3。1  INPUTS AND OUTPUTS

Inputs to, and outputs from, a PLC are necessary to monitor and control a process。 Both inputs and outputs can be categorized into two basic types: logical or continuous。 Consider the example of a light bulb。 If it can only be turned on or off, it is logical control。 If the light can be dimmed to different levels, it is continuous。 Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control。In small applications it’s often cost saving to use relay-based circuits instead of PLC-s。 

Progress in process automation systems is aiming at so called complete automation when all the human has to do is to enter the parameters of the product wanted and everything else is carried out by machines (Rosin, 2000)。 Although the destination lies far ahead, trends indicate movement in that direction。  Firstly, systems become more and more standardized。 Big manufacturers organize their products into families。 The aim is to reduce amount of knowledge needed for configuration and maintenance works of different devices from same company。 It’s also important that this way built applications are easily expandable。 

Secondly, importance of communication is rising (Hughes, 2000)。 There are many reasons, some of more essentials。

There is no need for processor module, resources of PC are used。 Some Soft PLC-s are still formed as processor cards for PC 。 Reliable communication networks are essential。 Soft PLC-s are well suited for data acquisition applications because of data storage possibilities of PC-s。 

Fourthly, combo-devices (figure 2) that contain both operator panel and medium size processor module gain popularity (Siemens AG, 2002)。 In this solution possibility of disturbances is low and reuse of some components make whole package cheaper。 

Fifthly, processor software takes over properties from PC software。 Data collected from production can be easily transferred into office applications (Siemens AG, 2000)。 

As a result most controls applications (and PLCs) use logical inputs and outputs for most applications。 Hence, we will discuss logical I/O and leave continuous I/O for later。 Outputs to actuators allow a PLC to cause something to happen in a process。 A short list of popular actuators is given below in order of relative popularity。 Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow。 Lights - logical outputs that can often be powered directly from PLC output boards。Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays。 Servo Motors - a continuous output from the PLC can command a variable speed or position。 Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs。 Continuous outputs require special output cards with digital to analog converters。 Inputs come from sensors that translate physical phenomena into electrical signals。

Typical examples of sensors are listed below in relative order of popularity。Proximity Switches - use inductance, capacitance or light to detect an object logically。 Switches - mechanical mechanisms will open or close electrical contacts for a logical signal。 Potentiometer - measures angular positions continuously, using resistance。 LVDT (linear variable differential transformer) - measures linear displacement continuously using magnetic coupling。 Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs。 Sourcing and sinking inputs are also popular。 This output method dictates that a device does not supply any power。 Instead, the device only switches current on or off, like a simple switch。 Sinking - When active the output allows current to flow to a common ground。 This is best selected when different voltages are supplied。 Sourcing - When active, current flows from a supply, through the output device and to ground。 This method is best used when all the devices use a single supply voltage。 This is also referred to as NPN (sinking) and PNP (sourcing)。 PNP is a more popular。 This will be covered in detail in the chapter on sensors。