Siemens S7 CPU Comparison: S7-1200 vs S7-1500 vs S7-300 vs S7-400
Selecting a Siemens SIMATIC S7 CPU is the first architectural decision in any PLC project, and it constrains everything downstream: rack sizing, distributed I/O topology, communication partners, motion capability, and the engineering toolchain. This reference consolidates the key parameters that differentiate the S7-1200, S7-1500, S7-300, and S7-400 families so that a controls engineer can build a one-page comparison matrix for project kickoff and produce a defensible BOM without re-reading every datasheet.
The methodology below is built around the official SIMATIC comparison list for S7-300 / S7-400 / S7-1200 / S7-1500 (Siemens Support entry 109778377), the SIMATIC S7-1500 / ET 200MP "in a nutshell" presentation, and the S7-1200 / S7-1500 system manuals published on the Siemens Industry Online Support portal.
1. Family Positioning and Application Class
Siemens organizes SIMATIC S7 controllers into three tiers. The tier you choose determines the ceiling on memory, I/O, and integrated communication, as well as the engineering license required.
| Family | Positioning | Typical Use | Status (2024-2025) |
|---|---|---|---|
| S7-1200 | Basic controller | Standalone machines, OEM skids, cost-sensitive serial machines | Active, current product |
| S7-1500 | Advanced controller | Standard-to-high-end machines, plant-level automation, motion | Active, current product |
| S7-300 | Mid-range modular (legacy) | Existing installations, spares, brownfield expansion | Phase-out / spare-parts |
| S7-400 | High-end (legacy) | Large process plants, high-availability redundant systems | Phase-out |
For a brand-new high-pressure unit with a low-cost target and a peer-to-peer data link to a second PLC, the S7-1200 and S7-1500 families are the relevant candidates. The S7-300/S7-400 only enter the picture when matching an existing fleet or a specific redundant requirement (S7-400H / S7-400F/FH).
2. Selection Methodology: The Seven Parameters That Matter
Datasheets list 60+ fields, but only a small subset drives a controller decision for a green-field project. Capture these seven in the comparison matrix and leave the rest for the BOM stage.
- Work memory (code + data) – determines program size ceiling and recipe/data buffer size.
- Bit instruction execution time – sets the scan budget; ~0.03 µs (S7-1500) vs ~0.1 µs (S7-1200) vs ~0.05 µs (S7-300) vs ~0.018 µs (S7-400).
- Maximum number of central / distributed I/O modules – defines the local rack size and ET 200SP / ET 200MP density.
- Integrated PROFINET / PROFIBUS interfaces – count, port count, switch capability, MRP support.
- Programming environment and license – STEP 7 Basic (TIA) vs STEP 7 Professional (TIA) vs classic STEP 7 V5.x.
- Technology / motion functions – PID, high-speed counters, PTO/PWM, camming, kinematics.
- Security and diagnostics – know-how protection, access levels, integrated web server, trace, system diagnostics.
3. S7-1200 (Basic Controller) – Specifications
The S7-1200 is a compact, single-rack controller with the CPU, signal board slot, and up to 8 signal modules (SM) in a fixed local configuration. The signal board sits on top of the CPU and adds a few extra digital or analog channels or a communication module (RS-485, RS-232, BB) without consuming an SM slot.
| Parameter | S7-1200 range |
|---|---|
| CPU models | CPU 1211C, 1212C, 1214C, 1215C, 1217C |
| Work memory (code) | 50 KB (1211C) up to 250 KB (1217C) |
| Work memory (data) | 50 KB up to 250 KB |
| Bit instruction time | 0.08 µs – 0.1 µs typical |
| Local SM slots (signal modules) | 1 (1211C), 1 (1212C), 8 (1214C), 8 (1215C), 8 (1217C) |
| Signal board slot | 1 on every CPU (BB or CB) |
| Communication module (CM) / CP | up to 3 slots in 1214C/1215C/1217C |
| Integrated PROFINET | 1× PROFINET interface (2-port switch on 1215C/1217C) |
| PROFIBUS | Optional via CM 1243-5 (master) or CP 1242-5 (slave) |
| Program / data retention | Yes, retentive bit memory / data blocks |
| Web server | Yes, standard pages + custom |
| Engineering | STEP 7 Basic V16+ in TIA Portal (bundled with CPU purchase) |
The S7-1200 covers low-end PLC duty with integrated motion primitives (PTO up to 1 MHz on 1217C, 100 kHz on most), up to 6 high-speed counters, and PID_Compact for closed-loop control. For a high-pressure unit with a few proportional valves, RTDs, pressure transducers, and one peer-PLC link, a CPU 1215C or 1217C is usually the right size.
S7-1200 programming environment
- STEP 7 Basic V18 / V19 in TIA Portal.
- LAD, FBD, SCL (SCL is included in STEP 7 Basic since V14).
- No Graph (S7-GRAPH), no CFC. These require STEP 7 Professional.
4. S7-1500 (Advanced Controller) – Specifications
The S7-1500 is the strategic replacement for both S7-300 and S7-400 in new applications. It uses a U-shaped DIN rail with snap-in modules, a separate system power supply (PS), and either an integrated or plug-in display for diagnostics.
| Parameter | S7-1500 Standard CPU range |
|---|---|
| CPU models (Standard) | CPU 1511-1 PN, 1513-1 PN, 1515-2 PN, 1516-3 PN/DP, 1517-3 PN/DP, 1518-4 PN/DP, 1518-4 PN/DP MFP |
| Work memory (code) | 150 KB (1511) → 6 MB (1518 MFP) |
| Work memory (data) | 1 MB → 60 MB (incl. optimized block support) |
| Bit instruction time | 0.06 µs (1511) down to 0.001 µs equivalent on 1518 (SIMATIC S7-1500 catalog values) |
| Central rack modules (incl. PS + CPU) | up to 32 modules on the mounting rail |
| Distributed I/O via PROFINET | up to 256 IO devices / 1024 submodules typical (CPU dependent) |
| PROFINET ports (integrated) | 1 (1511/1513), 2 (1515), 2+1 (1516/1517), 2+1+1 (1518) |
| PROFIBUS | On 1516-3 PN/DP, 1517-3 PN/DP, 1518-4 PN/DP only (DP master if configured) |
| Technology objects | Speed-controlled axes, positioning axes, synchronizing axes, cam, camming (count varies per CPU, 1517/1518 support 128+) |
| OPC UA server | Yes (companion specification, configurable from TIA) |
| Web server | Yes, with custom user pages and standard diagnostic views |
| Engineering | STEP 7 Professional V18+ in TIA Portal (licensed separately) |
S7-1500 fail-safe and redundant options
- S7-1500F – integrated fail-safe CPUs (1515F, 1516F, 1517F, 1518F) certified to IEC 61508 SIL 3 / ISO 13849 PL e for safety functions in the same program as standard logic.
- S7-1500R/H – redundant CPUs (1515R, 1517H) for high-availability process applications with bumpless switchover.
- S7-1500 TM – technology module variants (count, cam, SSI) for high-speed tasks that exceed CPU onboard performance.
5. S7-300 (Legacy Mid-Range) – Specifications
The S7-300 uses a vertical backplane bus rack with slots numbered left-to-right. The power supply (PS) occupies slots 1, the CPU slot 2 (rack 0), and the IM (interface module) on rack 0 occupies slot 3 if expansion racks are used.
| Parameter | S7-300 range |
|---|---|
| CPU models | CPU 312, 314, 315-2 DP, 315-2 PN/DP, 317-2 DP, 317-2 PN/DP, 319-3 PN/DP, plus F variants |
| Work memory (code + data) | 32 KB (312) → 2 MB (319-3 PN/DP) |
| Bit instruction time | 0.1 µs (312) → 0.01 µs (319) |
| Central rack I/O module slots | 8 SM/FM/CP slots on a single central rack (typical 0-1-2-3-4-5-6-7 arrangement; CPU and PS use their own slots) |
| Expansion racks | up to 3 (IM 365 / IM 360/361) |
| Distributed I/O | PROFIBUS-DP up to 125 slaves (or PROFINET via 315-2 PN/DP, 317-2 PN/DP, 319-3 PN/DP) |
| PROFIBUS / PROFINET | CPU-dependent; F-CP variants for PROFIsafe |
| Engineering | STEP 7 V5.5 / V5.6 (classic) or TIA Portal (limited device library) |
6. S7-400 (Legacy High-End) – Specifications
The S7-400 is the high-end SIMATIC for process plants. Its rack is larger (UR1 = 18 slots, UR2 = 9 slots, CR2 / CR3 = compact), and the CPU occupies a specific fixed slot per rack. The power supply is a 5 A, 10 A, or 20 A module sized to the rack's expected load.
| Parameter | S7-400 range |
|---|---|
| CPU models | CPU 412-1, 412-2, 414-2, 414-3, 416-2, 416-3, 417-4; plus F/H variants |
| Work memory (code + data) | 512 KB (412-1) → 30 MB (417-4) |
| Bit instruction time | 0.075 µs (412-1) → 0.018 µs (417-4) |
| Central rack (UR1, 18 slots) | PS + CPU + 16 I/O / FM / CP modules (slot count varies by CPU) |
| Expansion racks | up to 21 (mixed ER1 / ER2, requires IM 460 / IM 461 receivers) |
| Distributed I/O | PROFIBUS-DP up to 124 slaves; PROFINET via CP 443-1 |
| Redundancy (S7-400H) | Yes (CPU 412H, 414H, 417H); hot-standby with fiber sync modules |
| Engineering | STEP 7 V5.6 (classic) – S7-400 not supported in current TIA Portal versions |
The S7-400 remains in plants for the same reason the S7-300 does: installed base, spare parts, and tools. New builds should target S7-1500 or S7-1500H unless there is a contractual, certification, or migration-blocked reason to remain on S7-400.
7. I/O Expansion Architecture: Central Rack vs Expansion Rack vs Distributed I/O
This is the parameter most often confused in green-field design. The S7-300 / S7-400 / S7-1500 all have a central rack defined by the backplane. They differ in how many SM/FM/CP slots that rack exposes, how many additional expansion racks can be chained through an interface module, and how much distributed I/O can hang off PROFINET / PROFIBUS.
| Topology | S7-1200 | S7-1500 | S7-300 | S7-400 |
|---|---|---|---|---|
| Central rack SM/FM/CP slots | 1 / 1 / 8 / 8 / 8 (CPU-dependent) | ≤ 30 (out of 32 total incl. PS + CPU) | 8 | 16 (UR1) or 7 (UR2) excl. PS + CPU |
| Expansion racks (IM-based) | Not supported | Not supported (use ET 200MP instead) | ≤ 3 | ≤ 21 (mixed ER1/ER2) |
| Distributed I/O over PROFINET | ≤ 16 IO devices typical (CPU dependent) | ≤ 256 IO devices (CPU dependent) | Via 315-2 PN/DP, 317-2 PN/DP, 319-3 PN/DP | Via CP 443-1 Advanced |
| Distributed I/O over PROFIBUS | Via CM 1243-5 master / CP 1242-5 slave | Via CM 1542-1 (PN) or CPU with DP port (1516/1517/1518) | Up to 125 DP slaves (master) or 32 (slave) | Up to 124 DP slaves (master) or 32 (slave) |
On a high-pressure unit that needs growth headroom, the practical recommendation is:
- Local I/O on SMs / signal modules in the central rack — fast, deterministic, no extra configuration.
- Field I/O on an ET 200SP / ET 200MP station connected over PROFINET to the CPU. This scales without ever opening the central rack again.
8. Communication Interface Comparison
The communication checklist for a new project is usually: (a) HMI panels, (b) peer PLC, (c) drive / inverter, (d) cloud / SCADA, (e) field instruments. Map each of these to a supported protocol on the candidate CPU.
| Protocol | S7-1200 | S7-1500 | S7-300 | S7-400 |
|---|---|---|---|---|
| PROFINET controller (IO) | Yes (integrated) | Yes (integrated, multiple ports) | Yes (315-2 PN/DP, 317-2 PN/DP, 319-3 PN/DP) | Via CP 443-1 |
| PROFINET device | Yes (configured as iDevice) | Yes (iDevice) | Yes (PN CPU only) | Via CP 443-1 |
| PROFIBUS DP master | Via CM 1243-5 | On DP variants, or via CM 1542-5 | Yes (DP CPU or CP 342-5) | Yes (integrated on some CPU, or CP 443-5) |
| PROFIBUS DP slave | Via CP 1242-5 | Via CM 1542-5 | Via CP 342-5 | Via CP 443-5 |
| Modbus TCP / RTU | MB_Server / MB_Client libraries; CP 1242-7 for RTU master | MB_Server / MB_Client; CP 1542-1, CM 1542-5 | Via CP 340 / CP 341 / Modbus FB libraries | Via CP 441 / Modbus FB |
| OPC UA server | Yes (companion spec, data access only) | Yes (DA, HA, methods, companion spec) | Via CP 343-1 Lean / Advanced / OPC UA Server | Via CP 443-1 OPC UA Server |
| S7 communication (PUT/GET) | Yes (CPU 1200 acts as server; PUT/GET via TIA blocks) | Yes (server and client) | Yes | Yes |
| Open User Communication (TCP/UDP / ISO-on-TCP) | TSEND_C / TRCV_C / TCON | TSEND_C / TRCV_C / TCON | AG_SEND / AG_RECV (CP 343-1) | AG_SEND / AG_RECV (CP 443-1) |
| MQTT (cloud) | Via CP 1243-1 / CP 1243-7 LTE | Via CP 1543-1 / CP 1545-1 | Via CP 343-1 Advanced / IE/PB Link | Via CP 443-1 Advanced |
9. Programming Software and Engineering Tools
| Function | STEP 7 Basic (TIA) | STEP 7 Professional (TIA) | STEP 7 V5.6 (classic) |
|---|---|---|---|
| Target CPUs | S7-1200, S7-1500, ET 200SP (limited) | All S7-1200 / S7-1500 / ET 200SP / ET 200MP / S7-300 / S7-400 (partial) | S7-300 / S7-400 / WinAC |
| Languages | LAD, FBD, SCL | LAD, FBD, SCL, Graph, CFC | LAD, FBD, SCL, Graph, CFC, HiGraph |
| License | Bundled free with S7-1200 / basic S7-1500 kits | Perpetual or subscription license (per user) | Perpetual license, separate media |
| WinCC Comfort / Advanced / Professional | Comfort (panel) included with Basic | Advanced / Professional for PC HMI | WinCC flexible 2008 / WinCC V7.x |
For a new project in 2024-2025, use TIA Portal V18 or V19. The S7-1500 / ET 200MP in a nutshell presentation is the fastest on-ramp for first-time S7-1500 users; it covers the basic configuration of a TIA project, slot rules, and the technology objects.
10. Application Case: Low-Cost High-Pressure Unit with Peer-to-Peer PLC Link
The project context from the user’s question is a high-pressure unit (HPU): a stand-alone machine with a pump, accumulator, pressure transducer, proportional or servo valve, safety block, and HMIs. The cost target is low, with margin for future I/O additions and a peer-to-peer data link to a second PLC (likely an OEM pack or a line controller).
Build the comparison matrix for the two credible candidates — S7-1200 and S7-1500 — against the seven parameters from Section 2:
| Parameter | CPU 1215C DC/DC/DC | CPU 1515-2 PN | Comment |
|---|---|---|---|
| Work memory | 100 KB code / 150 KB data | 500 KB code / 3 MB data | 1215C fits an HPU + recipes; 1515 leaves headroom for diagnostics blocks |
| Bit instruction time | 0.08 µs | 0.03 µs | HPU scan budget is not tight; either CPU is fine |
| Local SM slots | 8 | ≤ 30 in central rack | 1515 scales to future I/O without an ET 200SP |
| Integrated PROFINET | 2-port switch (PN interface 1) | 2 PN interfaces (port 1 + port 2) | 2 separate PROFINET networks on 1515 (ring + line) |
| Peer PLC link | S7 communication PUT/GET (server only by default; client with block calls) | S7 communication PUT/GET (server + client), OPC UA | 1515 makes the link symmetric and adds OPC UA later |
| Motion / PID | PID_Compact, PTO 100 kHz | PID_Compact, technology objects (axis, cam) | 1515 if proportional valve needs a positioning axis |
| Engineering | STEP 7 Basic (free w/ CPU) | STEP 7 Professional (paid) | 1215C avoids STEP 7 Professional license cost |
| Approx. list price (2024) | lowest | ≈ 3× 1215C | Cost benefit clearly on S7-1200 side |
For a cost-sensitive HPU, the engineering decision typically lands on the CPU 1215C with a CM 1243-5 reserved as an option for PROFIBUS, and an ET 200SP PN station brought in only when the local SM slot count of 8 saturates. The peer PLC link uses PUT / GET blocks (or a TCP TSEND_C / TRCV_C pair) to a single PUT/GET partner on the second PLC.
Switch to the CPU 1515-2 PN if any of the following is true: motion is more than simple PID, OPC UA is required, the project will later merge into a larger TIA project with other S7-1500 stations, or the customer is standardising on S7-1500 fleet-wide.
11. CPU Selection Decision Matrix
Use this decision flow when reviewing candidate CPUs against project requirements:
- Will the machine integrate into an existing fleet? If yes, match the family. Brownfield S7-300 → S7-300. Existing S7-1500 → S7-1500.
- Is fail-safe logic required (SIL 2/3 / PL d/e)? Yes → S7-1500F or S7-300F. (S7-1200F is being introduced on selected firmware versions; check the catalog.)
- Is redundant high-availability required? Yes → S7-1500H or S7-400H.
- Is motion more than PID? More than PID → S7-1500 (technology objects and cam). PID only → S7-1200 fits.
- Is OPC UA server a requirement today or in 3 years? Yes → S7-1500 (preferred) or S7-1200 (basic DA only).
- Is the cost target the primary driver? Yes → S7-1200 with ET 200SP PN for distributed I/O.
- Are the local I/O and growth clear? If local > 8 SMs, go S7-1500. If ≤ 8, go S7-1200.
12. Verification Checklist Before Issuing the BOM
After the family decision, validate the specific CPU against project requirements:
- Memory headroom: TIA Portal → Program info after a first compile shows code/data bytes; target < 60% of nominal work memory for growth.
- Slot count: Central rack total slots = PS + CPU + IM (if present) + SM + FM + CP. For S7-1500, confirm with the TIA Portal device configuration that no slot is shown red.
- PROFINET port count and topology: Two independent PROFINET networks (CPU + CP) are needed if the HMI and the drive are on physically separate networks. A single PROFINET interface with a 2-port switch is enough when star/line topology is acceptable.
- Peer link partner capability: PUT/GET must be enabled on the partner CPU (TIA: Properties → Communication → Permit access with PUT/GET). Without this, the partner refuses the request with SF / BF.
- Retentive memory budget: Count the retentive bits, bytes, and DB areas; the CPU datasheet lists the retentive boundary. A recipe-heavy HPU may exceed 1215C retentive data; bump to 1515.
- Firmware compatibility: The CPU firmware version must be on the TIA Portal device catalog. S7-1500 firmware V2.9 / V3.0 is the baseline for TIA V18. Update firmware with the SIMATIC Automation Tool before commissioning.
- Engineering license: If the customer owns only STEP 7 Basic, do not select a CPU that requires STEP 7 Professional (e.g. S7-1500 with S7-GRAPH state machine).
13. Frequently Asked Questions
What is the maximum number of I/O modules in an S7-300 central rack?
An S7-300 central rack accepts up to 8 signal, function, or communication modules in addition to the power supply and CPU. To add more I/O, use up to 3 expansion racks with IM 360 / IM 361 pairs, or migrate to PROFINET ET 200 stations which is the recommended approach for new designs.
How many modules can an S7-1500 hold in a single rack?
An S7-1500 mounting rail holds up to 32 modules including the system power supply, CPU, I/O modules, technology modules, and communication modules. The actual count depends on the CPU firmware and the power supply sizing; verify with TIA Portal hardware catalog.
Can an S7-1200 communicate with another S7-1200 or S7-1500 over PROFINET?
Yes. S7-1200 and S7-1500 both support S7 communication (PUT/GET), TCP/UDP open user communication, and OPC UA. The simplest peer-to-peer data exchange is a PUT/GET block pair, with PUT/GET access enabled on the partner CPU under Properties → Communication in TIA Portal.
Do I need STEP 7 Professional for an S7-1500 project?
Yes. S7-1500 requires STEP 7 Professional in TIA Portal (V18 or newer). STEP 7 Basic, which is bundled with S7-1200, only supports S7-1200. Engineering laptops that need both S7-1200 and S7-1500 in the same project must run STEP 7 Professional.
Is the S7-300 or S7-400 still usable for new projects in 2025?
Siemens has both families in phase-out / spare-parts status. S7-300 is supported for spare parts and known service contracts; S7-400 is in formal phase-out. New builds should target S7-1200 or S7-1500 unless a fleet standard, safety certification, or migration constraint forces the older platform.