result, some input and output devices provide a decimal display where each decimal digit corresponds to four PLC binary inputs or outputs. The most common system used by input and output devices of this type is referred to as Binary-Coded Decimal
(BCD).
One example of a BCD device is a type of four-digit thumbwheel switch. Each thumbwheel digit controls four PLC inputs. This means that for a four-digit thumbwheel, 16 inputs are required. Because each thumbwheel digit only needs to represent decimal values from 0 through 9, only ten corresponding binary values are required for each digit.
7. Hexadecimal
Hexadecimal is another system used in PLCs. The hexadecimal system has the following characteristics :
16 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F
Base 16
Weights Powers of base 16 (1, 16, 256, 4096….)
The ten digits of the decimal system are used for the first ten digits of the hexadecimal system. The first six letters of the alphabet are used for the remaining six digits.
The hexadecimal system is used in PLCs because it allows the status of a large number of binary bits to be represented in a small space such as on a computer screen or programming device display. Each hexadecimal digit represents the exact status of four binary bits. To convert a decimal number to a hexadecimal number the decimal number is divided by the base of 16. To convert decimal 28, for example, to hexadecimal:
Decimal 28 divided by 16 is 1 with a remainder of 12. Twelve is equivalent to C in hexadecimal. The hexadecimal equivalent of decimal 28 is 1C.
The decimal value of a hexadecimal number is obtained by multiplying the individual hexadecimal digits by the base 16 weight and then adding the results. In the following example the hexadecimal number 2B is converted to its decimal equivalent of 43.
8. Conversion of Numbers
The following chart shows a few numeric values in decimal, binary, BCD, and hexadecimal representation.
Terminology
Developing an understanding of PLCs requires learning some basic terminology commonly used to describe these devices and related components.
1. Sensors
Sensors are devices that convert a physical condition into an electrical signal for use by the PLC. Sensors are connected to the input of a PLC. A pushbutton is one example of a sensor that is connected to the PLC input. An electrical signal is sent from the pushbutton to the PLC indicating the condition (open/closed) of the pushbutton contacts.
2. Actuators
Actuators are devices that convert an electrical signal from the PLC into a physical condition. Actuators are connected to the PLC output. A motor starter is one example of an actuator that is connected to the PLC output. Depending on the output PLC signal the motor starter will either start or stop the motor.
3. Discrete Input
A discrete input, also referred to as a digital input, is an input that is either on or off. Pushbuttons, toggle switches, limit switches, proximity switches, and contact closures are examples of discrete sensors which are connected to the PLCs discrete or digital inputs. In the ON condition a discrete input may be referred to as a logic 1 or a logic high. In the OFF condition a discrete input may be referred to as a logic 0 or a logic low.
A normally open (NO) pushbutton is used in the following example. Many PLCs require a separate power supply to power its inputs; however, this example is for a PLC that powers its own inputs. One side of the pushbutton is connected to the first PLC input. The other side of the pushbutton is connected to an internal 24 VDC power supply. With the pushbutton in the open state, no voltage is present at the PLC input. This is the OFF condition. When the pushbutton is depressed, 24 VDC is applied to the PLC input. This is the ON condition.
4. Analog Inputs
An analog input is a continuous, variable signal. Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts. In the following example, a level transmitter monitors the level of liquid in a tank. Depending on the level transmitter, the signal to the PLC can either increase or decrease as the level in the tank increases.
5. Discrete Outputs
A discrete output is an output that is either on or off. Solenoids, contactor coils, and lamps are examples of actuator devices connected to discrete outputs. Discrete outputs may also be referred to as digital outputs. In the following example, a lamp can be turned on or off by the PLC output it is connected to.
6. Analog Outputs
An analog output is a continuous, variable signal. The output may be as simple as a 0-10 VDC level that drives an analog meter. Examples of analog meter outputs are speed, weight, and temperature. The output signal may also be used on more complex applications such as a current-to-pneumatic transducer that controls an air-operated flow-control valve.
7. CPU
The central processor unit (CPU) is a microprocessor system that contains the system memory and is the PLC decision-making unit. The CPU monitors the inputs, outputs, and other variables and makes decisions based on instructions held in the program memory. Typical CPU operations include relay, counting, timing functions, data comparison, sequencing, and arithmetic operations.
8. Programming
A program consists of instructions that accomplish one or more tasks. The degree of complexity of the PLCs program depends upon the complexity of the task to be performed, the number and type of input and output devices, and the types of instructions used. Some PLC programs are made up primarily of ladder logic instructions, but other approaches such as statement lists or function block diagrams are also common.
9. Ladder Logic
Ladder logic (LAD) is one programming language used with PLCs. Ladder logic uses components that resemble elements used in a line diagram format to describe hard-wired control. Refer to the STEP course Basics of Control Components for more information on line diagrams.
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10. Ladder Logic Diagram
The left vertical line of a ladder logic diagram represents the power or energized conductor. The output element or instruction represents the neutral or return path of the circuit. The right vertical line, which represents the return path on a hard-wired control line diagram, is omitted. Ladder logic diagrams are read from left-to-right, top-to-bottom. Rungs are sometimes referred to as networks. A network may have several control elements, but only one output coil.
In the example program shown example I0.0, I0.1 and Q0.0.
represent the first instruction combination. If inputs I0.0 and I0.1 are energized, output relay Q0.0 energizes. The inputs could be switches, pushbuttons, or contact closures. I0.4, I0.5, and Q1.1 represent the second instruction combination. If either input I0.4 or I0.5 are energized, output relay Q0.1 energizes
11. Statement list
A statement list (STL) provides another view of a set of instructions. The operation, what is to be done, is shown on the left. The operand, the item to be operated on by the operation, is shown on the right. A comparison between the statement list shown below, and the ladder logic shown on the previous page, reveals a similar structure. The set of instructions in this statement list perform the same task as the ladder diagram.
12. Function Block Diagrams
Function Block Diagrams (FBD) provide another view of a set of instructions. Each function has a name to designate its specific task. Functions are indicated by a rectangle. Inputs are shown on the left-hand side of the rectangle and outputs are shown on the right-hand side. The function block diagram shown below performs the same function as shown by the ladder diagram and statement list.
13. PLC Scan
The PLC program is executed as part of a repetitive process referred to as a scan. A PLC scan starts with the CPU reading the status of inputs. The application program is executed using the status of the inputs. Once the program is completed, the CPU performs internal diagnostics and communication tasks. The scan cycle ends by updating the outputs, then starts over. The cycle time depends on the size of the program, the number of I/Os, and the amount of communication required.
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