Why This Is a Genuine Architectural Decision, Not Just a Sizing Output
Once required I/O point counts, module counts, and chassis requirements are calculated, a further architectural decision remains: should all I/O be housed locally with the CPU (in the same or directly cabled expansion chassis), or should some or all I/O be distributed to remote racks located nearer the actual field devices, communicating back to the CPU over an industrial network? This decision has real cost and design implications extending well beyond the basic I/O sizing calculation itself.
What Local I/O Architecture Actually Means
Local I/O architecture houses I/O modules in the same physical location as the CPU — either directly in the CPU's own chassis or in expansion chassis physically cabled nearby using a direct backplane extension connection. This architecture requires every individual field device's wiring to run all the way back to this central location, which can mean substantial cable length and cable cost for field devices physically distant from the control room or main equipment room.
What Remote I/O Architecture Actually Means
Remote I/O architecture instead places I/O racks physically near clusters of field devices — in a field junction box, a local equipment room, or a motor control center closer to the actual process equipment — with these remote racks communicating back to the central CPU over an industrial Ethernet network (EtherNet/IP, PROFINET, Modbus TCP, or similar) rather than requiring individual field device wiring to run all the way to the central CPU location. Only a single network cable connects each remote rack back to the broader control network, rather than dozens or hundreds of individual field device cables.
Why Field Cable Cost Is Often the Deciding Factor
For a process with field devices geographically dispersed across a large physical area — a sprawling outdoor process unit, a large water treatment plant, a distributed manufacturing facility — the cable cost savings from remote I/O can be very substantial: instead of running many individual instrument cables over long distances back to a central location, only a single network cable per remote rack location is needed, with local, short field wiring runs from each remote rack to its nearby cluster of field devices. For a compact facility with field devices already located close to the control room, this cable-cost advantage shrinks or disappears, and local I/O's architectural simplicity becomes the more attractive default.
Why Remote I/O Adds Its Own Costs and Complexity
Remote I/O architecture is not simply "free" cable savings — each remote rack requires its own power supply (sized independently for that specific rack's I/O load, exactly as this site's PLC I/O Sizing Tool calculates on a per-rack basis), its own enclosure or cabinet, and connection to the broader industrial network, which itself requires network switches, network cable infrastructure, and ongoing network management and troubleshooting capability that a purely local I/O architecture does not require at all. This is a genuine infrastructure investment that has to be weighed against the field-cable savings remote I/O provides.
Why Network Reliability and Determinism Also Factor Into the Decision
Beyond raw cost tradeoffs, remote I/O introduces a network communication dependency between the CPU and each remote rack that local I/O, communicating directly over a backplane, does not have — network latency, potential communication interruption, and the need for appropriately robust, often redundant network infrastructure for critical applications are real design considerations remote I/O architecture has to address, particularly for time-sensitive or safety-relevant control applications where communication reliability and determinism matter significantly.
Why Real Systems Often Use a Hybrid Approach
Many real industrial control system designs use neither purely local nor purely remote I/O architecture exclusively, but a hybrid combining both — I/O for field devices clustered near the main control room or equipment room housed locally, while I/O for geographically distant process areas is distributed to remote racks specifically where the cable-cost and practical-installation benefits genuinely justify the added remote I/O infrastructure investment. This hybrid approach reflects that the local-versus-remote decision is best made area-by-area based on actual field device geography, not as a single all-or-nothing architectural choice for the entire facility.