How to Create a PLC Variable/Tag Table from Scratch

When you inherit a PLC program with no symbol table, every address is a mystery. E 0.0 could be a start button, a proximity sensor, or an emergency stop. Creating a variable table is the single most valuable documentation step — it transforms cryptic addresses into readable code. This guide covers the complete process.

What You Need Before Starting#

• The PLC program (backup or online connection)
• Electrical schematics / wiring diagrams (the most important document)
• Access to the machine (for physical tracing if schematics are incomplete)
• A cross-reference list from the PLC program
• 1–3 days of time, depending on program size

Step 1: Generate a Cross-Reference List#

The cross-reference list shows every address used in the program and where it is used.

In STEP 5: Extras → Cross-Reference (Querverweisliste / QL)

In STEP 7 Classic (SIMATIC Manager): Extras → Reference Data → Cross References

In TIA Portal: Project → Cross-references (in the Inspector window)

This gives you a complete list of every I/O address, marker, timer, counter, and data block element used in the program. Many addresses in the hardware may not be used in the program — the cross-reference list tells you which ones actually matter.

Step 2: Map I/O Addresses to Physical Devices#

This is the core work. For every input and output address in the cross-reference list, identify the physical device:

From wiring diagrams (preferred):

• Find the PLC terminal assignments
• Trace each terminal to its field device
• Record: PLC address → Terminal → Cable → Device → Device function

From the machine (if no wiring diagrams):

• Put the PLC in STOP mode (or use the monitoring function in RUN)
• Activate each sensor/actuator manually and observe which PLC address changes
• For outputs: Force individual outputs (with extreme caution!) and observe which device activates

Record format:

Step 3: Name Markers, Timers, and Counters#

Markers (M), timers (T), and counters (Z) do not have physical connections — their function can only be determined by reading the program logic.

Common patterns:

Step 4: Choose a Naming Convention#

There is no universal standard, but consistency is key. IEC 61131-3 specifies that variable names must consist of letters, digits, and underscores, starting with a letter or underscore.

Recommended structure: [Location]_[Device]_[Function]

Examples:

Rules to follow:

• Use underscores as separators (not spaces or camelCase)
• Be descriptive: Conveyor_1_Motor beats M1
• Include location when the plant has multiple identical stations
• Maximum length: 24 characters in STEP 5, 128 in STEP 7, 128 in TIA Portal
• Avoid special characters (ä, ö, ü, ß) in international projects

Step 5: Enter the Table#

In STEP 5: The symbol table (Zuordnungsliste) is edited in the ZL editor.

In STEP 7 Classic: Symbol Editor (Extras → Symbol Table). Can export/import as .ASC file.

In TIA Portal: PLC tags (in the project tree under "PLC tags → Default tag table"). Can export/import as .XML file.

Bulk entry tip: Export the tag table to Excel/CSV, fill in names and comments, then import back. This is much faster than typing each entry in the PLC software.

Step 6: Verify#

After creating the table, verify it:

• Open the program with symbols displayed
• Read through the most critical blocks — does the logic make sense with the names?
• Monitor the program online — do the named variables behave as expected?
• Have an operator walk through a machine cycle while you watch the named I/O

How PLCcheck Pro Helps#

PLCcheck Pro can accelerate steps 1–4 dramatically:

• Generates the complete cross-reference automatically
• Identifies common logic patterns and suggests variable purposes
• Proposes naming based on usage patterns
• Exports a ready-to-import symbol table

Upload your PLC code →

Frequently Asked Questions#

How long does it take to create a tag table for a typical machine?

For a small machine (32 I/O, 20 blocks): 4–8 hours. For a medium machine (128 I/O, 50 blocks): 1–2 days. For a large system (500+ I/O, 100+ blocks): 3–5 days. The time depends heavily on the availability of wiring diagrams.

Should I name every address or only the used ones?

Name every used address (from the cross-reference list). Unused addresses can be marked as "spare" or left unnamed. Do not name addresses that are reserved for future use unless you know their intended function.

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Maintained by PLCcheck.ai. Last update: March 2026. Not affiliated with Siemens AG.