Types of PLC IO signals and DATA Types

Programmable Logic Controller(PLC) receives Input signals from the sensors and controls the output devices by sending control or output signals, to achieve the desired operation in a system.

For Basic details of PLC & Programming click these links What is Programmable Logic Controller? & Understanding the PLC Programming

Types of PLC Input:

1. Digital Input (DI) – Digitals inputs are Logic High & Logic Low signals (Two States either ON or OFF) from the Digital Input devices. Input devices are ON/OFF switch, Push Button, Level switch, Limit switch, Pressure switch, Temperature switch, etc.,
2. Analog Input (AI) – Analog Inputs are ranges of electrical signals (Not just two states but signals between Minimum and Maximum range) from the Analog Input devices such as Pressure Transmitter, Level Transmitter, Analyzer Transmitter, Temperature transmitter, etc., The range electrical signals of an AI from the Transmitters are depended on its working principle, check the following Analog input types.
   • Current inputs ( 4-20 mA or 0-20mA)
   • Voltage inputs ( 0-10 V or 0-5 V)
   • RTD inputs ( Resistance)
   • Thermocouple (Resistance)

Types of PLC Output:

1. Digital Outputs (DO) – Digital outputs are commands to Output devices to operate it in two positions (two states) ON or OFF. Digital output devices or actuators such as Lamp, Solenoid Valve (ON/OFF valve), Single speed motor (motor without VFD), Sounder (Buzzer or Hooter for audible alarms), etc.,
2. Analog Output (AO) – Analog outputs are command to the output devices to operate it between the ranges of positions (multiple states). Analog output devices such as Control valves, Variable speed motor (motor with VFD), etc., Types of Analog output to output devices are,
   • Current output ( 4-20 mA or 0-20 mA)
   • Voltage output (0-10 V)

Data Types

PLC Tag names are Memory reference to IO signals in PLC. Each type of IO signal has its own data type. The above block diagram shows the relations between Data types and IO signals.

Data types are standard memory format which is assigned to Tags/variables to store the values of IO signals.

Refer above block diagram,

DI_Tagname and DO_Tagname have a Data type of BOOL. BOOL is the 1 Bit memory it can store either 1 (Logic High) or 0 (Logic Low). We know that Digital devices have only two states ON (Logic High) and OFF (logic Low) and these two states can be stored in 1 bit, so we need one Tag with a BOOL data type to store the value of one Digital IO device.

AI_Tagname & AO_Tagname has a Data type of INT. INT or Integer is 16-Bit memory type, it can store decimal values from 0 to 65535. For example consider pressure transmitter which gives electrical signals ranging from 4 to 20 mA to AI module. AI module converts 4 – 20 mA electrical signals to 0 – 65535 decimal value (digital signal) which is readable by the PLC. So we need Tag with Integer data type to store the values from the Analog IO devices.

Note: Conversion of 4-20mA to Decimal value (0-65535) is depends on PLC manufacturer.

PLC not only handles IO signals but also handles Control logics like sequences, calculations, comparisons, etc. So this requires more types of Data formats. The following are the commonly used data types in most PLCs.

1. BOOL or Bit.
2. INT – Integer 16 bit memory (Decimal value, 0 to 65535).
3. SINT – Signed Integer 16 bit memory ( Decimal value, -32767 to 32768).
4. WORD – 16 bit Memory (Binary, 16-bit locations).
5. DINT – Double Integer 32-bit memory (Decimal value).
6. DWORD – Double WORD 32-bit memory (Binary).
7. FLOAT or REAL – 32-bit memory (Decimal value with fractions like 10.7, 654.32, etc., ).
8. STRING or CHAR – To represent Alpha-Numeric characters.