I/O List to Wiring. Termination and Marshalling.

The I/O list and the termination schedule are related documents that often get treated as the same document. They are not. The I/O list says which channel a field instrument lives on. The termination schedule says which wire carries the signal from the field terminal to that channel. Both must be correct and must agree with each other, but they are produced at different times, owned by different people on many projects, and serve different purposes during construction and commissioning.

Understanding where the boundary between them falls matters because that boundary is also where rework concentrates when a project goes wrong.

The chain from field instrument to PLC channel

Every wired instrument on a P&ID travels a physical path to the control system. Starting at the field device and working back.

1. The field instrument, say, a pressure transmitter with a 4-20 mA output connects via a field cable to a junction box or directly to the marshalling cabinet.
2. In a two-tier architecture, the field cable lands on a terminal block inside the marshalling cabinet. A short inter-panel cable then carries the signal to the system cabinet.
3. In the system cabinet, the signal arrives at the I/O card terminal. Each channel on the card corresponds to one instrument.

The I/O list owns the right-hand end of this chain. The controller identity, the rack, the slot, and the channel number. The termination schedule owns the left-hand end and the middle. The field cable, the marshalling terminal, and the cross-connect to the card terminal. The two documents share the tag number and the card terminal as their linking keys.

When the I/O list assigns PT-101 to PLC-1, Rack 1, Slot 3, Channel 4, that assignment is what the termination schedule needs to write the card-terminal reference in its last column. Every upstream column in the termination schedule, cable ID, core number, marshalling terminal, is also derived indirectly from the I/O list through the signal class and the field location the I/O list records.

Home runs vs multicore

A home-run cable serves one instrument. Two conductors, or one twisted pair from the field terminal to the marshalling cabinet. A multicore cable serves a cluster of instruments in the same field area. 10, 20, or more pairs in a single sheathed cable, each pair dedicated to one instrument.

The choice between them comes down to instrument density, distance, signal type, and segregation rules.

Density and distance. A cluster of eight instruments within 30 metres of the marshalling cabinet is a strong multicore candidate. The cable tray fill is lower, the number of cable entries into the cabinet is lower, and the installation cost per instrument drops. A single instrument 200 metres from the cabinet in a different process area justifies its own home run. A multicore serving only one distant instrument is wasteful, and any future instruments in that area will not necessarily be in the same signal class or hazardous-area zone.

Signal segregation. IS, Intrinsically Safe circuits certified for Zone 1 or Zone 2 areas must be physically segregated from non-IS circuits. IS and non-IS signals must not share a multicore, must not share a conduit, and must land on separate terminal strips in the marshalling cabinet. This rule tends to fragment the multicore groupings, because a cluster of instruments may contain a mix of IS pressure transmitters and non-IS thermocouple extensions. You end up with two multicores serving the same area. One IS, one non-IS.

Signal type. Mixing signal types within a multicore needs care. A DC power cable run alongside a 4-20 mA analogue pair will induce noise if the cable is long or the shield treatment is inconsistent. Most house standards prohibit combining signal and power in the same multicore. The practical result is that a row of DO channels driving solenoids on a fused 24 V DC supply typically gets its own cable, separate from the AI transmitter loops in the same junction box.

Documenting the multicore groupings on the I/O list, even in a simple notes column, is useful. It tells the cable-schedule engineer which tags to bundle and prevents the field crew from routing IS and non-IS instruments into the same conduit.

Marshalling cabinet vs system cabinet

A marshalling cabinet is a cross-connect panel. Field cables arrive on one side, land on terminal blocks, and leave on the other side via a discrete inter-panel harness toward the system cabinet. The key property of the marshalling cabinet is that it provides a documented, labeled disconnect point for every field circuit, independent of the I/O card terminals.

A system cabinet holds the control system hardware. The PLC or DCS chassis, the CPU modules, the power supply modules, and the I/O cards. I/O card terminals face the back of the cabinet and accept wiring from the inter-panel harnesses coming from marshalling.

The separation exists because field modifications are routine over a plant's life. When a pressure transmitter needs to be moved to a new nozzle, the field cable gets re-routed, the marshalling terminal gets updated, and the rest of the system is untouched. Without a marshalling layer, that modification reaches all the way to the card terminal, which may mean partially de-wiring an I/O card that is still live on other channels.

The marshalling cabinet sizing article covers the terminal-density and zoning rules that determine how many marshalling cabinets a given I/O count requires. What matters here is how the I/O list's rack-slot-channel columns map onto card terminals. Each card has a fixed channel count, 16 AI channels on a typical analogue input card, for example. Each channel has a labelled terminal on the card face. The termination schedule fills in that terminal reference by knowing which card is in which slot and what the vendor's terminal numbering convention is for that card family.

A Siemens S7-1500 analogue card and a Rockwell Allen-Bradley FLEX I/O analogue card use different terminal designations for the same physical channel. The I/O list tells you which PLC platform and which slot. The card data sheet tells you which terminal label that maps to. The termination schedule carries the result.

What the termination schedule needs from the I/O list

The minimum set the termination schedule requires from the I/O list, per row, is.

• Tag number, the linking key
• Signal class, AI, AO, DI, DO determines wire count, IS requirement, and power arrangement
• Controller assignment, PLC-1 vs SIS-1, for example
• Rack, slot, and channel number, resolves to a specific card terminal

What the termination schedule adds, and cannot derive from the I/O list alone.

• Cable identifier, assigned by the electrical engineer in the cable schedule, referenced here
• Core or pair number, which conductor pair within the multicore serves this instrument
• Marshalling terminal block identifier and terminal number
• System cabinet card terminal, from the slot and channel via the card terminal convention
• Fused or unfused field power, and if fused, which fuse group and rating
• Screen and drain treatment, screened pair drained at one end, shield continuity, etc.

The cable schedule guide covers the electrical engineer's side of this. Cable routing, sizing, and the cable block diagram. The termination schedule borrows the cable identifier from that document. Any mismatch between the cable identifier in the termination schedule and the cable identifier in the cable schedule is a discrepancy that must be resolved before loop checking starts.

Fused vs unfused field power and IS segregation

Fused field power for DO channels. A digital output channel drives a solenoid valve, a motor contactor coil, or a lamp. The 24 V DC field supply that energises the solenoid passes through a fuse before it reaches the terminal block. The fuse protects the wiring and limits the fault current if the solenoid coil fails. On the termination schedule, each DO row carries a fuse group identifier, F-GRP-01, F-GRP-02, and so on and a fuse rating, typically 1 A or 2 A for solenoid loads. The grouping logic matters. One fuse per solenoid is cleanest and most maintainable. Grouping several solenoids on one fuse saves hardware but means a blown fuse silences multiple outputs.

AI loop power. A 4-20 mA loop can be powered from the card, the card supplies 24 V DC loop excitation, and the transmitter draws from it or from an external 24 V DC supply distributed at the marshalling cabinet. The choice depends on the card type. Cards with loop power provided per channel show "card powered" on the termination schedule. Externally powered loops show the distribution terminal where the field supply originates. The termination schedule must be explicit about this because the commissioning tech needs to know which supply to test before loop-checking.

IS segregation. Instruments in hazardous areas classified Zone 1 IIB T4 or Zone 2 IIC T3 require Intrinsically Safe circuits. IS circuits run through a Zener barrier or galvanic isolator, mounted in a safe-area cabinet, typically at the marshalling level before they reach the card. The IS barrier sits between the field terminal and the card terminal. The termination schedule must show it. IS circuits must also land on separate terminal strips from non-IS circuits in the same cabinet, and they must be wired in separately sheathed cables or conduit from the field.

The I/O list's signal class column is the trigger for IS designation. Wherever the field instrument is in a hazardous area, the termination schedule must route through the appropriate barrier. This is why the I/O list needs a hazardous-area classification column in addition to signal class. Without it, the termination schedule engineer cannot know which AI channels need IS treatment.

Worked walkthrough. A small instrument group on PID-001 and PID-002

Take five instruments from two P&IDs. PT-101, AI, 4-20 mA, Zone 2 IIC, FT-102, AI, 4-20 mA, non-IS, XV-301, DO, on, off solenoid, non-IS, FCV-302, AO, 4-20 mA positioner, non-IS, and a position switch ZSO-501, DI, dry contact, non-IS. All are assigned to PLC-1.

**Home run or multicore. ** PT-101 is in a Zone 2 area and must be IS. It gets a dedicated IS home-run cable, one twisted shielded pair, coloured to the IS convention used on the project. The other four instruments are in general-purpose areas within 25 metres of the marshalling cabinet. FT-102 and FCV-302 are both analogue signals and can share a multicore, say, a 4-pair overall-screened cable designated MC-001. XV-301 and ZSO-501 are a DO and a DI. They can share a multicore for discrete signals, MC-002, provided the 24 V DC field supply for the DO is carried in the same cable or run alongside it.

Marshalling layout. In the marshalling cabinet, PT-101 lands on IS terminal strip TS-IS-01, separate from everything else. FT-102 arrives on TS-AI-01 via MC-001 pair 1. FCV-302 arrives on TS-AO-01 via MC-001 pair 2. XV-301 arrives on TS-DO-01 via MC-002. ZSO-501 arrives on TS-DI-01 via MC-002. IS and non-IS strips are on opposite sides of a marked segregation barrier.

Card terminal resolution. PLC-1, Rack 1, Slot 3 holds a 16-channel AI card. PT-101 is channel 1, IS barrier output to card terminal A1 plus, A1-. FT-102 is channel 2, card terminal A2 plus, A2-. FCV-302 is on Rack 1, Slot 5, AO card, channel 1, terminal B1 plus, B1-. XV-301 is on Rack 1, Slot 7, DO card, channel 3. Its field power comes from fuse group F-GRP-01 at 24 VDC, 2 A. ZSO-501 is on Rack 1, Slot 6, DI card, channel 4.

Termination schedule rows. PT-101 gets one row. Cable HR-PT101, home run, 1x2, pair 1, terminal TS-IS-01-01, IS barrier output to card A1 plus, A1-, IS, screened, drain at marshalling end. FT-102. Cable MC-001, pair 1, terminal TS-AI-01-01, card A2 plus, A2-, non-IS, screened overall, drain at marshalling end. FCV-302. Cable MC-001, pair 2, terminal TS-AO-01-01, card B1 plus, B1-, non-IS. XV-301. Cable MC-002, core 1, terminal TS-DO-01-01, card channel 3, fused F-GRP-01 at 2 A. ZSO-501. Cable MC-002, core 2, terminal TS-DI-01-01, card channel 4, no field power.

Five rows, but notice that if the I/O list had not confirmed the rack-slot-channel assignments before this work started, the card terminal column would be blank or provisional, and the whole schedule would be speculative.

What goes wrong

Termination schedule built before the I/O list is frozen. This is the most common source of rework. If the controls engineer starts the termination schedule against preliminary I/O assignments and those assignments change, a rack gets added, channels are reallocated to balance card loading, every affected row has to be re-done. The field crew wires to the issued termination schedule. If the schedule changes after termination is complete, the field crew must re-terminate. Even a partial freeze of the I/O assignment by area, issued as an interim IFC, is better than working against a moving target.

IS and non-IS sharing a multicore. This is a safety and regulatory issue, not just a tidy-wiring preference. Discovered during construction, it means cutting and re-routing cables. Discovered during a safety audit, it means a formal non-conformance and potential delay to commissioning. The I/O list's hazardous-area column, cross-checked against the multicore groupings at the start of the termination schedule, prevents it.

Missing core or pair IDs so the field crew guesses. A termination schedule that shows the cable identifier but leaves the core or pair number blank forces the field crew to count conductors and assume. On a 20-pair multicore with faded print marks this produces wiring errors that show up as reversed signals or floating channels during loop check. Fill in the core and pair numbers before issuing for construction.

The loop folder and the termination schedule disagree. A loop drawing shows the cable identifier as HR-FT102. The termination schedule shows MC-001. One document was not updated when the design changed from home run to multicore. The field tech holding both documents is now deciding which one is correct. A cross-reference check before issue, comparing every cable identifier in the termination schedule against the corresponding loop drawing, catches these discrepancies at a desk instead of at a junction box. See the cable schedule reading guide for the cable attributes the two documents must agree on.

Fused power grouping not carried through from the I/O list. The I/O list's DO rows might carry a note "fused field power, 24 VDC" but not specify fuse groups. The termination schedule engineer assigns all DO channels to a single fuse group because nothing told them otherwise. A solenoid failure that blows the fuse now takes down a dozen outputs. The grouping decision belongs in the I/O list, or at minimum in a power distribution note attached to it, so the termination schedule can carry it faithfully into construction.

From I/O list to field-ready package. The short version

The I/O list finalises the channel assignment. The termination schedule turns that channel assignment into a wiring instruction. The two documents are produced in sequence, not in parallel, and the termination schedule cannot be correct until the I/O list is frozen.

The columns the I/O list owes the termination schedule are. Tag, signal class, hazardous-area classification, and rack-slot-channel. Every other detail in the termination schedule, cable ID, core number, marshalling terminal, card terminal, fused power grouping, IS status, flows from those four. Get them right and frozen early. The rest of the termination package follows from them.

For a deeper look at how marshalling cabinet terminal counts and IS zoning affect the panel scope, see Marshalling cabinet sizing from an I/O list. For the I/O list structure that feeds this process, the I/O list creation guide covers the column set from scratch. If you need a refresher on what the cable schedule contributes to the same chain, reading cable schedules covers the electrical engineer's document in detail.

Tagsight and the instrument list that starts this chain

Tagsight extracts the tagged instrument list with signal class, loop number, and field-location data from P&ID drawings and produces a structured export in Excel or CSV format. The result is the starting document for the termination package. Every row carries the tag and signal class the termination schedule needs, and tags that need verification are checked before the data moves downstream. From that base the controls engineer adds the channel assignments and the termination schedule engineer builds the wiring instructions. The output formats include the column structure that feeds PLC programming tools directly, so the same frozen I/O list that drives the termination schedule also seeds the tag database. Start a workspace to see the extraction and export workflow, or read the I/O list creation guide first.