SIMATIC S7-1500 CPU Comparison Table: What Changes (and When It Matters)

Most “CPU comparisons” are just spec dumps. In real projects, the CPU choice is a bet on the next 18–36 months: Will the machine grow? Will you add more I/O racks? Will a customer demand OPC UA to a historian? Will somebody insist on a second network, “because IT said so”?

This page is a practical comparison of common S7-1500 Standard CPUs—with the columns that actually drive pain or peace during commissioning and maintenance. Use it as a quick decision aid, then confirm the exact order number / firmware generation in Siemens’ current documentation for your project.

Important note on specs:
Siemens has multiple hardware revisions and variants (PN vs PN/DP, plus C/F/T/R/H/MFP families). Some values (especially memory) vary by order number / generation. Treat the table below as a planning-level comparison, then verify the exact CPU you’re buying.

Browse SIMATIC S7-1500 (CPUs / I/O / Comms) I/O Planning (ET 200SP/ET 200MP) 24V Power Budgeting (Load Planning) Communication Planning

Think in “decision gates,” not model numbers. Most upgrades happen for I/O scale, comms load, or network separation—rarely for pure logic speed.

How to read this table (what actually matters)

Interfaces drive architecture. The jump from “one PROFINET” to “two PROFINET with separation” is often more important than a small speed bump. If you expect an IT/OT boundary discussion, read the interface columns first.
Work memory is where “it worked in the lab” turns into “why can’t we add one more feature?”. If your project will evolve, memory is usually the cheapest insurance you can buy.
Bit operation time is real, but it’s not the whole story. Communication, diagnostics, technology objects, logging, and OPC UA can dominate CPU load in modern systems.
Variant choice (PN vs PN/DP, C/F/T/MFP) can be more decisive than the performance tier. Don’t “accidentally” need PROFIBUS after you’ve already standardized on PN-only hardware.

S7-1500 Standard CPU comparison (planning-level)

CPU tier	Typical model(s)	Integrated comms (at-a-glance)	Work memory (ballpark)	Bit ops (typ.)	When it’s a good fit	Common “why we upgraded later” reasons
Entry / small-mid	CPU 1511-1 PN	1× PROFINET (IO RT/IRT)	~1–2 MB class	~tens of ns class	Compact machines, moderate I/O, limited comms partners, clean architectures (one cell network).	Added OPC UA + historian, more devices, more diagnostics/logging, or “just one more feature” pushed memory/CPU margin too tight.
Mid-range	CPU 1513-1 PN	1× PROFINET (IO RT/IRT)	~3 MB class	~tens of ns class	The “default safe pick” for many machine builders: enough headroom for typical expansions without overbuying.	Second network needed; comms load increased (many HMI/SCADA clients or multiple third-party services).
Mid-large	CPU 1515-2 PN	1× PROFINET IO RT/IRT + 1× additional PROFINET IO RT (separate interface)	~5–6 MB class	~single-digit ns class	You want separation: e.g., one PN for I/O/drives, one PN for HMI/SCADA/IT gateways. Great when you expect topology growth and “network politics.”	Needed PROFIBUS later; needed even more comms + devices; cell grew into a line with multiple subcells.
High / demanding comms	CPU 1516-3 PN/DP	PROFINET (multi-interface behavior) + PROFIBUS DP (integrated on PN/DP variants)	~9–10 MB class	~single-digit ns class	Plants with mixed generations: you need PROFIBUS, or your comms load is heavy (multiple networks, many partners, more integration).	Even heavier data/logging/IT integration; very large tag sets; very short reaction-time requirements.
High-end	CPU 1517-3 (PN/DP variants common)	Higher comms capacity tier (variant-dependent) Often used when the “communication tasks” column is the real driver	~10 MB+ class (variant-dependent)	~few ns class	When the PLC is not just controlling a machine, but acting like a hub: more clients, more services, more data exchange.	Required extreme performance, very short reaction times, or advanced open development / co-processing needs.
Top-end	CPU 1518-4 (PN/DP) / 1518-4 MFP	Highest tier + advanced options (variant-dependent) MFP adds C/C++ execution via ODK/Target 1500S	~tens of MB class (and up, variant-dependent) Note: some MFP memory is reserved for runtime libraries.	~1 ns class	High-performance lines, demanding comms, very short reaction times, or projects needing C/C++ blocks (MFP) for specialized control/compute.	Usually not upgraded from—this is where you land when you want “headroom without negotiations.”

Planning tip (quietly important):
The S7-1500 uses a SIMATIC Memory Card as program/load memory and it is required for operation. When you plan firmware updates (especially on higher-tier CPUs), choose a card size that won’t become a maintenance trap later.

The “real” upgrade triggers (what teams learn the hard way)

1) Communication grows faster than logic

Modern S7-1500 projects are rarely isolated. Once you add OPC UA, a historian, a second HMI, a gateway, and a handful of third-party devices, the PLC starts spending time managing connections, servicing clients, and moving data—not just scanning I/O. That’s when a comfortable CPU/memory margin stops being “nice to have” and becomes reliability.

2) Network separation becomes a requirement, not an option

Many projects begin with one flat network and end with “we need to separate machine I/O traffic from IT traffic.” CPUs with a second PROFINET interface (or architectures that support clean separation) are often chosen specifically to keep commissioning sane: one side stays deterministic for I/O/drives, the other side can host HMIs, SCADA clients, and service laptops.

Two-network separation is one of the most common “non-negotiable” late-stage requirements—plan for it early if you smell it coming.

3) “We’ll add it later” usually means memory later

Features like trace/debug tooling, richer diagnostics, recipe handling, logging, and big tag structures don’t feel large when written on a whiteboard. They feel large when you ship a machine and the customer asks for “just a few more signals,” “just a new report,” and “just one more device.” CPU memory headroom is cheap compared to a redesign.

OPC UA: yes, it’s real—and it changes CPU selection conversations

OPC UA is often the line between “controls owns it” and “IT is now in the room.” On S7-1500, the OPC UA server is available on CPUs as of firmware V2.0 (with details and feature growth across firmware versions), and access is via the CPU’s integrated Ethernet interfaces.

OPC UA is not just a protocol checkbox—it's a boundary decision (clients, security, certificates, and who owns the data model).

Quick picks (if you just need a starting point)

“Standard machine, likely to evolve”: start at 1513 if budget allows. It’s a common “safe default” that avoids early regret when scope grows.
“We need network separation / more comms headroom”: look hard at 1515 class options. The second interface is often the difference between a clean architecture and a future retrofit.
“Legacy PROFIBUS exists (or will exist)”: plan PN/DP variants early (often 1516 PN/DP class and up).
“Data hub + demanding integration + very short response times”: you’re in 1517/1518 territory.

Where this page fits in your S7-1500 selection workflow

Plan I/O first: S7-1500 I/O Planning (because the I/O structure dictates topology and comms reality).
Plan the 24V budget: S7-1500 Power Budgeting (because “it boots in the shop” is not the same as “it survives production transients”).
Plan comms: S7-1500 Communication Planning (because networks drive CPU pain more than people expect).
Then choose CPU with this table, and validate in current Siemens documentation for the exact order numbers you’re buying.

Key takeaways

CPU selection is a growth bet, not a spec exercise Interfaces & topology often matter more than raw speed Communication load becomes the limiter faster than logic Memory headroom prevents painful “just one more feature” moments Verify exact specs by order number / firmware generation

If you want, the next page (Compatibility Matrix) will turn this into a “no surprises” build plan: CPU ↔ rack/rail ↔ ET 200 family ↔ comms modules ↔ firmware/TIA portal constraints. That’s where selection stops being opinion and becomes a repeatable engineering process.