PLC I/O Count and Module Calculator
Count PLC I/O from device types and quantities. The sheet totals AI, AO, DI and DO, adds spares and calculates how many modules are needed for each signal type.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 20 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Device types
| Device type | Signals | AI each | AO each | DI each | DO each | Quantity | Total points | Remarks |
|---|---|---|---|---|---|---|---|---|
| Pressure transmitter | AI | 1 | 0 | 0 | 0 | 24 | 24 | |
| Temperature transmitter | AI | 1 | 0 | 0 | 0 | 18 | 18 | |
| Control valve | AO | 0 | 1 | 0 | 0 | 12 | 12 | |
| On off valve (2 limit switches + solenoid) | DI/DO | 0 | 0 | 2 | 1 | 10 | 30 | |
| Motor (run, trip, start, stop) | DI/DO | 0 | 0 | 2 | 2 | 8 | 32 | |
| Level switch | DI | 0 | 0 | 1 | 0 | 6 | 6 |
About this Calculator
Early I/O counts set PLC hardware cost and cabinet size. Counting by device typicals is fast, consistent and easy to update as the design changes.
Basis: Totals = Σ quantity × points per device. Modules = ROUNDUP(total × (1 + spare) ÷ channels per module).
How to use
- Set spare % and channels per module for your PLC family.
- List device types with their AI, AO, DI and DO per device.
- Enter quantities from the instrument index and motor list.
- Read totals, totals with spare and modules required.
- Transfer to the IO list template for detailed allocation.
Formulas used
- Total = Σ quantity × points.
- With spare = ROUNDUP(total × (1 + spare)).
- Modules = ROUNDUP(with spare ÷ channels).
Related tools
Frequently Asked Questions
How much spare?
Commonly 10 to 20 % installed spare plus space for future modules.
What is a typical?
A standard set of I/O for a device type, such as two limit switches and one solenoid for an on off valve.
Should HART count as extra I/O?
No, HART uses the same AI channel, but needs HART capable cards.
Learn more on the blog
IO list: 10 essential columns for a flawless DCS design
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article