In this article13 sections
ISA 5.1 instrument identification is the ANSI/ISA-5.1 convention that names every instrument on a P&ID with a letter code and a loop number. The first letter is the measured variable. The letters after it are what the instrument does with that measurement. The number is the loop the instrument belongs to. FIT-101 reads as flow, indicating, transmitter, on loop 101.
| First letter | Measured variable | Example tag | Class, once wired |
|---|---|---|---|
| F | Flow | FIT-101 | AI |
| P | Pressure, vacuum | PIT-301 | AI |
| T | Temperature | TIT-401 | AI |
| L | Level | LIT-201 | AI |
| A | Analysis, pH, O2, composition, density | AIT-301 | AI |
| W | Weight, force | WIT-101 | AI |
| Z | Position, dimension | ZT-101 | AI |
| V | Vibration, mechanical analysis | VT-101 | AI |
| S | Speed, frequency | ST-101 | AI |
| H | Hand, manual | HV-101 | none for a hand valve; DO only if an actuator or solenoid is tagged with it |
| Q | Quantity, totalize | QI-101 | Review |
| J | Power | JI-101 | AI |
| U | Multivariable | UT-101 | AI |
| X | Unclassified, user-defined | XT-101 | varies |
The last column is where a device with that first letter most often lands once its wiring is known. It is a starting point for the review and never an assignment. For the standard itself, what ISA 5.1 covers and where its scope ends, start with the definition.
What the letters settle, and what they do not
ISA 5.1 is an identification scheme. It tells you what an instrument measures and what function it performs. It does not tell you which PLC or DCS channel the device lands on, and reading a signal class straight off the last letter is the most common way an I/O list goes wrong.
The letter narrows the candidates. The class is settled by the wiring and the project's control architecture, which is why an engineer confirms it against the design.
| Last letter | What it tells you | What still has to be checked |
|---|---|---|
| T | A transmitter, so a continuous measurement exists | Whether the measurement reaches an input card directly, through a multiplexer, or over a fieldbus |
| E | A primary or sensing element | Whether it is wired at all. An orifice plate has no electrical connection. A thermocouple may go straight to a card, or through a transmitter |
| C | A controller function | Whether it is a physical device or a block inside the control system. A shared-display controller has no channel of its own, and the analog output belongs to the path out to the final element |
| S | A switch function | Whether the contact is wired to an input, or the state is derived in software |
| A | An alarm function | Whether the alarm is a wired contact or a software comparison on a value the system already reads |
| V | A valve | The actuator and its interface. Modulating, on and off, several position contacts, or fieldbus. A hand valve has none of these |
| Y | A relay, solenoid or computing function | Whether it is an energised coil, a signal converter, or a software calculation |
| I, G, R | A local readout | Usually no wired channel. Confirm against the signal line |
A worked case. FIC-201 drawn as a shared-display bubble with a dashed line to FCV-302 is one AO on the path to the valve, not two channels. The same tag drawn as a discrete field-mounted controller is a different arrangement. The letters are identical in both. The drawing is what separates them.
When the sheet leaves the class open, mark the row for review rather than assigning a class. A guessed class becomes a wrong card count.
Tag structure at a glance
[First Letter][Modifier Letters]-[Loop Number][Suffix]
Example. FIT-101A breaks down as F, Flow, I, Indicating, T, Transmitter, 101, loop number, A, redundancy suffix.
The letters carry all functional information. The loop number identifies which control loop the instrument belongs to.
Tag number structure
Every P&ID instrument tag follows this format.
[First Letter][Modifier Letters]-[Loop Number][Suffix]
Example. FIT-101A
- F. Measured variable, Flow
- I. Function, Indicating
- T. Function, Transmitter
- 101. Loop number
- A. Redundancy suffix
The letters carry all functional information. The loop number identifies which control loop the instrument belongs to.
How to read a compound tag
Some tags carry more information than a simple two-letter code. Learning to parse these is essential for I/O list accuracy.
FY-2034A-2 breaks down as follows.
- F. Measured variable, Flow
- Y. Computing relay or conversion, the instrument performs a mathematical function on the flow signal, such as ratio or totalisation
- 2034. Loop number
- A. Redundancy designator, the first of multiple similar instruments in the loop
- -2. Secondary number identifying a second element within the same tagged device, common when one physical enclosure contains two independent outputs
PT-2034A is simpler. Pressure, P, Transmitter, T, loop 2034, first of a redundant pair.
PAHH-301 parses as Pressure, P, Alarm, A, High-High, HH. Full read: Pressure Alarm High-High on loop 301. The letters say what it is, not how it is wired. If it is a physical contact landed on an input card it is a DI, and whether that card belongs to the shutdown system is decided by the cause and effect matrix or an explicit marking on the drawing. If it is an alarm computed in software from the value PT-301 already delivers, it carries no channel at all. The loop sheet settles it; the tag cannot.
The suffixes that follow the loop number deserve particular attention. A single letter, A, B, C almost always indicates a redundant or parallel instrument in the same loop. A numeral suffix, -1, -2 usually identifies a sub-element within one physical device. Two independent outputs from one flow computer, for example.
First letters. What it measures
The first-letter table sits at the top of this page. The class column in it is a starting point for the review and never an assignment; the letters alone do not decide it, and the modifier table below names what does.
Letters B, C, D, G, M, N, O are formally "user's choice". Facilities assign them to site-specific variables. Common assignments. C, conductivity, D, density, G, gauging, M, moisture. Because the meaning comes from the project rather than from the standard, a user's-choice letter is read off the drawing's own legend or the project tagging manual, never assumed.
The remaining first letters
The table above covers the letters that carry a signal on most drawings. These twelve complete the set. The class column is deliberately absent: it is decided the same way as above, by what the drawing wires, and on these letters the drawing decides it far more often than the letter suggests anything.
| Letter | Variable | Also seen as | Common example |
|---|---|---|---|
| B | Burner, combustion | User's choice on some specifications | BS-101 |
| C | User's choice, commonly conductivity | Concentration, capacity | CT-201 |
| D | User's choice, commonly density | Differential, when it prefixes another letter, as in DPT for differential pressure | DT-401 |
| E | Voltage | Any electrical measurement | ET-501 |
| G | User's choice, commonly gauging or gas | Position or dimension on some specifications, rare | GT-301 |
| I | Current, electrical | Indicator, when it sits in the second slot, as in FI, PI | IT-505 |
| K | Time, time schedule | Time rate of change | KY-301 |
| M | User's choice, commonly moisture | Mass, rare | MT-401 |
| N | User's choice | No standard assignment | NT-105 |
| O | User's choice | No standard assignment | OT-405 |
| R | Radiation | Resistance, rare | RT-901 |
| Y | Event, state, presence | Computing relay, when it sits in the second slot, as in FY, PY | YS-201 |
Six of these are the reason the legend sheet matters more than the standard. B, C, D, G, M, N and O are the user's-choice set, so the same letter can mean conductivity on one plant and capacity on the next. Four more (I, S, V, W, Y) mean one thing in the first slot and something else entirely in the second, which is the single most common misreading of an unfamiliar tag: IT-505 is a current transmitter, while the I in PI is an indicator.
More examples by first letter
Flow, F. FIT-101, flow transmitter, AI, FIC-101, flow indicating controller, AO, FCV-101, flow control valve, AO, FSH-101, flow switch high, DI, FFY-101, flow ratio relay, computing.
Pressure, P. PT-301, pressure transmitter, no indicator display, AI, PIT-301, pressure indicating transmitter, AI, PSH-301, pressure switch high, DI, PSHH-301, pressure switch high-high, DI, on the shutdown system if the C&E matrix says so, PIC-301, pressure indicating controller, AO, PCV-301, pressure control valve, AO.
Temperature, T. TE-401, temperature element, RTD or thermocouple, AI, TIT-401, temperature indicating transmitter, AI, TW-401, thermowell, no signal, TSH-401, temperature switch high, DI, TIC-401, temperature indicating controller, AO, TCV-401, temperature control valve, AO.
Level, L. LIT-201, level indicating transmitter, AI, LSH-201, level switch high, DI, LSHH-201, level switch high-high, DI, LSLL-201, level switch low-low, DI, both on the shutdown system only where the drawing or the C&E matrix assigns them, LIC-201, level indicating controller, AO, LCV-201, level control valve, AO, LG-201, level gauge or glass, no wired signal.
Analysis, A. AIT-301, analyzer indicating transmitter, AI, AT-301, analyzer transmitter without local display, AI, ASH-301, analyzer switch high, DI, AIC-301, analyzer indicating controller, AO.
Modifier letters. What it does
The signal column below is the class a device of that function most often carries once the drawing has been read. It is a starting point for the review, not an assignment. Each row names what still decides it.
| Letter | Function | Often | What decides it |
|---|---|---|---|
| T | Transmitter | AI | Whether the output reaches a card directly or over a fieldbus |
| E | Sensing element | AI or none | An orifice plate is not wired. A thermocouple may go direct or via a transmitter |
| C | Controller | AO or none | A shared-display or software controller carries no channel. The AO belongs to the path out to the final element |
| V | Valve, final element | AO, DO or none | The actuator and its interface. A hand valve carries none |
| S | Switch | DI | Whether the contact is wired, or the state is derived in software |
| Y | Relay, solenoid, converter | DO or none | A solenoid coil is a DO. A signal converter or a computing function may be neither |
| I | Indicator | Review | Local readout, usually not wired |
| R | Recorder | Review | Chart or digital record |
| G | Gauge, glass | Review | Local sight device |
| H | High, alarm modifier | Used in combination | Modifies the setpoint, SH, SHH |
| L | Low, alarm modifier | Used in combination | Modifies the setpoint, SL, SLL |
| A | Alarm | DI or none | A wired alarm contact is a DI. An alarm computed in software on a value already read is neither |
| K | Control station | AO or none | Whether the station is a physical device or an operator faceplate |
| N | User's choice | varies | Site-defined. Read the legend |
| U | Multifunction | varies | A multivariable transmitter may carry several signals, or one fieldbus connection |
| Z | Driver, actuator, unclassified | varies | Read the legend. Conventions differ |
Signal classification reference
AI. Analog Inputs, transmitters, 4-20mA
| Code | Instrument | Notes |
|---|---|---|
| PT, PIT | Pressure Transmitter | PIT indicates local display present |
| FT, FIT | Flow Transmitter | FIT. Orifice plus transmitter combo common |
| LT, LIT | Level Transmitter | DP, radar, guided-wave radar |
| TT, TIT | Temperature Transmitter | Usually with TE, thermocouple as separate element |
| TE | Temperature Element | RTD or thermocouple. May have no transmitter |
| AT, AIT | Analyzer Transmitter | pH, O2, conductivity, chromatograph |
| PDT | Differential Pressure Transmitter | Also used for level via DP |
| WT, WIT | Weight Transmitter | Load cell assembly |
| ZT | Position Transmitter | Valve positioner feedback, linear position |
| VT | Vibration Transmitter | Machinery protection |
| JT | Power Transmitter | Watt or kW measurement |
| ST | Speed Transmitter | Tachometer output |
DI. Digital Inputs, switches, contact closure
| Code | Instrument | Notes |
|---|---|---|
| PSH, PSL | Pressure Switch High, Low | Process value crosses setpoint |
| PSHH | Pressure Switch High-High | Often a trip input; SIS membership comes from the C&E matrix, not the letters. Two out of three voting common |
| FSH, FSL | Flow Switch High, Low | No-flow detection common for pump protection |
| LSH, LSL | Level Switch High, Low | Float or tuning fork switch |
| LSHH, LSLL | Level Switch High-High, Low-Low | Vessel overfill or pump dry-run protection; on the SIS where the drawing or the C&E matrix assigns it |
| TSH, TSL | Temperature Switch High, Low | Bimetallic or electronic |
| ZSO, ZSC | Valve Position Open, Closed | Limit switch feedback from XV valve |
| XS | Vibration Switch | Machinery shutdown trigger |
| HS | Hand Switch | Operator pushbutton, maintained or momentary |
AO. Analog Outputs, controller outputs, 4-20mA
A controller tag names the control function. Where that function lives inside the DCS or PLC, it takes no channel of its own, and the analog output listed here belongs to the signal path from the control system out to the final element. Check the symbol: a shared-display bubble is a software block, a discrete field or panel controller is a device. Both are drawn with the same letters.
| Code | Instrument | Notes |
|---|---|---|
| FIC | Flow Indicating Controller | Output path to FCV. The AO is on that path, not on the block |
| PIC | Pressure Indicating Controller | Output path to PCV. The AO is on that path, not on the block |
| LIC | Level Indicating Controller | Output path to LCV. The AO is on that path, not on the block |
| TIC | Temperature Indicating Controller | Output path to TCV. The AO is on that path, not on the block |
| FCV | Flow Control Valve | Receives 4-20mA from FIC or split-range |
| PCV | Pressure Control Valve | Receives 4-20mA from PIC |
| LCV | Level Control Valve | Receives 4-20mA from LIC |
| TCV | Temperature Control Valve | Receives 4-20mA from TIC |
| HIC | Hand Indicating Controller | Operator-set manual output |
| FY | Flow Relay, Converter | I, P converter or ratio relay, analog output |
DO. Digital Outputs, on, off commands
| Code | Instrument | Notes |
|---|---|---|
| XV | Block Valve, on, off | 24VDC or pneumatic solenoid commanded |
| SOV | Solenoid Valve | Directly commanded coil |
| MOV | Motor Operated Valve | Motor-actuated. May also have ZSO, ZSC feedback |
| ESDV | Emergency Shutdown Valve | SIS-commanded. Separate DO from BPCS |
| SDV, BDV | Shutdown, Blowdown Valve | Process isolation or blowdown on SIS trip |
| HV | Hand Valve, power-operated | Manual or operator-initiated open, close |
| HS | Hand Switch output | Start, stop command to a motor or pump |
Review. May not be wired
| Code | Instrument | Notes |
|---|---|---|
| PI, TI, LI, FI | Indicators | Local gauge. No wired signal unless DCS-connected |
| PG, TG | Pressure, Temperature Gauge | Mechanical local only |
| LG | Level Gauge, Glass | Sight glass. No signal |
| TW | Thermowell | Mechanical protection element. No signal |
| FR | Flow Recorder | May be a DCS trend tag, not a physical instrument |
Panel-mount vs DCS vs PLC vs computer-display symbols
ISA 5.1 uses line styles through and around the instrument circle to encode where an instrument's signal is processed or displayed. Understanding this prevents misclassifying a DCS function block as a field-wired I/O point.
| Symbol | What it means | I/O implication |
|---|---|---|
| Circle, no line | Field-mounted instrument | Hardwired to PLC, DCS AI or DI |
| Circle with single horizontal line | Panel-board-mounted, primary control room | Hardwired to panel-mounted controller |
| Circle with double horizontal line | Panel-board, rear-of-panel, local panel | Hardwired to local panel |
| Circle with dashed horizontal line | DCS or shared-display device | Software tag. May or may not be hardwired field device |
| Hexagon | Computer function, software block | Pure software. No wired I/O point |
| Square | PLC function | Software tag in PLC program |
The dashed-line circle is the one that causes the most confusion. A TIC on a 2009-era drawing with a dashed line is a DCS function block. It represents PID logic running in a distributed control system, not a field-mounted temperature controller. The temperature transmitter, TT feeding it is the wired I/O point.
When extracting an I/O list from a P&ID, only field-mounted instruments and direct-hardwired panel instruments generate I/O rows. Hexagons and squares do not.
How loop numbers are assigned
ISA 5.1 does not mandate a loop-numbering format. Owner-operators define this in their house standard, and conventions vary significantly.
Area-based numbering. The most common approach. The loop number encodes an area or unit prefix followed by a sequential number. 101 is the first loop in area 1, 2034 is the 34th loop in area 20. Some facilities prefix the first letter of the unit name. C-101 for a compressor area loop.
Train or unit designation. Multi-train plants, LNG, ethylene crackers often include a train identifier. T1-FIT-2034 for Train 1. The train prefix prevents tag collision when the same loop appears on parallel trains.
Redundancy suffixes. When a loop has more than one instrument of the same type, a letter suffix distinguishes them. FIT-101A and FIT-101B are two flow transmitters in the same loop, often for voting or redundancy. This is distinct from FIT-101-1 and FIT-101-2, which identifies sub-elements of a single device.
See ISA letter codes for the full first-letter index. The instrument index master class guide covers how those loop numbers and tag structures feed into a complete instrument index across a multi-drawing plant.
Common dialects and regional variants
ISA 5.1 is the dominant convention in North America, and widely used in international projects. Several regional or industry-specific variants modify or extend it.
KKS, German power industry. The KKS classification system uses a multi-segment tag structure where the segment sequence encodes plant, system, function, and item. A typical KKS tag looks like 10LAC01CT001. 10, plant unit, L, system type. Electrical, AC, aggregate. Current, 01, first aggregate, C, function. Measurement, T, kind of item. Transmitter, 001, item number. The function letters partly overlap ISA letter codes but follow a different table. KKS is common in German and European power station work. See /standards for corpus coverage.
IEC 81346, European reference designations. Where ISA 5.1 names an instrument by what it measures and does, IEC 81346 designates an object by aspect, and marks each aspect with its own prefix character. = opens the function aspect, what the object does. - opens the product aspect, what the object is. + opens the location aspect, where it sits. A single item can therefore carry more than one designation depending on which aspect the drawing is describing. The letter codes that classify the object, the kenletters in Dutch practice, come from IEC 81346-2, and they do not map one to one onto ISA first letters. This is the standard behind RDS-PP in power generation and behind NEN-EN-IEC 81346 on Dutch installation drawings, so a European project set will often mix it with ISA tags on the same sheet. What IEC 81346 is and how to read a reference designation walks the aspects and the letter codes.
NORSOK I-005, Norwegian offshore. The Norwegian oil industry standard extends ISA 5.1 conventions and adds requirements for functional location numbering aligned with facility topology. Tag structure adds a functional location prefix ahead of the ISA tag. 21-FIT-0101 where 21 is the functional location code. NORSOK is essentially ISA-compatible at the letter-code level. The main difference is the mandatory location prefix and the specific alphanumeric loop-number format.
Operator-specific house standards. Major owner-operators, Shell, ExxonMobil, BP, ADNOC publish engineering specifications that constrain ISA 5.1 choices. Fixed area numbering ranges, mandatory signal classes for certain devices, specific loop-number formats, and defined suffixes for SIS versus BPCS instruments. These house standards are built on ISA 5.1 but are not identical to it. When extracting from a drawing set, the house standard's legend block is the authoritative reference.
Dutch industrial practice. Some Dutch-language P&IDs use local notation for certain instrument types, particularly in HVAC and industrial buildings. Tags like AB2WV05 follow a system-function-number format that looks nothing like ISA but encodes similar functional information. These are not errors. They are a different convention applied to the same drawing type.
When you encounter a tag that does not fit the ISA pattern, check the drawing legend block before assuming an error.
Common mistakes
T vs P confusion. These letters look similar at small font sizes on P&IDs. It is the most common transcription error in manual I/O list creation.
Truncated codes. Reading TIC, Temperature Indicating Controller, AO as TI, Temperature Indicator, Review. If the instrument connects to a control valve, it is a controller.
Split instruments. Some P&IDs show FE-101, flow element and FT-101, flow transmitter separately. These are one loop and should consolidate as FIT-101 in the I/O list.
FCV classification. Flow Control Valves are AO, analog output, not DO. They modulate position via 4-20mA, unlike XV valves which are strictly on, off.
DCS function blocks in the I/O list. A hexagon or dashed-line circle is a software block, not a field instrument. Including it in the I/O list inflates the I/O count and confuses the cabinet layout. For how those correctly classified tags flow into a complete I/O list, the I/O list creation guide covers column structure and review conventions.
Missing redundancy rows. FIT-101A and FIT-101B are two separate wired AI points. Both go in the I/O list. Collapsing them to one row underestimates card count.
Related
- P&ID to I/O list. Upload your drawings, get a classified I/O list back
- P&ID to TIA Portal. Direct Siemens tag import
- P&ID to Rockwell L5X. Direct Studio 5000 tag import
- ISA letter index. First-letter reference pages with examples per variable type
- Signal Class on the I/O List: How a Bubble Becomes AI, AO, DI, DO. The full classification chain from ISA letter to wired signal type
- PLC tag naming conventions. Turning the ISA tag into the PLC tag database
- Instrument index vs I/O list vs line list. Which document is which
- What changed in ISA 5.1. The 2022 and 2024 revisions, in ISA's words
- NORSOK Z-DP-002 explained. ISA-compatible Norwegian offshore tagging conventions
- KKS coding explained. The hierarchical power-plant tagging system used alongside ISA tags on international projects
- What is IEC 81346. Reference designations and kenletters, the European scheme you meet on RDS-PP and Dutch drawing sets
- Free I/O list Excel template. 14-column template with signal class dropdowns
- How to extract instruments from a P&ID PDF. Turning the tags you have just decoded into a working list
- ISA 5.1 cheat sheet. The first and succeeding letters on one printable page
- Instrument index. The master register every tag on the drawing belongs to
- P&ID symbol library. The circle, bubble, and valve shapes that carry these tags
- Instrument installation details and hook-up drawings. How each class of device is mounted and connected once the tag is settled
FAQ
How do I handle a tag where the instrument type is ambiguous from the letters alone.
Check the drawing symbol first. The physical symbol, circle type, line style tells you whether the device is field-mounted or a DCS function block. If the tag is on a field instrument, no line or a solid line through the circle, it is a wired I/O point. If it sits on a hexagon or has a dashed line, it is a software block. When the symbol is unclear, the drawing legend and the P&ID notes block are the next reference. If neither resolves the ambiguity, the instrument index, if one exists for the project will list the device type and signal type explicitly.
What is the difference between FT-101 and FIT-101 on the same drawing.
FT-101 is a flow transmitter with no local indicator. FIT-101 is a flow indicating transmitter. It has a local display in addition to the transmitted signal. Both are AI points in the I/O list. The distinction matters for procurement, FIT costs more but not for signal classification. Some facilities use FT everywhere and note the display requirement in the instrument data sheet rather than the tag.
How do I handle a tag where the same loop number appears on two drawings.
Loop numbers should be unique across an entire drawing set. If the same loop number appears on two P&IDs, one of three things is true. The drawings reference an off-page connector and the loop physically spans both drawings, correct and expected. The loop number was duplicated by error, a real problem for the I/O list. Or the drawings come from different units or trains where each unit uses its own numbering range, loop 101 exists in Area 1 and also in Area 2. In the last case, the area prefix in the tag normally distinguishes them. Flag any duplicate loop numbers without an area qualifier for review.
When should a PSV, pressure safety valve appear in the I/O list.
Usually it should not. A spring-set pressure safety valve is a purely mechanical device. It has no wired connection and generates no I/O point. It does appear on the P&ID for documentation, but the I/O list captures only wired instruments. The exception is a PSV with a pilot valve, position monitor, or rupture disc indicator that feeds a DI back to the control system. In that case, the monitoring switch, typically ZS or PSH goes in the I/O list. The PSV body does not.
What is the correct classification for a valve positioner.
A valve positioner, sometimes tagged ZT or IT in the position feedback loop receives a 4-20mA command signal, AO from the controller and returns a 4-20mA position feedback, AI. In the I/O list, the command channel is AO and the position feedback is AI. On a HART-capable positioner, the feedback may be read as a HART secondary variable on the same wiring pair as the command, so there may be only one wired AO channel with HART feedback, not a separate AI channel. Clarify this with the positioner specification before finalising the I/O count.