PLC related Terminologies

Terminologies associated when discussing Programmable Logic Controllers (PLC) are sometimes confusing. With this in mind, let’s discuss some of the technical terms, provide a simple definition and a common example.

1. PLC – Programmable Logic Controller
As discussed in the previous chapters, PLC means Programmable Logic Controllers (PLC).

2. DCS – Distributed Control System
DCS is different from PLC. Ang DCS ay isang kumpletong system na kapag binili, may kasama nang PLC at Visualization system (SCADA o HMI). Sa DCS, isa lamang ang PLC sa mga components.

3. Input and Output
Input and Output or sometimes called I/O. Tumutukoy ito sa mga devices na magbibigay ng signal sa PLC upang maging reference nito sa pagproseso ng logic.

4. Discrete Input
Kilala rin sa tawag na Digital o Binary input. These are signals that provides Two (2) signal positions – ON and OFF. Ilan sa mga halimbawa ng mga devices na nagbibigay ng Discrete Input signal  ay Pushbutton switches, Limit Switch, Magnetic contactor / Relay auxiliary contacts, Overload relays, level switch, Proximity switch, Photo-Electric sensors at iba pa.

The PLC receives the Discrete input signal from the instrument via the Discrete input module.

Push button and Switches – These devices produce Discrete Signals

5. Discrete Output
Similar to Discrete Input, sometimes called Digital or Binary Output. These are signals that have Two (2) actuating positions – ON and OFF. The ON/OFF states are used to drive devices such as Motors, Pumps, ON/OFF Valves, Pilot Lamps, Magnetic Contactor and Relay coils, Buzzer, etc.

The PLC Sends the Discrete output signal to the instrument via the Discrete output module.

Pilot Lamps – Example of Discrete Output Devices

6. Analog Input
This Input signal type has multiple states or values. Ginagamit ang analog input signal sa measurement ng Process Variable tulad ng Pressure, Level, Flow at Temperature. Examples of these type of signals are 4-20 mA, 1-5 Vdc, 0-10Vdc,  -10 – +10Vdc, etc. Instruments capable of producing an analog signal are Transmitters, Transducers, Thermocouple, RTD and others.

The PLC receives the Analog Input signal from the device or instrument via the Analog Input Module.

Analog Input Devices – Transmitters and Transducers produces Analog signals in reference to the actual process measurement.

7. Analog Output
This Output signal type, similar to analog input has multiple states or values. However, an analog output signal is used to drive modulating devices or equipment. Ilan sa halimbawa ng mga instruments na tumatanggap ng analog output ang mga VFD, Control Valves, Positioners, Signal converters, Dampers, Louvers at iba pa. Examples of these type of signals are 4-20 mA, 1-5 Vdc, 0-10Vdc,  -10 – +10Vdc, etc.

The PLC Sends the analog output signal to the instrument via the analog output module.

A Control Valve with positioner modulates in reference to Analog Output sent by the PLC.

8. Upload
Program Upload refers to the action where the program stored inside the PLC is transferred to a programming device or a computer. Ginagawa ang pag-upload ng program upang makakakuha ng kopya ng aktuwal na gumaganang program mula sa PLC.

Normally, an upload operation shall not switch the PLC to STOP mode however, precautions must be observed when performing this task. Incorrect handling of the PLC hardware, may trigger PLC Stoppage.

9. Program Download
Program Download refers to the action where the program prepared in the programming device or a computer is transferred to the PLC. Ginagawa ang download upang mailipat ang ginawang program sa PLC.

Precautions must be applied when downloading a program to an existing PLC in RUN mode. Download may cause PLC Stoppage.

10. RUN Mode
Run mode refers to the PLC state when the program inside is executed. Kapag nasa RUN mode ang PLC, ang logic sa loob ng PLC ay pinoproseso na siyang nagpapagana ng process.

11. STOP Mode
STOP mode refers to the PLC state when the program inside is not executed. Kapag nasa STOP mode ang PLC, ang logic sa loob ng PLC ay hindi gumagana.

12. Remote or Remote Run
Ang Remote o Remote Run ay PLC state kung saan pinahihintulutan ang pagbabago o pag-edit sa PLC program mula sa isang remote programming device o computer. Mula sa programming device na ito, maaring ilipat ang mode ng PLC sa STOP at REMOTE RUN gamit ang programming software.

13. Online
Online means connecting to a PLC using a programming device. Kapag online sa PLC, makikita ang aktuwal na paggana ng PLC at takbo ng program. Nakakatulong ang pag-online sa PLC sa troubleshooting, testing at pag-obserba sa process.

When online with the PLC, program elements animate to show which part of the program is active.

14. Online Edit
Online edit refers to a task done by the programmer where the program is being revised or edited online. Precautions must be applied when performing online edits as the revisions may trigger programming errors which in turn, may change the PLC mode to STOP.

15. SCAN
The PLC process where the inputs are examined, logic is executed, outputs are activated and internal PLC diagnostics are conducted is called a PLC SCAN.

PLC SCAN Procedure source:

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