What is an Instrument I/O list?
- I/O is an abbreviation for Input/Output. I/O refers to any device that sends data to or from a PLC/DCS system.
- An I/O list is a document that comprises a list of the Input and Output utilized in a control system.
- The I/O modules are linked to field devices such as sensors, transducers, actuators, control valves, and so on. These I/O modules are internally connected with the PLC/DCS system.
- The I/O list is generally referred as I/O list for PLC or I/O list for DCS.
Detailed Explanation of I/O List:
- An I/O (In/Out) list is a document with a list of the instruments that information processing systems use to communicate to each other.
- Signals that are received by the system are its inputs, and signals that are sent by the system are its outputs. Any program, activity, or device that transfers data to or from a computer or another device is referred to as an I/O (In/ Out) list. One of the main deliverables of the CSI (Control System and Instrumentation) discipline is an I/O List.
- I/O list only displays the tag number that actually has a cable connecting to the control system. The I/O list must clearly state which instruments are assigned to which control system when there are many control systems in a plant or it may segregate the instruments into different sections of the document.
What is included in an I/O list?
The following information should be included, but not limited to, in the I/O list:
- Tag number
- Loop Number
- Service description
- P&ID Number
- Type of Instrument
- Location
- I/O Signal Type
- Control System
- Range of the instrument
- setpoint value
The information in the I/O list column may be as simple as what is shown above, but some projects need the I/O list to be more detailed by including an alarm list, controller action, and logic “0”/”1″descriptor. I/O list would include I/O number assignment (rack, slot, channel number), which information would be supplied by the control system vendor.
How do you create an I/O list?
To generate the I/O list, filter all tag numbers with an Input or Output from the instrument index.
What is the purpose of an I/O list?
- I/O list is used to determine the size of the control system necessary for a project by determining the number of I/O within a plant for each control system. It serves as the foundational material for designing PLC and DCS.
- The I/O List is used to specify the instruments’ input & output signal types. Inputs and outputs of PLC/DCS are categorized as follows:
- Digital Output – DO
- Analogue Input – AI
- Digital Input – DI
- Analogue output – AO
How to prepare an IO list?
An IO (Input/Output) List is an important document for projects that involve industrial automation and instrumentation. It is a master reference that shows all the I/O points that are linked to the plant’s control system, whether it is a DCS (Distributed Control System), PLC (Programmable Logic Controller), or SCADA (Supervisory Control and Data Acquisition) system.
The IO list is usually made during the detailed engineering phase and is utilized by teams working in instrumentation, electrical, automation, control system programming, and commissioning.
Depending on the size of the project and how complicated the system is, an IO list might have several structures. The example below shows a format that is often used and has 16 important columns, each of which is explained in depth.
1. Sr. No. (Serial Number)
- A simple, sequential number that uniquely identifies each I/O entry.
- Helps in tracking the total number of I/Os in the project or a specific unit.
2. Tag No.
- The unique identifier assigned to the field device (e.g., TT-101 for Temperature Transmitter).
- Follows the plant tag numbering philosophy based on ISA or project-specific standards.
3. System Tag No.
- The tag used in the DCS or PLC controller database.
- Often matches the Tag No. but may differ in format depending on the system configuration or hierarchy.
4. Service
- Describes the location or purpose of the instrument in the process (e.g., “Heat Exchanger Inlet”, “Pump Discharge”, etc.).
5. Instrument Type
- Uses standardized short forms:
- FIT – Flow Indicating Transmitter
- TE – Temperature Element
- LT – Level Transmitter
- PSV – Pressure Safety Valve
- A quick reference to the kind of instrument.
6. Instrument Description
- A brief functional description (e.g., “Temperature Transmitter for Reactor Jacket”, “Level Switch for Sump Pump Low Level”).
7. P&ID Reference
- The Piping and Instrumentation Diagram (P&ID) number where the instrument is shown.
- Crucial for cross-referencing and verification during design and loop checking.
8. Line No. / Equipment No.
- Indicates the pipeline number or equipment number the instrument is associated with.
- Aligns with P&ID and piping isometric drawings.
9. Signal Type
- Categorizes the nature of the I/O signal:
- AI – Analog Input
- AO – Analog Output
- DI – Digital Input
- DO – Digital Output
- Also includes special types such as HART, Pulse, or Modbus signals.
10. Signal Power / Wiring Type
- Defines both the signal standard and the wiring method:
- Signal Standards: 4-20 mA, 1-5 VDC, 3-15 PSI, Modbus, etc.
- Wiring Type: 2-wire, 3-wire, or 4-wire transmitters.
- Also mentions Wet Contact (powered signal) or Dry Contact (potential-free signal).
11. Source / Destination
- Source: Where the signal originates (e.g., field device, junction box).
- Destination: Where the signal terminates (e.g., PLC input card, DCS analog module).
12. Scale Range (Low / High)
- Defines the measurable range of the analog signal:
- Example: 0 to 100°C, 0 to 10 bar.
- Often divided into two separate columns: Low Range and High Range.
13. Setpoint
- Applicable for closed-loop control systems.
- Indicates the desired control value where a loop maintains the process (e.g., 50°C for a jacketed vessel).
14. Engineering Unit
- Specifies the unit of the measured value:
- Examples: °C, bar, m³/h, %, ppm, etc.
15. Alarm Limits
- Defines the alarm thresholds:Low Alarm (LA), High Alarm (HA), Low-Low Alarm (LLA), High-High Alarm (HHA)
- Critical for both process alarms and safety shutdowns (ESD).
16. Alarm Priority
- Assigns priority levels for alarms: Example: Low, Medium, High, or numerical scale (e.g., 1-3).
- High-priority alarms may trigger interlocks or emergency actions.
Tips for Preparing the IO List
- Begin with the P&IDs and the Instrument Index: Check all of the instruments against each other.
- Work with Control System Vendors: Make sure that the IO list matches the PLC/DCS setup.
- Always include the version and date to keep track of changes.
- Add a status column (optional) to keep track of the engineering, purchasing, and commissioning stages.
- Use filters or color coding: Excel can be helpful for seeing analog, digital, critical, or safety signals.
Example Instrument I/O list excel is below:
