In this article9 sections
Every controls engineer has built one. Open a P&ID PDF, find the instrument bubbles, read the tag numbers, classify each signal type, and enter it all into a spreadsheet. The exercise is well understood. The errors are predictable.
What goes into an I/O list
An I/O list maps every field instrument to a signal type for DCS, PLC configuration. Each row captures.
- Tag number. The unique identifier, FIT-101, PSH-200, LCV-301
- Instrument type. What the ISA letter code means, Flow Indicating Transmitter, Pressure Switch High
- Signal class. AI, AO, DI, or DO
- Description. What it measures or controls
- Drawing reference. Which P&ID page it appears on
This list drives procurement, loop wiring, PLC programming, and commissioning. Errors here propagate through the entire project.
Common classification mistakes
| Instrument | Correct class | Common mistake |
|---|---|---|
| FIT, Flow Indicating Transmitter | AI | Classified as AO because "I" is misread as indicating a controller |
| FCV, Flow Control Valve | AO | Classified as DO. It's analog because it modulates position |
| PSH, Pressure Switch High | DI | Classified as AI. Switches are discrete, not analog |
| XV, Shutoff Valve | DO | Classified as AO. On, off valves are digital outputs |
| ZSO, Valve Position Open | DI | Missed entirely. Position feedback is easy to overlook |
These mistakes surface during FAT or commissioning. The most expensive time to find them.
File preparation before extraction
What you upload determines the quality of what you get back. These steps take a few minutes and reduce review time significantly.
Confirm the P&ID revision. Extract instruments from the controlled revision, not a working draft. The controlled revision number should appear in the title block. If you are unsure which revision is controlled, check the drawing index before starting.
Check resolution on scanned drawings. For scanned PDFs and image files, 300 DPI minimum produces readable instrument bubbles. At 150 DPI, small tag text becomes ambiguous. At 72 DPI, web resolution, tag text is too degraded for reliable reading. Check the scan resolution by looking at the pixel dimensions relative to the paper size. An A1 drawing at 300 DPI is approximately 9933 x 7016 pixels. If the image is significantly smaller than that, request a higher-resolution scan.
Flatten markups before uploading. Some PDF viewers save markup annotations as a separate layer. If the annotation layer contains correction information that differs from the drawing, only the base layer will be read. Flatten the PDF, print to PDF, or use Acrobat's flatten annotations function to merge all layers before uploading.
Split very large drawing sets. Drawing sets with more than 30 to 40 pages per PDF process correctly but take longer. If a drawing set is a single multi-hundred-page PDF, splitting it into groups of 20 to 30 pages by process area speeds up review because results come back in smaller, reviewable batches rather than one large output.
Orient the pages correctly. Some scanned PDFs contain pages that are rotated 90 or 180 degrees. Rotate pages to portrait or landscape as intended before uploading so the output positions match the drawing as you read it.
Scanned vs vector PDFs. What changes
Vector PDFs, exported directly from CAD software such as AutoCAD, SmartPlant, or AVEVA, hold the drawing as lines and text rather than pixels. They stay sharp at any zoom, so the smallest tag text is as legible as the title block. Some CAD exports keep the text as searchable characters and some turn it into outlines, and both stay crisp. A vector PDF gives the cleanest result.
Scanned PDFs are raster images, typically produced by scanning a paper drawing or a physical plotted copy. Small tag text, faded lines, skewed scans, or bleed-through from double-sided originals all produce more tags that need manual review.
For projects where both a scanned PDF and a CAD file exist, always prefer the CAD source. If only the scanned version is available, the quality of the scan is the primary variable. A clean 400 DPI scan with no handwritten annotations reads close to a CAD export. Heavily annotated or low-resolution scans need more checking.
Where Tagsight fits
Upload the P&ID set and Tagsight returns the I/O list read from the drawings, every row marked on the sheet it came from, for an engineer to confirm before export. The reviewed list exports to the format your team uses.
- Excel (.xlsx). The standard format for procurement, engineering, and commissioning
- TIA Portal XML. Tag table import package for Siemens S7-1200 and S7-1500 projects
- Rockwell L5X. Import into Studio 5000 for Allen-Bradley systems
- PLCCreator CSV. Signal IO List for single-rack and fieldbus projects
- DEXPI XML. Plant model for process industry data exchange
When this matters most
- Brownfield projects. Legacy P&IDs exist only as scanned PDFs, no CAD source
- Control system migrations. DCS-to-DCS or DCS-to-PLC, need fresh I/O counts
- Bid estimation. Count instruments to size a control system proposal
- Documentation audits. Verify that as-built I/O lists match current P&IDs
After extraction, validate the resulting I/O list against the P&IDs using the structured checklist in how to validate an I/O list against P&IDs.
Working example on a real drawing
Upload a P&ID and Tagsight returns a classified I/O list ready to review and export. The best test is your own drawings, because every facility has its own conventions.
For format-specific export guidance, see the I/O list template reference and the instrument index vs I/O list vs line list comparison.
FAQ
What resolution should I scan P&ID drawings.
300 DPI is the practical minimum for reliable tag text. 400 DPI is preferred for drawings with small text or fine linework. Anything below 200 DPI produces ambiguous tag numbers that require significant manual correction. For A0 or A1 drawings with many instruments per page, scan at 400 DPI. For A3 or A4 drawings with larger text, 300 DPI is sufficient. Always scan in grayscale or color, not pure black-and-white, 1-bit, which loses anti-aliasing and makes curved text harder to read.
How do I handle instruments with non-ISA tag formats.
Tag formats that do not follow ISA 5.1, such as Dutch HVAC tags like AB2WV05, German DIN formats, or KKS plant coding like 10LAC01CT001, keep the form the drawing prints. The ISA second-letter rule does not apply to them, because the letter positions mean different things, so take the signal class from the instrument type and the facility's own tagging convention document.
How do I deal with revision clouds on P&IDs.
Revision clouds, the curved lines that highlight changed areas on a drawing, do not change what belongs on the instrument list. An instrument shown deleted inside a cloud, with an X through the bubble, comes off the list, so check each clouded area against the revision block.
What is the best way to handle instruments that appear on a vendor package drawing rather than the main P&IDs.
Vendor package drawings, skid P&IDs supplied by the equipment vendor should be processed separately from the main facility P&IDs. The tag numbering convention may differ, and the SIS, BPCS classification for vendor package instruments must be confirmed against the package instrument index supplied by the vendor. After processing, the vendor package I/O list is merged into the main project I/O list with a clear source flag, for example, a "Package" column noting the vendor package reference. Duplicate tag numbers between the vendor package and the main facility list must be resolved before merging.
Related
- Supported file formats. PDF, scanned, and image inputs the extraction accepts
- Instrument index. The master register the extracted tags populate
- Instrument index vs I/O list vs line list. Which document each tag belongs on
- Building an I/O list from P&IDs. The full workflow, start to finish
- Excel I/O list template guide. The 14 columns and how to fill them