When to use: Building a PLC I/O point list station by station — every physical input/output, its tag name, signal type, and voltage/current level. Live digital vs. analog counts feed directly into panel/I/O card sizing, and duplicate tag names are flagged automatically.
The I/O list is the foundational document of any PLC-based control system — a complete inventory of every physical input and output each station's PLC needs to read or control, organized by device, tag name, signal type, and voltage/current level. This tool builds that list station by station, live-counts digital vs. analog points for downstream I/O card sizing, and flags duplicate tag names before they cause confusion during programming or commissioning.
A photoeye is a simple digital (on/off) input; a fill-level sensor or VFD speed reference is typically an analog (4-20mA or 0-10V) signal requiring a different I/O card. Getting the digital-vs-analog count right at the I/O list stage is what makes panel design — sizing the actual I/O card count and rack layout — an accurate exercise instead of a guess. Undercounting analog points at this stage is a common source of late panel design changes.
A well-built I/O list assigns each point a unique tag name following a consistent convention (station prefix + device type + sequence number) before any programming happens. That tag list is what SCADA screens and 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.
DI (digital input) and DO (digital output) are simple on/off signals — a photoeye or a motor start command. AI (analog input) and AO (analog output) carry a continuous value, typically 4-20mA or 0-10V — a level sensor reading or a VFD speed reference. Each type requires a different PLC I/O card.
A consistent convention — station prefix, device type abbreviation, and a sequence number (e.g. INF_PE_001 for the first infeed photoeye) — keeps the tag list readable across a multi-station project and prevents the same physical point from getting inconsistent names between the PLC program, the SCADA screen, and commissioning documentation.
The point counts (especially digital vs. analog) feed directly into panel design — selecting the actual I/O card models and count, sizing the enclosure, and estimating power supply and thermal load. A complete, accurate I/O list at this stage prevents late-stage panel design changes.
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