in output modules, which in turn control various output field devices such as solenoids, motor starters, etc.
Figure 1.3: Input modules
1.3.3Output Modules
Output modules provide the physical connection between the processor unit and output field devices to facilitate the PLC processor sending the control signals to output devices (Figure 1.4). Output modules perform the following functions:
•Provide connection terminals for output field devices
•Receive control data from the PLC processor and pass on to control output field devices
•Give isolation between the PLC system and field voltages
Figure 1.4: Output modules
Figure 1.5: Hand-held programming device
Figure 1.6: Programming terminal
1.3.4Programming Devices
The main purpose of a programming device is to enter the control programs to PLCs. There are several ways you can perform this task. The available programming devices you can use for a particular PLC vary from manufacturer to manufacturer. There are three basic types of programming devices: hand-held programmers, programming terminals, and personal computers.
Hand-held programmers are an inexpensive and portable way to program small PLCs (Figure 1.5). They have a display and keyboard with numeric keys and programming instruction keys for input. Programming terminals are used in larger PLCs. They are proprietary products from PLC manufacturers and are more costly. They look like a combination of computer monitor and keyboard (Figure 1.6). One programming terminal can serve multiple PLCs.
Personal computers can work together with PLC programming software to create PLC programs for any PLC types. Each brand name PLC has more than one type of programming software for you to prepare programs; the software allows you to download programs to PLCs and monitor the status of the systems.
1.4 PLC Operation Principle
The successful implementation of a PLC project depends on integrating field devices, the PLC program, and the PLC system. The keys to integrate and coordinate these three elements are interface wiring and address assignment (Figure 1.7).
Figure 1.7: PLC operation principle
1.4.1Input Wiring
There are two types of interface wiring: input wiring and output wiring. Input wiring connects input devices to the input modules. There are a fixed number of connection terminals in each input module. Each connection terminal has its assigned address in the PLC memory (Figure 1.8). The actuation of an input device generates a digital ON (1) or OFF (0) signal to appear at its connected terminal of the input module (Figure 1.9). The PLC processor picks up the signal while it reads the input module and stores the signal in the assigned memory address. In this way, any status change in input devices can be immediately reflected on the PLC memory.
Figure 1.8: PLC memory address for input module
Figure 1.9: Signals produced from input module
1.4.2Output Wiring
Similarly, output wiring connects output devices to the output modules. Each connection terminal has its assigned address in the PLC memory. The PLC processor sends the digital signal to connection terminals based on the content in their corresponding memory address (Figure 1.10). An ON (1) signal to a particular output terminal causes the wired output field device to be energized or turned ON. An OFF (0) signal to the output terminal de-energizes the output field device (Figure 1.11).
Figure 1.10: PLC memory address for output module
Figure 1.11: Signals from PLC to output module
1.4.3PLC Program
The PLC program is the road map of the operation. In most PLC systems, PLC programs are written in ladder diagram format. A ladder diagram consists of two rails and several rungs (Figure 1.12). Two rails, arranged vertically, represent the power lines. Program instructions are arranged along the horizontal rungs. The number of rungs increases as the program becomes larger and complicated. Each input and output instruction is assigned a memory address. A PLC program is implemented in cycles. Each cycle involves three steps:
Figure 1.12: A PLC program
1.Reading the status of input field devices in input modules and writing these signal data to their memory address
2.Scanning the program to update the status of input instructions and placing the output results to their address
3.Sending the control signals to the output modules
1.4.4PLC Scanning
The PLC processor scans a program in cyclic manner. The scanning cycle starts from left to right at the top rung and proceeds to the second rung until reaching the bottom rung to complete a cycle. It then returns back to the top rung to continue the next cycle (Figure 1.13). During scanning, the processor simultaneously updates the status of input and output instructions, in both the program and the memory, as well as interacting with the input and output modules.
Figure 1.13: PLC scanning cycle
1.5 PLC Applications
1.5.1PLCs versus Hard-Wired Relay Systems
PLCs initially were intended to replace hard-wired relay systems. They offer many advantages over their hard-wired counterparts. Table 1.2 compares these two types of control systems.
Table 1.2: Comparison between PLC control and relay control
| Feature | Hard-Wired Relays | PLCs |
|---|---|---|
| Functions | Limited to relay types of control. | Extended and advanced control functions available. |
| Feasibility | Complex
|
