Instrumentation basics · 1.6
The instrument deliverables: index, datasheet, hook-up, loop diagram, I/O list
25 October 2026 · 10 min read · ISA 5.1, ISA 5.4, ISA 20, IEC 61987
In short
- Instrument engineering produces a family of documents. Every one is keyed on the tag number, and every one must agree with the others.
- The instrument index is the master list. Datasheets, hook-ups, the I/O list, cable schedules and loop diagrams are all built from it.
- They are produced in a fixed order, because each document needs data from the ones before it.
- Most late-project rework comes from one change that was not carried through to every document.
The P&ID says what instruments a plant needs. The instrument deliverables say exactly what to buy, where and how to install it, how to wire it and how to configure it. This article maps the main documents, what each holds, where its data comes from and who uses it.
Where this matters
- Procurement buys from datasheets. A wrong material or range means the wrong instrument arrives.
- Construction installs from hook-ups, location plans and cable schedules.
- Control system engineers configure from the I/O list and the instrument index.
- Commissioning checks loops against loop diagrams and datasheets.
- Operations and maintenance inherit the full set as the plant's permanent record.
How it works
The main deliverables
| Deliverable | What it holds | Main inputs | Main users |
|---|---|---|---|
| Instrument index (list) | One row per tag: type, service, line or equipment, P&ID, location, signal and system, and the status of every related document | P&IDs | Everyone, as the master |
| Instrument datasheets | Full specification of each instrument: process conditions, range, materials, connections, electrical, hazardous area, accessories | Index, process data, line list, piping classes | Procurement, vendors |
| Calculations | Flow element, control valve, thermowell and relief valve sizing | Process data, line data | Datasheets, vendors, reviewers |
| I/O list | Every signal into and out of the DCS, SIS and PLCs, with its type (AI, AO, DI, DO), range, units and system | Index, control narrative, cause and effect | Control system engineers |
| Hook-up drawings | Typical installation details: impulse lines, valves, fittings and supports, with a bill of materials | Datasheets, piping classes | Construction, material take-off |
| Instrument location plans | Where each instrument and junction box sits on the plot | Index, piping layouts | Construction, cable routing |
| Cable and junction box schedules | Every cable: from, to, type, cores, length, gland. Every junction box and its terminals. | Index, I/O list, location plans | Electrical construction |
| Loop diagrams | Each loop from field device to control system: every terminal, cable, core and junction box | I/O list, cable schedules, vendor drawings | Commissioning, maintenance |
| Cause and effect charts | Which inputs cause which trips and actions | Process safety studies, the SRS | SIS engineers, commissioning |
The order they are made in
P&ID → index → process data + calculations → datasheets → I/O list → hook-ups + locations → cables + JBs → loop diagrams
Work overlaps in practice, but the dependency order holds. A loop diagram cannot be finished before its cable numbers exist, and cables cannot be scheduled before the I/O and the locations are known. Each step is issued at a revision (for example IFR, then IFA, then IFC), and a change upstream re-opens every document downstream.
What the standards and practice say
| Deliverable | Common references |
|---|---|
| Tags and symbols | ISA 5.1, IEC 62424 |
| Datasheets | ISA 20 specification forms; IEC 61987 property lists (the basis of IEC 62683 product data) |
| Loop diagrams | ISA 5.4 |
| Binary logic and cause and effect | ISA 5.2, plus project formats |
| Safety requirements behind the trips | IEC 61511 safety requirements specification (SRS) |
| Hook-ups, cables and JBs | Company and project standards; IEC 60079-14 for hazardous-area installation |
Most operators impose their own templates and numbering. The engineering contractor's job is to deliver a consistent set in the client's format.
Worked example: one drum, traced through the deliverables
Take the compressor suction drum from article 1.3 and count what it adds to each document.
| Tag | Datasheet | I/O (system, type) | Hook-up | Cables |
|---|---|---|---|---|
| LIT-4101 | Yes | DCS AI | Level, DP type | 1 |
| LV-4101 | Yes (with sizing) | DCS AO | Valve, positioner air | 1 |
| LT-4102 | Yes | SIS AI | Level, DP type | 1 |
| PDIT-4103 | Yes | DCS AI | DP, gas service | 1 |
| TE/TT/TW-4105 | Yes (TW with calculation) | DCS AI | Thermowell, head-mounted transmitter | 1 |
| XV-4106 + XY | Yes | SIS DO | Valve, actuator air | 1 |
| ZSO / ZSC-4106 | On the XV datasheet | SIS DI × 2 | None | 1 (shared multicore) |
| PSV-4104 | Yes (with sizing) | None | None | 0 |
Totals for this drum: 7 datasheets (more if the TE, TT and TW get separate sheets), 3 sizing calculations (LV, TW, PSV), 8 I/O points (DCS: 3 AI and 1 AO; SIS: 1 AI, 1 DO and 2 DI), 5 hook-up types, 7 field cables and 5 loop diagrams (loops 4101, 4102, 4103, 4105 and 4106).
What this example shows
- One P&ID bubble can mean several document entries. XV-4106 brings a solenoid, two position switches and three I/O points.
- Counting early is easy. Tags, I/O and cables can be estimated directly from the index, long before any detail design.
- Every column is a dependency. If LT-4102 moved from the SIS to the DCS, the I/O list, cable schedule, JB schedule, loop diagram and cause and effect chart would all change.
When it fails: the fixes in order
- Make the index the single source. Every other document should take its tag list, service and location from the index, not from its own copy.
- Track the revision of each document against the index. Know which documents are out of date after each P&ID change.
- Freeze the inputs in order. Process data before datasheets, I/O before cables, cables before loop diagrams.
- Cross-check before each issue. Tags, ranges, I/O types and cable numbers should match across documents.
- Carry changes through every document. Use a change log that lists each document affected.
Common mistakes
- Keeping separate tag lists in the datasheets, the I/O list and the cable schedule, so they drift apart.
- Issuing datasheets before the process data and line data are firm.
- Forgetting accessory tags (solenoids, limit switches, thermowells) in the I/O and cable counts.
- Updating a range on the datasheet but not in the I/O list.
- Drawing loop diagrams before the cable and JB numbers are fixed.
- Treating the as-built update as optional. Maintenance relies on it for decades.
Checklist
- The instrument index exists and every other document takes its tags from it.
- Every tag on the P&IDs is in the index, and nothing in the index is missing from the P&IDs.
- Every instrument needing one has a datasheet, and every valve, flow element, thermowell and PSV has a calculation.
- The I/O list matches the index: signal type, system, range and units.
- Every field instrument has a hook-up and a location.
- Every I/O point has a cable, a JB terminal and a loop diagram.
- The cause and effect chart matches the SIS I/O.
- The revision status of every document is tracked against the latest P&ID revision.
- As-built updates are planned and resourced.
Quick reference
- Order: P&ID → index → data and calculations → datasheets → I/O → hook-ups and locations → cables and JBs → loop diagrams.
- Master: the instrument index.
- Count early: about 1 cable per field device, with one I/O for a transmitter and three for an on/off valve with two switches.
- Standards: ISA 5.1 (tags), ISA 20 and IEC 61987 (datasheets), ISA 5.4 (loops), IEC 61511 (SRS).
Further reading
- ISA 5.4, Instrument Loop Diagrams
- ISA 20, Specification Forms for Process Measurement and Control Instruments
- IEC 61987 series, Industrial-process measurement and control: data structures and elements in process equipment catalogues
- In this series: 1.1 What instrumentation does · 1.3 P&ID symbols and tags · later, Module 8 on control systems, marshalling and loop diagrams
The drum example is illustrative, and real counts depend on the project's typicals and wiring philosophy. ISA and IEC are named for identification only.