INTRODUCTION TO THE PLC-5
FORCE LED
The FORCE LED is amber. It indicates that a force exists within the processor. The FORCE LED is on steady when forces are installed and enabled, blinks when forces are installed but not enabled, and off when no forces are installed.
Battery
The processor houses one AA lithium battery. If power is not applied to the processor module, the battery retains the processor memory for up to one year. The battery is held beneath a cover on the front of the processor module. The date the battery was installed should be written on the front of the module.
Processor Module DIP Switches
The processor module is configured for operation through three groups of DIP switches.
These switches, labeled SW1, SW2, and SW3, are located inside the processor module as illustrated in Figure 4.
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Figure 4: Processor Module Switches
Switch assembly SW1 is an eight-switch assembly. It is used to determine the station number of the processor module when it is configured in a peer communications link (data highway plus). This switch assembly also configures the processor for scanner or adapter operation.
Switch assembly SW2 is also an eight-switch assembly. It sets the number of words exchanged between the host processor and the PLC-5 processor when the PLC-5 processor is in adapter mode. The PLC 5/15 can transfer eight words between the host PLC-5 and the adapter module per scan. This switch assembly also establishes the beginning I/O group number assigned to the PLC-5 processor, and the I/O rack number of the processor module when it is in adapter mode.
Switch assembly SW3 is a four-switch assembly that connects a terminator across the line when the processor module is the last device in a peer communications link remote I/O link. The specific switch settings for this module are found in the processor technical bulletin.
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Memory Modules
Each processor module contains a base memory. This is usually an adequate level for most applications. However, due to system expansion and increased needs, additional memory may be required. These memory modules are installed into the memory-module slot on the bottom of the processor memory-module. There are three memory memory-modules that may be added to the processor:
• EEPROM Module (1785-MJ) - Provides up to 6K words of nonvolatile memory backup.
• CMOS RAM Module (1785-MR) - Provides 4K words of RAM memory in addition to the processor’s base memory.
• CMOS RAM Module (1785-MS) - Provides 8K words of RAM memory in addition to the processor’s base memory.
The EEPROM module may be used in any processor. The two CMOS RAMs are only available for use with the PLC 5/15 and 5/25 processors.
Input Modules, Output Modules, and Field Wiring
Input modules accept input signals from field devices and condition them to meet the power requirements of the processor. Output modules accept the control signals from the processor and energize the designated output module point. Field wiring connects the modules to signaling or control devices in the facility.
Input Modules
An input is any signal that supplies information to the programmable controller. The interface between all physical inputs and the controller is the input module. The input module receives the signal from the input device, transforms the signal to a format that is recognizable by the ladder logic, and then passes the information on to the controller through common connection in the equipment rack. Common types of input devices are push buttons, limit and proximity switches, control relays, sensors, and operator controls.
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There are several types of input modules. Input modules are available in 8-point (8 input signal terminals), 16-point, and 32-point designs and accept AC or DC input signals. The type of input module selected for a particular application depends on the type of input signal. This includes analog inputs, digital inputs, and specialty modules for inputs from thermocouples, resistance-temperature devices, and encoders. Table 14 summarizes the rating characteristics of the various AC and DC input modules commonly used with the PLC-5.
Table 14: AC and DC Input Modules
Model Number Input Voltage Rating Number of Input Points
A typical input module is the 1771-IAD. This number provides descriptive information about the module. The “1771” identifies the PLC-5 family and indicates that the module fits into a 1771-series universal chassis. The “I” indicates an input module, the “A”
indicates an AC module, and the “D” indicates high density. A high-density module is a module with 16 or more points.
This module converts sixteen individual 120VAC inputs to a logic level compatible with the processor. Typical field device inputs to this module are proximity switches, limit switches, and push buttons. The input signals are filtered within the module to limit the effects of voltage transients caused by contact bounce and electrical noise. This prevents false data input to the processor. The input circuits within the input module are optically isolated from the back plane of the chassis.
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The power used to operate the logic circuitry within the input module is drawn from the chassis back plane. Each input module requires approximately 0.25 Amperes of current.
Figure 5 illustrates the 1771-IAD module.
Figure 5: 1771-IAD AC Input Module
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The 1771-IAD module occupies one slot in the universal chassis and can be placed in any location within the universal chassis except for the very first slot to the left, which is reserved for the processor. To install the module, slide it into the slotted track located within the chassis. To remove the module, pull outward on the tab located on the top of the module.
The field devices are wired to the terminal block on the front of the module. This terminal block is hinged on the bottom and connected to the universal chassis. This eases the removal and replacement of a module. Note that the first four terminals (A, B, C, D) are not used on input modules. The next sixteen terminals are numbered 00 through 17 (octal). The last terminal (E) is for the common ground connection. A hinged plastic cover protects the terminals.
The input status indicators are located on the front of the module above the terminal strip.
The status indicators show the condition of the module and its inputs. The green ACTIVE LED when the module is powered and the opto-isolator data paths are functioning properly. The remaining sixteen LEDs (00 to 17) illuminate red when the associated input has power present on the terminal.
The input module fault mode selection configuration plug is located on the top of the module. The purpose of this plug is to determine the status of the inputs to the processor during a module failure. The plug has two positions: “state” and “reset.” In the last-state position, the inputs to the processor from the module remain in the last known valid state when a failure is detected. In the reset position, the inputs are reset to the off position when a module failure occurs.
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Output Modules
An output from the programmable controller causes an external event to occur. The interface between the controller and a physical output is the output module. The output interface module interprets the control signals controller from the controller then outputs the signals that actually change the position of equipment or modify processes. Typical output devices include relays, solenoids, lamps, and system displays or monitors.
There are several types of output modules. As with the input module, the type of output module depends on the application. Types of output modules include those for analog and digital signals, and linear position transducers. Table 15 summarizes the rating characteristics of the various AC and DC output modules commonly used with PLC-5.
Table 15: AC and DC Output Modules Model Number Output Voltage Rating Number of
Output Points
A typical output module is the 1771-OAD. This number provides descriptive information about the module. The “1771” identifies the PLC-5 family and indicates that the module fits into a 1771-series universal chassis. The “O” indicates an output module, the “A”
indicates AC module, and the “D” indicates high density. A high-density module is a module with 16 or more points.
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The power used to operate the logic circuitry within the output module is drawn from the chassis back plane. Each output module requires approximately 0.7 Ampere of current.
Figure 6 illustrates the 1771-OAD module.
Figure 6: 1771-OAD AC Output Module
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The 1771-OAD module occupies one slot and can be placed in any location within the chassis except for the very first slot to the left, which is reserved for the processor. The module is installed and removed in the same manner as its corresponding input module.
The field devices are wired to the terminal block on the front of the module. This terminal block is hinged on the bottom to allow easy module removal without removing the field device wiring. A hinged plastic cover protects the terminals. AC power is supplied to this module through the four terminals labeled L1. These four terminals should be jumpered together to prevent overstressing any single point. Power is supplied to all four points to protect from exceeding the total surge rating of the module. Field devices are connected to terminals 00 to 17 (octal). The connection paths are from the module to the field device to ground. The last terminal (L2) may or may not be used as a common ground with the field device. If it is not used, no connection to this point is necessary.
The output status indicators operate in a manner similar to the input module. The ACTIVE LED indicates power to the output module and opto-isolation data path operation. The red output LEDs (00-17) indicate that the processor has commanded an output on. They do not indicate the presence of power on a given terminal. One additional indicator is present on the status panel. It is the FUSE indicator. When illuminated, it indicates that the output fuse has blown.
The output module fault mode selection configuration plug is located on the bottom of the module. This plug determines the state of the outputs following a module failure. The possible plug positions are “last state” and “reset.” In the last-state position, the outputs will remain in the last known current state should a module failure occur. In the reset position, the outputs will reset to off following a module failure.
The module configuration plug operates independently of the last-state switch on the I/O chassis back plane. The module plug position takes precedence when a module fault occurs. The I/O chassis back plane plug takes precedent if a rack fault occurs.
Field Wiring
All inputs and outputs are connected to the programmable controller by field wiring.
Field wiring is all wiring, junction boxes, and connectors used to connect the programmable controller to external devices. Field wiring completes the PLC-5
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Remote I/O Adapter Module
The 1771-ASB Remote I/O Adapter module is an interface between remote racks and the processor module. Essentially, the remote I/O adapter takes the place of the processor module in the remote racks. The adapter communicates with the other I/O modules in the remote rack, and the processor module communicates with the adapter.
The adapter occupies one slot in the universal chassis and must be placed in the left-most slot, just as with the processor module. The power to operate the module is drawn from the chassis back plane. The module requires 1.2 Amperes of current. Figure 7 illustrates the Remote I/O Adapter module.
Figure 7: Remote I/O Adapter Module
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The terminal block on the front of the module is used for connection of external I/O communication cables and an optional chassis restart button. The module has built-in fault detection capabilities. If a fault should occur in a remote I/O chassis containing inputs, the inputs to the processor will remain in their last pre-fault state. As a result, when a fault occurs, the outputs in an un-faulted local or remote rack will remain in the last state ordered prior to the fault.
Two switch assemblies are located inside the 1771-ASB Remote I/O adapter module.
These switches are labeled SW-1 and SW-2. They are used to set group numbers and rack numbers in both a complimentary and non-complimentary I/O configuration. The positioning procedures for these switches are contained in the equipment technical bulletin.
The module has three status indicators. The ACTIVE indicator is green. When on, it indicates: that there is active communication between the processor and the adapter module, that DC power is on and supplying the entire I/O rack, and that the I/O adapter module is actively controlling the modules. When it is OFF it indicates there is no communication between the processor and the adapter module. When flashing it indicates that a communication link is established between the processor and the remote I/O adapter module, the processor is in the program or test mode, and the remote I/O adapter module is not actively controlling the I/O modules.
The ADAPTER FAULT indicator is red. When on it indicates that the module is not operating properly, there is a fault, and that the I/O rack response is in the manner denoted by the last state switch (switch number one of the I/O chassis back plane switch assembly). When it is flashing, it shows that the processor restart lockout switch on the I/O chassis back plane switch assembly is on. Depress the I/O rack restart push button (if installed) to clear the restart lockout.
The I/O RACK FAULT indicator is red. When on, it indicates that a fault has been detected at the remote I/O adapter module on the logic side of the I/O modules.
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PLC-5SYSTEM OPERATION
The major components of a PLC are the equipment chassis, processor module, input module, output module, and power supply. A programming terminal is used to program the processor, but it is not considered a major component because once the processor is programmed, the terminal may be disconnected. The operation of these major components is best illustrated by developing a hypothetical hardwired circuit, then implementing the same circuit using the major PLC components.
Figure 8 illustrates the hypothetical circuit for this example. This circuit controls two different lamps. Switch 1 and Switch 2 are normally open push button switches. Lamp 1 illuminates when switch 1 is closed, and lamp 2 illuminates when switch 2 is closed.
Figure 8: Hypothetical Circuit
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Figure 9 shows the same switches and lamps under the control of a PLC system. The push button switches connect to an input module in the PLC system instead of directly to the lamps. The lamps are connected to the output module. Notice also that the input module is indirectly connected to the output module via the processor.
Figure 9: Hypothetical Circuit Controlled by PLC System
The processor is programmed to connect Switch 1 to Lamp 1, and Switch 2 to Lamp 2 through software. This software is also known as ladder logic since it appears similar to a standard electrical ladder diagram. The processor is programmed using a terminal (laptop) connected to a communication port on the processor. The operation of the hardwired lamp system and the PLC-controlled system appear identical. When Switch 1 is closed, Lamp 1 lights, and when Switch 2 closes, Lamp 2 lights. The major differences between the two models relate to the signal flow paths.
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Signal Flow Paths
When a push button is pressed in the hardwired system, power moves from the voltage source through the switch to the lamp, and then to ground. Electrical power simply follows the wire conductors to the lamp. When the switch is opened, power is interrupted and the light goes out.
In the PLC controlled system, power moves from the voltage source, through the switch, into the input module. The input module senses the presence of this voltage and in turn, sends a small signal voltage into the processor through the back plane connections to the equipment chassis. The voltage from the switch is isolated from the voltage signal that the module sends into the processor. This isolation is necessary since the fragile processor chip operates at very low voltage and current levels.
The signal received by the processor is analyzed and interpreted by the ladder logic.
The ladder logic generates a low-voltage output signal from the processor to the output module. This output signal not only contains the ON signal to the lamp, but also tells the output module to which terminal the lamp is connected to on the module. This allows the output module to discriminate between Lamp 1 and Lamp 2. An observer of both hardwired and PLC controlled systems would not notice any difference in the system operation. In both systems, Switch 1 controls Lamp 1, and Switch 2 controls Lamp 2.
The greatest advantage of a programmable logic controller becomes evident when a change is needed in the circuits previously discussed. For example, if you needed to change the circuits of a hardwired system to have Switch 1 control Lamp 2, and Switch 2 control Lamp 1, it would take several minutes to rewire them, and would involve exchanging the wires at the switches or the lamps. With a PLC, a simple editing operation can make these changes internal to the program. This eliminates the need for rewiring and this process takes only a fraction of the time required to change a
hardwired system
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Ladder Logic and I/O Control
A practical application demonstrating the flexibility of a PLC ladder program is illustrated in the next example. Figure 10 shows a vat containing a liquid. In this system, a motor is energized to rotate the stirrer and mix the contents of the vat when certain conditions of temperature and pressure are met.
Figure 10: Vat Control System
Figure 11 illustrates the hardwired method for vat control. In this example, a pressure switch and a temperature switch are hardwired in series. This means both switches must be energized at the same time before the motor will start. A manual override push button is also installed in order to bypass the temperature and pressure switches and start the motor on demand.
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Figure 12 illustrates the vat control circuit implemented in PLC ladder logic. Notice that the three different inputs (pressure switch, temperature switch, and manual override) are represented by the contacts 000, 001, and 002, respectively. The actual pressure switch and the temperature switch would be hardwired to two different terminals on an input module. The manual override push button would be hardwired to a third input terminal. The motor, represented by the coil labeled 110, would be hardwired to a terminal on an output module.
Figure 12: PLC Vat Control System
Figure 12: PLC Vat Control System