Spare I/O.
Spare I/O is the proportion of input and output channels and cabinet capacity deliberately left unused when a control system is sized, so future instruments can be added without re-engineering the hardware. It is specified as a percentage of each signal type and as physical slot and cabinet space.
Three drawings included on a new account.
What Spare I/O means.
No control system is sized to fit only the instruments on the current drawings, because plants always grow. Spare I/O is the headroom built in on purpose. It is a percentage of unused channels on each signal type, plus unused slots and cabinet footprint, so the next transmitter or valve can be wired in without replacing cards or cabinets. It comes in two forms. Installed spare is channels that are already wired and terminated, ready to accept a field device with no hardware work. Space spare is empty card slots and cabinet space reserved for cards that have not been bought yet. The spare requirement rides directly on the I/O count. The live count taken from the drawings sets the working channels, and the spare percentage is added on top of each signal type to size the cards, the marshalling, and the cabinets. How much spare is carried is set by the owner's standard and applied to each signal type separately.
Why spare I/O exists
Plants change after start-up, and adding instruments is far cheaper if the control system was sized with headroom. Spare I/O lets a future device be connected without replacing cards or cabinets, and it gives management of change somewhere to land small additions without a hardware project.
Installed spare versus space spare
Installed spare is channels already wired and terminated, ready to take a field device immediately. Space spare is empty card slots and reserved cabinet footprint for cards not yet purchased. Most specifications call for some of each. Installed spare covers quick additions, and space spare covers larger future growth.
Sizing from the count plus spare
Card, marshalling, and cabinet sizing start from the live I/O count taken from the drawings, with the spare percentage added on top of each signal type. Because spare is a function of the count, an accurate count is the precondition for sizing the hardware correctly.
Typical spare requirements
Project specifications commonly call for around twenty percent spare on each signal type, with separate figures sometimes given for installed versus space spare. The exact requirement varies by owner standard and by how much future expansion the plant anticipates.
Worked example, sizing a cabinet from a signal count.
A unit counts 240 analogue inputs, 60 analogue outputs, 300 digital inputs and 100 digital outputs from the instrument index. Apply a 20 percent installed channel spare to each type and the design counts become 288, 72, 360 and 120. Now divide by the card density, which is a property of the chosen platform rather than anything the drawings say. At 16 channels per card that is 18 analogue input cards, 5 analogue output cards, 23 digital input cards and 8 digital output cards, rounding each one up because a part card does not exist. That is 54 cards. Apply a 20 percent card slot spare on top and the racks have to hold 65 slots, which at 16 slots per rack is 5 racks rather than the 4 the bare count would have suggested. The rack count is what sets the cabinet count, the power supply sizing and the heat load, so the spare policy decides the footprint long before it decides anything about signals. Two allowances are commonly missed at this point: terminal spare in the marshalling cabinet, which has to match the channel spare or the spare channels cannot be reached, and spare pairs in the field home runs, which cannot be added later without pulling another cable.
Spare capacity by level, and what each level actually buys
Typical allowances a project specification states. The percentage alone means nothing until the level it applies to is named, because installed spare and space spare cost different amounts to use.
| Level | What is counted | Typical allowance | What it takes to use one |
|---|---|---|---|
| Installed channel spare, per card type | Unused channels on cards that are already fitted and wired to terminals | 10 to 20 percent | A field cable and a configuration change |
| Card slot spare, per rack | Empty slots in a rack that is already fitted and powered | 10 to 20 percent | A card, terminals to land it on, and a field cable |
| Cabinet space spare | Free rail, terminal and cable entry space in the cabinet | 20 to 25 percent | A card, terminals, a cable and cabinet work |
| Marshalling terminal spare | Free terminals on installed strips | 20 percent | Cross wiring only, if there is a spare channel behind it |
| Junction box terminal spare | Free terminals in a field junction box | 20 percent | A field cable to the box, if a spare pair reaches the marshalling cabinet |
| Multicore spare cores or pairs | Unused pairs in an installed home run | 20 percent | Terminating an existing pair; without one, a new cable pull |
| Controller capacity | Scan time, memory and communications headroom | 30 to 50 percent | Configuration only, until the headroom runs out |
| Power supply capacity | Spare current on the cabinet supplies | 20 percent | Nothing, until it is exhausted and a supply has to be added |
Why the allowance differs by card type
A single flat percentage across every card type is the usual shortcut, and it is why the digital input cards fill first on almost every project.
| Card type | What tends to drive late additions | Practical effect on the allowance |
|---|---|---|
| Analogue input | Added measurements, and instruments split out of packages | The most commonly exhausted card type after the first year |
| Analogue output | Added control valves and variable speed drives | Fewer late additions than analogue input |
| Digital input | Status, limit switches and package alarms that arrive with vendor documents | Frequently under-estimated, because package signal counts arrive late |
| Digital output | Added solenoids, lamps and permissives | Moderate; usually tracks digital input |
| Safety rated I/O | Changes only through the safety lifecycle | Spare is held, but using it is never only a configuration change |
| Intrinsically safe I/O | Hazardous area instruments added after the area classification is issued | Constrained by barrier and terminal segregation as much as by channels |
| Fieldbus or digital bus | Devices added to an existing segment | Limited by segment power budget and spur count rather than by channel count |
Common questions
Is spare I/O calculated on used channels or total card capacity.
What is the difference between installed spare and space spare.
How does spare I/O affect cabinet sizing.
Is spare I/O specified per signal type.
Get the I/O list template.
Fourteen columns with the signal class as a dropdown, auto-filter on every column, frozen header. Plain .xlsx.