The project: a packaging line automation retrofit for a mid-size food/beverage manufacturing plant — 3 PLC-controlled stations (infeed/accumulation, filling/capping, case packer), networked to a plant-floor SCADA system. Every downstream step in this series is built against the I/O list this step produces.
The I/O list is the foundational document of any PLC-based control system: a complete inventory of every physical input (sensors, switches, encoders) and output (motors, valves, indicator lights) each station's PLC needs to read or control, organized by device, signal type (digital or analog), and voltage/current level. For this 3-station line, the I/O list is built station by station: the infeed/accumulation station needs product-present photoeyes, an accumulation-zone encoder, and conveyor motor start/stop/speed control; the filling/capping station needs fill-level sensors, cap-torque feedback, a servo-driven capper, and product-in-position photoeyes; the case packer needs case-present sensors, a pick-and-place robot's I/O interface, and reject-bin-full detection.
Digital vs. analog signal type matters at this stage because it drives the PLC's I/O card selection later: a photoeye is a simple digital (on/off) input, while a fill-level sensor or a variable-frequency drive's speed reference is typically an analog (4-20mA or 0-10V) signal requiring an analog input/output card, not a digital one. Getting the digital-vs-analog count right at the I/O list stage is what makes Step 2's panel design — sizing the actual I/O card count and rack layout — an accurate exercise instead of a guess.
A well-built I/O list also assigns each point a unique tag name (following a consistent naming convention across all 3 stations — e.g. station prefix + device type + sequence number) before any programming happens. That tag list is what Step 4's SCADA screens and Step 6's FAT/SAT testing both reference directly — a photoeye tagged inconsistently between the PLC program and the SCADA screen is exactly the kind of small early mistake that turns into hours of confusion during commissioning.