Data and I/O

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2.2 Input and Output (I/O)

The control of a system reacts to input information and configures output(s) accordingly. Input and output information can be in the form of physical signals such as electrical and pneumatic pulses or levels or in a virtual form such as text instructions or data. A controller may react to switches or fluid levels by turning on valves or running motors at a given speed, or a computer may react to text or mouse-click type instructions by changing display screens or running a program. These are both cause and effect illustrations of automation at work.

2.2.1 Discrete I/O

Most control systems on a manufacturing plant floor use discrete I/O (or Input/Output) in some form on both the input and output side of the process. Digital signals such as switches, pushbuttons and various types of sensors are wired to the inputs of a system while the outputs can drive motors or valves by turning them off and on.

Typical electrical I/O uses low voltage and current signals for inputs and outputs. 24 volt DC (24vdc) and 120 volt AC (120vac) signals are the most widely used though this can vary by application and by country. In some systems that need even lower electrical energy due to a hazardous environment low voltage systems called “intrinsically safe” circuits are used. These are typically about 8 volts DC or less. When a system is shielded from outside effects such as signals inside a controller or on a circuit board signals of 5 volts DC or less are common.

Due to personnel safety concerns, 120VAC I/O is not as widely used, however systems which have sensors and actuators spread over a large physical area still sometimes use AC. Process plants with AC valves and motor starters or large conveying systems still occasionally use 120VAC, but distributed communication or network based I/O is becoming more common.

Other types of discrete I/O are used for special cases. Pneumatic valves can be plumbed in a configuration called “Air Logic” where switches may allow air to flow in a circuit actuating valves and other air switches to serve much the same purpose as electrical signals. Air logic is used in some cases where electricity can be hazardous but is not as common as electrical signals.

2.2.2 Analog I/O

Analog inputs and outputs typically take the form of changes in either voltage or current. Analog inputs may represent the position of a device, an air pressure, the weight of an object or any other physical property that can be represented numerically. Most measurement systems use analog inputs. Analog outputs may be used to control the speed of a motor, the temperature of an oven or many other properties.

Common analog ranges in industrial applications are 0-20mA or 4-20mA when using current or 0-10vdc for voltage. Current control is considered to be less susceptible to electrical noise, while voltage control can be used over longer distances.

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