The PLC That Ran Out of I/O
We picked a PLC (a compact, 16 I/O) for a machine. On the bench, it worked. On the floor, we added 2 more sensors (a spare for a new feature). The PLC ran out of inputs (no room). We had to add an expansion module (more I/O). The problem: we sized the PLC for the known I/O (16) without spare. We needed 20 (16 + 4 spare). The mistake was not leaving spare I/O (for future changes, or spares).
PLC I/O counting sizes the I/O (with spares). This article covers the method.
Count the I/O
List every input and output:
- Inputs (sensors): Home (article 114), limit (article 114), part-present (article 108), safety (article 93). Count them.
- Outputs (valves/motors): Cylinder A (article 85), cylinder B, alarm light (article 98). Count them.
- Analog: Pressure (article 122), temperature. Count them (they need analog cards).
Step 1: The Count
For example: 12 inputs (sensors) + 8 outputs (valves) + 2 analog = 22 I/O. A compact PLC (16 I/O) is too small. Use a 32 I/O PLC (or a 16 + expansion).
Step 2: The Spare (20%)
Add a 20% spare (for future). For 22 I/O, the spare is 4 (20% of 22). Total: 26. Use a 32 I/O PLC (it has room). The spare is used for a new sensor, or a replacement (a broken I/O).
The I/O rule: Count the known I/O, then add 20% spare. The PLC that ran out had 16 I/O on a 12-input/8-output design (no spare). Size for the count + 20%. A 32 I/O PLC handles it. Don’t go tight (you’ll need the spare).
Step 3: The Card Types
- Digital (DI/DO): The sensors (PNP/NPN, article 181) and valves. Most I/O.
- Analog (AI/AO): The pressure (article 122) and temperature. Need an analog card (a few channels).
- High-speed (HSI): The encoder (article 166). Needs a high-speed input card (for the pulse counting).
Step 4: The Expansion
A compact PLC (16 I/O) can add expansion cards (more I/O). But expansion cards cost (and need space in the panel, article 168). Better to start bigger (32 I/O) than to expand later. For a big machine, use a modular PLC (rack + cards).
| Count | Sizing |
|---|---|
| 12 DI + 8 DO + 2 AI = 22 | 32 I/O PLC (with spare) |
| 20 DI + 12 DO + 4 AI = 36 | 40 I/O (or modular) |
| 50 DI + 30 DO = 80 | Modular rack (cards) |
An I/O Checklist
- List all inputs? (Sensors?)
- List all outputs? (Valves?)
- Are there analog? (Pressure?)
- Is there a high-speed? (Encoder?)
- Count the total? (I/O?)
- Add 20% spare? (For future?)
- Pick the right PLC? (Size?)
- Is there room in the panel? (Article 168?)
- Is the spare used? (Test?)
- Is the wiring documented? (Article 170?)
The Bottom Line
PLC I/O counting counts the known, then adds 20% spare. The PLC that ran out had no spare. Count the I/O. Add 20%. Pick the size. The PLC that had room wasn’t the smallest one — it had spare.
