GGSTech ToolsI/O count calculator
Build the I/O list before buying hardware
| Tag | Device | Signal | Fail state |
|---|---|---|---|
| LSL-101 | Low-level switch | DI 24 VDC | Define whether loss of signal means alarm/trip. |
| PT-101 | Pressure transmitter | AI 4–20 mA | Define underrange/overrange behavior. |
| XV-101 | Solenoid valve | DO via relay | De-energize-open or de-energize-close depends on process risk. |
What you should understand first
Channel count alone is not enough. Every point needs signal type, electrical characteristics, isolation, fail philosophy, terminal assignment and diagnostic expectation.
Signal and energy boundaries
A PLC is a controller, not the power source for a motor. Separate the low-energy logic side from the field power circuit. Digital outputs often command an interposing relay, contactor coil, valve solenoid or VFD input. The motor power still passes through the MCC/MCCB/contactor/VFD power circuit.
Fail-safe thinking
For critical shutdowns, do not rely only on software logic. Emergency-stop, trip and safety functions may require dedicated safety-rated hardware and a risk assessment. A normal PLC output should not be assumed to provide a safety function unless the architecture is specifically designed and certified for it.
Common mistakes
- Counting only present I/O with zero spare
- Mixing 24 VDC and 230 VAC points on unsuitable modules
- Ignoring high-speed inputs or pulse signals
- No allowance for relay isolation or intrinsic safety barriers
- Assuming all analog modules accept every sensor type
Commissioning checklist
- Freeze a tag-based I/O list.
- Separate DI/DO/AI/AO and special-function channels.
- Add spare channels and spare terminal blocks.
- Check module commoning and isolation groups.
- Confirm power-supply capacity and heat dissipation.
Frequently asked questions
How much spare I/O should I keep?
There is no universal percentage. 10–20% is common in preliminary planning, but project standards and expansion expectations should govern.
Can RTD connect to a normal 4–20 mA AI?
Not directly unless the input module supports RTD. Often an RTD transmitter converts temperature to 4–20 mA.
Do safety I/O channels count separately?
Yes. Safety-rated I/O must be selected according to the safety system architecture and certification requirements.