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All you need to know about IO List


The IO List is a very important document for any instrument engineer. You can also download the Free industrial standard IO List sheet.

The most common skills mentioned in every instrumentation design engineer job vacancy include the preparation of the Instrumentation index and IO list documents.

I already shared information on the Instrument Index in my previous article You will simply visit by clicking Here

Lastly, I will also share an example IO list format that I used for my projects so you can directly download it for your reference.


The IO List gives overall information about all the instruments that are used in the plant or any production unit.

IO list will help you in troubleshooting in the control system because it has all the general information related to Input and Output that is connected to the plant control system.

Importance of IO List

Procurement of the control system is directly dependent on IO List. IO list is the most important document when you have to order or design a plant control system.

IO list is created before procurement of the PLC/DCS system. IO list works as a boundary line for how big my plant control system will be.

When your IO list is prepared that you have to share it with automation vendors to get commercial offers.

IO list is also a reference document for your cause-and-effect charts because all the setpoint and alarm limits are mentioned in this document.

Preparation of IO List

An IO list is a type of document that contains all the IOs and their related information connected to the plant control system.

The format that I am using has a total of 21 columns and it may change according to project requirements and type. I will explain all the columns one by one.

1. Sr. No.: This column shows the simple serial number and the last serial number shows how many instruments are used or will be used in a specific plant/unit.

2. Tag No.: As you guess in tag no. the column you will find the tag no of the instrument as per your plant tag number philosophy.

3. System Tag No: In this column, you have to put the Tag numbers that you considered in PLC/DCS controller database. Generally, it is the same as tag numbers.

4. Service: The service column shows that this particular tag is used in which services like a storage tank, heat exchanger, etc.

5. Instrument Type: In the Instrument Type Column you can find the short name of the instrument. like flow indicating transmitter is FIT, and TE means Temperature Element.

6. Instrument Description: In the Instrument Description Column you can find out what type of instrument.

7. P&ID: In P&ID Column you have to write the P&ID drawing number where the instrument is present.

8. Line No./ Equipment No.: In this column, you have to write the standard pipeline number or Equipment No.

9. Signal type: In this column, you will find what type of signal it is like Analog Input, Analog Output, Digital Input, and Digital output.

10. Signal Power: In this column, you have to mention the transmitter wiring and signal type. The type of signal is as follows 1-5VDC, 4-20mA, Modbus and 3-15PSI Wiring type is as follows 2, 3, and 4 Wire method.

You also have to mention whether it is Wet or Dry contact.

11. Source/Destination: In these columns, you will find where this signal comes from and where it is going. like the field to DCS.

12. Scale Range: The scale range is divided into two columns High and low range of instruments.

13. Setpoint: This these columns, you have to mention the setpoint of a particular close loop.

14. Engineering Unit: In this column, you will find the engineering unit for measuring variables.

15. Alarm Limit: So, in these columns, you have mentioned alarm limits where your safety/ESD system will respond.

16. Alarm Priority: In this column, you have to mention which alarm gets the highest priority.

Example of the IO List:

Example Instrument index


I hope these resources and Knowledge will become helpful to you in the future. You can also give your input through the comment section.

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If you like this article, you can also like my previous article on Temperature measurement without thermowell.

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