How to Extract an I/O List from P&ID PDFs

Every controls engineer has done it: open a P&ID PDF, find the instrument bubbles, read the tag numbers, classify each signal type, and enter it all into Excel. For a brownfield plant with 50+ drawings, this takes a week.

This guide covers the process, common mistakes, and a faster approach.

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.

The manual process

1. Open the P&ID PDF — or print it and mark it up with a highlighter
2. Scan each page systematically for instrument bubbles
3. Read the tag number (often 5-8pt text inside a small circle)
4. Decode the ISA 5.1 letter code to determine instrument type
5. Classify the signal: transmitter → AI, controller output → AO, switch → DI, solenoid valve → DO
6. Type everything into a spreadsheet
7. Repeat for every instrument on every page

A 50-page P&ID set with 500+ instruments takes an experienced engineer 3-5 working days. The error rate is typically 5-15% — misread characters (T vs P, I vs L), missed instruments, or wrong signal classifications.

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.

Automated extraction

Automated tools read P&ID pages as images, identify instrument bubbles, extract tag numbers, and classify signals. The workflow becomes:

1. Upload the PDF
2. Review extracted instruments — each tagged with a confidence score
3. Correct the 10-20% that need attention
4. Export as Excel or CSV

The time comparison:

Method	50-page set	100-page set
Manual	3-5 days	1-3 weeks
Automated + review	30-60 min	1-3 hours

The key insight: you're not eliminating human review — you're eliminating the tedious extraction step. The engineer's time goes to reviewing and correcting, not squinting at PDFs.

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 — quickly count instruments to size a control system proposal
• Documentation audits — verify that as-built I/O lists match current P&IDs

Try it on your own drawings

Tagsight gives you 5 free pages to test. Upload a P&ID, review the extraction, and export an I/O list — takes about 5 minutes. The best test is your own drawings, because every facility has its own conventions.