The programmable logic controller (PLC) is a microprocessor-based system that accepts input data from switches and sensors, processes that data by making decisions in accordance with a stored program, and then generates output signals to devices that perform a particular function based on the application. The traditional motor control circuit is normally a hardwired system; therefore, any required circuit design change is a rather involved process in terms of material and labor.

With roots established in the manufacturing and automotive industries, the PLC was born out of a desire to automate the motor control process in a way that offered flexibility to make circuit design changes easier. The interesting challenge early on in its development was to design a programming language that would allow the industrial electrician a familiar way to communicate with the electronics of the PLC. This programming language would use symbols encountered in conventional ladder diagrams of the hard-wired variety.

The original purpose of the PLC was to allow electro-mechanical and electronic input devices to communicate with a computer that would perform logical operations on the input data and output a corresponding signal to some form of output device. (See PLC Logic Functions below.) To truly understand how a PLC works, you have to speak the same language. Let's take a look at how to make the translation.

Understanding inputs and outputs

The magnetic motor starter is the controller that operates the connected motor load. The 2-wire and 3-wire control circuits use various types of input devices to energize the starter's coil. These input devices use a momentary or maintained contact configuration.

Typical input devices are push buttons, proximity sensors, liquid level sensors, photoelectric sensors, selector switches, and pressure transducers. Typical output devices are contactors, magnetic motor starters, solenoids, pilot lights, and intelligent display panels. These output devices behave according to the connection of the input devices.

PLC operation is a function of these inputs and outputs. In the standard 3-wire control circuit, as shown in Fig. 1 on page 18, you'll notice a normally closed (N.C.) momentary stop push button, and a normally open (N.O.) momentary start push button. These contact devices represent the input function. The coil of the magnetic motor starter represents the output function.

The N.O. start push button energizes the coil of the magnetic motor starter, and the N.C. stop push button de-energizes it. Here, the PLC recognizes two input functions, which are the stop and start push buttons, and one output function, which is the coil of the magnetic motor starter.

In very simple terms, a PLC is designed to perform three tasks: check the input status, execute the program, and update the output status. The PLC checks the input status by scanning each input to determine if the connected device is on or off and then records that information in memory. Next, the PLC has to execute the user program (one line at a time) to make decisions. For example, suppose the user program tells the PLC to turn on an output device if Input #1 is on, and then turn off another output device if Input #2 is off. The PLC will analyze these conditions, execute the appropriate action, and then store that information in memory.

Lastly, the PLC has to update the output status. This means it will send data to an output device, such as the coil of a magnetic motor starter, to enable some type of manufacturing process to begin. The time it takes the PLC to go through this cycle is called the scan time.

The PLC uses a programming language based upon readily identifiable symbols common to motor control. Hand-held programmers or PCs are the most common methods for programming the PLC. Figure 2 on page 18 is an example of a programming code setup to perform an “AND” operation. Switch #1 and Switch #2 are connected in series to the coil of a relay. The first rung of the ladder diagram shows two inputs, namely Switch #1 and Switch #2, and an output, namely the relay. Each rung of the ladder diagram should contain input(s) and output(s). The input(s) should be the first listed instruction and the output(s) the last listed instruction. Usually, programming code requires the END command to be listed as the last instruction on the last rung of the ladder diagram.

The following specifications will give you a sense of the type of information that is important in the selection and application of the PLC:

  • 80188 CPU 8MHZ clock speed

  • Input points — 16

  • Output points — 12

  • 8226; High-speed counter — 10KHZ

  • Maximum user program — 1K

  • Registers — 256 words

  • Internal coils — 2560

  • Memory backup with lithium battery — five years

  • LED status indicators for I/O and CPU status

  • Scan rate — 18mS/1K of logic.

For more background on PLC operations, you can review past Motor Facts articles that deal with magnetic motor starters, input and output devices, and ladder logic by going to and searching by keyword.

Vidal is president of Joseph J. Vidal & Sons, Inc., Throop, Pa.

Sidebar: PLC Logic Functions

Ladder logic is the symbolic language of motor control. The binary number system has two numbers, namely “0” and “1.” The “0” refers to a logic state low (OFF), and the “1” refers to a logic state high (ON).

Two logic functions most common in motor control circuits are the “AND” operation and the “OR” operation. The “AND” operation occurs when two contact devices are connected in series. For example, if two switches are connected in series, Switch #1 and Switch #2 must both be in the ON position for the load to be energized. In terms of logic, this means Switch #1 (Input # 1) and Switch #2 (Input #2) can either be in the ON or OFF position. When both switches are in the ON position (logic state 1), the load is energized. When both switches are in the OFF position (logic state 0), the load is de-energized.

The “OR” operation occurs when two contact devices are connected in parallel. For example, if two switches are connected in parallel, either Switch #1 or Switch #2 can be in the ON position for the load to be energized. In terms of logic, this means Switch #1 (Input #1) and Switch #2 (Input #2) can either be in the ON or OFF position. When one of the switches is in the ON position (logic state 1), the load is energized. A truth table is a graphical way of showing how inputs and outputs behave according to logic function. The “ones” and “zeros” in the table are binary numbers that represent ON and OFF states. (See the Figure below for logic function “AND” and logic function “OR” and the corresponding truth tables.)