S7-315 and S7-1200 PROFINET VFD Connection Limits

Overview

Selecting a Siemens controller for a multi-drive application often comes down to one question: how many PROFINET variable frequency drives (VFDs) can the CPU address on a single PROFINET subnet, and what overhead does the configuration impose on cycle time, connection resources, and engineering effort? This reference documents the published device-count ceilings for the S7-315-2 PN/DP (order code 6ES7315-2EH14-0AB0) and the S7-1215C (6ES7215-1xx40-0XB0 family), explains how the limits interact with the PROFIdrive profile used by most modern VFDs, and walks through a representative TIA Portal integration using a third-party GSDML.

The two platforms are not in the same performance class, and the published limits reflect that:

• S7-315-2 PN/DP (firmware V3.3): up to 128 PROFINET IO devices as IO controller.
• S7-1215C (firmware V4.0 and later): up to 16 PROFINET IO devices integrated via the onboard PROFINET port.

For larger populations of drives on an S7-1200 line, the engineering workaround is to add a PROFINET switch and segment the topology, or to step up to an S7-1500. For the S7-300 family, the S7-315-2 PN/DP sits in the middle of the lineup and is the workhorse for mid-size drive skids that exceed what an S7-1200 can host.

S7-315-2 PN/DP PROFINET Specifications

The S7-315-2 PN/DP is part of the SIMATIC S7-300 product line and is shipped with an integrated PROFINET interface on the X2 port of the CPU. The relevant data sheet values for the 6ES7315-2EH14-0AB0 with firmware V3.3 are summarized below.

Per the official Siemens product support entry for the 6ES7315-2EH14-0AB0, the CPU is listed as supporting up to 128 PROFINET IO devices. This number is the maximum number of IO-Devices that the CPU can address as IO controller; it does not include non-IO nodes such as switches, programming devices, or HMI panels, which consume addressing slots but do not count against the IO device budget.

Reference: Siemens Product Support, CPU 315-2 PN/DP (6ES7315-2EH14-0AB0) — support.industry.siemens.com.

S7-1215C PROFINET Specifications

The S7-1215C is the high-end CPU in the S7-1200 family. It carries two PROFINET ports (X1 and X2) on the front of the CPU, with the second port intended for linear topology or sub-net separation. The relevant PROFINET numbers, as published in the S7-1200 System Manual and the Siemens Product Support entries, are summarized below.

The IO device limit on the S7-1200 is firmware dependent. The values above are taken from the S7-1200 System Manual revisions covering FW V4.0 through V4.5. The IO device limit refers to the number of PROFINET IO Devices the CPU can manage as an IO controller; it does not include the CPU's own connection to a higher-level controller when the S7-1215C is configured as IO Device on its X2 port.

The 16-device ceiling is sufficient for small skids (for example, four VFDs plus two ET200SP stations, one I/O island, and one safety block), but it is the binding constraint when the application starts scaling.

Reference: Siemens S7-1200 Programmable Controller System Manual (entry ID 109825007) and the S7-1200 Communication Function Manual (entry ID 48289408).

PROFINET Device Limits: Side-by-Side Comparison

The 8x gap in IO device ceiling is the deciding factor for many applications. A 30-drive conveyor cluster is a comfortable fit on the S7-315-2 PN/DP; the same 30 drives is impossible on a single S7-1215C CPU without changing the architecture.

Connection Resources: The Other Ceiling

The number of PROFINET IO devices is not the only ceiling. Each IO device consumes a slot in the PROFINET connection table, but separate from the IO count, every communication relationship on the CPU consumes connection resources. These are partitioned into:

• PG connections — used by STEP 7/TIA Portal for online access.
• OP connections — used by HMI panels (WinCC, WinCC Unified, third-party HMIs).
• S7 connections — used for PUT/GET, BSEND/BRCV, S7 routing.
• Open User Communication (OUC) — TCP, UDP, ISO-on-TCP, and Modbus TCP connections, programmed in the user program (TSEND_C, TRCV_C, MB_CLIENT, etc.).
• Web connections — HTTP client/server for the integrated web server.
• OPC UA connections — server sessions, when configured.

Per the Siemens TIA Portal Help entry "What you need to know about using connection resources (S7-1200)", each connection consumes a resource on both endpoints. If the resource is exhausted, new connection attempts return a 0x80A7 error in the user program (resource limit reached).

Reference: Siemens TIA Portal Help, "What you need to know about using connection resources (S7-1200)" — docs.tia.siemens.cloud.

For a VFD-rich project, two pitfalls are common:

1. Forgetting that HMI panels consume OP resources. A 10-line WinCC Unified project with one panel per line consumes all 10 PG/OP slots on the S7-300 even before any HMI-to-CPU S7 routing.
2. Forgetting that PG/OP and S7 connections both consume the same pool. On the S7-1200, raising the S7 count to 8 lowers the OP/PG count and vice versa; the device prevents you from oversubscribing the pool at compile time.

PROFINET Topology and Subnet Limits

The PROFINET specification caps a single subnet at 256 stations. That is the IEEE 802.1D limit on MAC address forwarding on most managed PROFINET switches (the limit of 8 192 MAC entries is the theoretical Ethernet cap, but PROFINET typically enforces 256). The 256-station cap includes:

• The IO controller (the CPU)
• All PROFINET IO devices (VFDs, distributed I/O)
• Switches (managed PROFINET switches such as Scalance XC206 count once each)
• Programming devices and HMI panels
• Third-party nodes

For the S7-315-2 PN/DP, the 128-device ceiling is the binding constraint. For the S7-1215C, the 16-device ceiling is the binding constraint; the 256-station cap is essentially unreachable on a 1215C.

Both CPUs have two PROFINET ports forming an internal switch (the X1 and X2 ports on the S7-1215C are an internal 2-port switch; the X2 port on the S7-315-2 PN/DP is also a 2-port switch). This means you can build a linear topology without external switches. For larger VFD skids, however, a star topology with a managed PROFINET switch (e.g., Scalance XC206-2SFP, XC208, or XC216) is preferred for diagnostics and for isolating a faulted segment.

For deterministic behaviour, all managed switches in the path between the IO controller and the IO device must be configured as PROFINET conformance class CC-B (or higher) and updated to a firmware that supports the desired PROFINET version. Use the Topology Editor in TIA Portal to draw the interconnections, and have the controller compare the planned and actual topology at start-up.

Connecting a Third-Party VFD via PROFINET

Most modern VFDs expose PROFINET via an option card (e.g., Siemens SINAMICS G120 with PN option, ABB ACS880 with FENA-11/-21, Danfoss FC 302 with EPN, Schneider ATV630 with VW3A3627). Integration procedure:

Prerequisites

• PN option card installed and parameterized (IP, device name, telegram selection).
• GSDML file (Generic Station Description Markup Language) for the VFD, downloaded from the drive manufacturer's website.
• TIA Portal V15.1 or later (V17 / V18 / V19 / V20 are current at the time of writing). Earlier TIA Portal versions do not import the latest GSDML revisions.
• The drive's PROFIdrive profile document, in particular the supported Standard Telegrams (1, 2, 3, 4, 5, 6, 7, 8, 9, 20, 102, 103, 105, 106, 220, 352, etc.).

Step-by-Step Configuration in TIA Portal

1. Install the GSDML. In TIA Portal, choose Options → Manage general station description files (GSD). Browse to the GSDML file from the drive manufacturer. TIA Portal installs the file into the Hardware Catalog under the appropriate vendor path.
2. Add the VFD to the project. In the Devices & Networks view, drag the VFD from the catalog into the network. Connect its PROFINET port to the CPU's PROFINET interface or to a Scalance switch in the path.
3. Assign IP and device name. In the VFD's properties → Ethernet addresses, assign a unique IPv4 address and PROFINET device name. The PROFINET device name must be unique on the subnet and must match the name programmed in the drive (typically via the drive's keypad or web server, or via PRONETA).
4. Select the PROFIdrive telegram. In the VFD's properties → Module parameters / IO tags, choose the Standard Telegram. For most pumps, fans, and conveyors, Standard Telegram 1 (PZD-2/2, 2 words control / 2 words status, 1 word speed setpoint / 1 word actual speed) is the default. For applications needing more data (e.g., encoder feedback, torque values, alarms), Standard Telegram 3 (PZD-5/9) or Standard Telegram 5 (PZD-5/15) are common.
5. Map the cyclic I/O. TIA Portal maps the cyclic I/O to hardware identifiers (HW IDs) in the device. The user program reads/writes the slots using LDPW/TXPW instructions (S7-300) or via a DeviceName ~ I/O data access (S7-1200, symbolic access).
6. Configure the watchdog time. Set the watchdog to 3× the bus update time. For a 2 ms update time, watchdog is 6 ms. This avoids nuisance trips from jitter.
7. Compile and download. Compile the project; download the hardware configuration to the CPU. Cycle power to the VFD or use PRONETA to push the new name and IP.
8. Verify IO is alive. In TIA Portal, go Online → Online & diagnostics on the VFD. The device should show "IO active" with green status; cyclic IO should be exchanging.

A worked example of installing a third-party GSD into a Siemens S7-1200 (using a Keyence vision system) is documented in the following third-party engineering reference. Reference: "S7-1200 to Third Party Device Over PROFINET" — theautomationblog.com.

Cyclic Messaging With a PROFIdrive Drive

On an S7-1215C, the I/O addresses of a PROFIdrive VFD are accessible as standard I/O symbols. The typical Telegram 1 layout is:

Example SCL (Structured Control Language) snippet for the S7-1200 — read status, latch fault, then issue run forward:

// Cyclic on OB1 (or a separate FC scheduled in the IO controller's cycle)
#StatusWord := "VFD_1" ~ ZSW1;  // symbolic IO access on S7-1200
#ActualSpeed := "VFD_1" ~ NIST_A;

// Fault handling
IF ("VFD_1".ZSW1.%X3 AND NOT #FaultLatched) THEN
    #FaultLatched := TRUE;
    #FaultPending := TRUE;
END_IF;

// Run command using STW1 bits per PROFIdrive
// Bit 0 = ON, Bit 3 = Enable, Bit 4 = Enable ramp generator, Bit 10 = Control by PLC
#ControlWord.%X0  := TRUE;        // ON
#ControlWord.%X3  := TRUE;        // Enable operation
#ControlWord.%X4  := TRUE;        // Enable ramp generator
#ControlWord.%X10 := TRUE;        // Control by PLC
"VFD_1" ~ STW1 := #ControlWord;
"VFD_1" ~ NSOLL_A := #SpeedSetpoint;

// Acknowledge
IF #FaultAck THEN
    #ControlWord.%X7 := TRUE;      // Fault acknowledge (rising edge)
END_IF;

On the S7-300 side, the same logic uses L / T on the hardware I/O identifiers (e.g., L PEW 256 for input word 256, the address of ZSW1 for the first VFD). For drives beyond the first, add the slot offset of the telegram to the base address.

Acyclic Messaging (DPV1 / PROFIdrive Parameter Channel)

Many VFDs expose parameters (ramp times, motor data, fault logs) only via the acyclic record-data channel. On the S7-1200, the RDREC and WRREC instructions are used. The slot index for PROFIdrive is 0/47 (record 47) for the parameter channel. A simple parameter read looks like:

// Read parameter r0021 (actual speed smoothed) from VFD_1
#Req := "VFD_1".RDREC(
    REQ        := TRUE,
    ID         := "VFD_1_Interface".HardwareID,
    INDEX      := 47,            // PROFIdrive parameter channel
    MLEN       := 10,            // max record length
    VALID      => #Valid,
    BUSY       => #Busy,
    ERROR      => #Error,
    STATUS     => #Status,
    RECORD     := #RecordBuffer   // VB buffer
);

The first 4 bytes of the record buffer are the parameter request/response header (function code, parameter number, subindex, number of elements). The PROFIdrive parameter channel specification is described in the PROFIdrive profile document available from PROFIBUS & PROFINET International (PI).

Commissioning and Verification Checklist

Use the following checklist to confirm a PROFINET VFD cluster is healthy at site acceptance:

1. Each VFD has a unique IPv4 address on the PROFINET subnet (for example, 192.168.0.20 + N, where N is the device index).
2. Each VFD has a unique PROFINET device name (e.g., vfd-pump-01). Names are case-insensitive on the wire but case-preserving in PRONETA.
3. The PROFINET device name is assigned to the drive, not just to the TIA Portal project. Use PRONETA or the drive's web UI to push the name; otherwise, the IO controller cannot find the device.
4. Topology editor in TIA Portal shows "Match" for every port-to-port connection.
5. Cyclic IO is active on every device (green check in online diagnostics).
6. The configured telegram matches the drive's parameter p0922 (Siemens drives) or equivalent.
7. The watchdog time is at least 3× the send clock; for a 2 ms send clock, the watchdog is 6 ms minimum.
8. Each drive is running with STW1 bit 10 (control by PLC) set true, so the drive ignores local setpoints and accepts the controller's setpoint.
9. One HMI panel per drive or per line is tested for OP connectivity.
10. Disconnection of one drive does not fault the others; the watchdog is set per device, not per subnet.

Troubleshooting Matrix

Limit Expansion Strategies

If the IO device ceiling is the binding constraint, the following options preserve investment in existing PLCs while expanding drive count:

1. Add a CP 343-1 / CP 343-1 Advanced to the S7-300 rack (only for the S7-315-2 PN/DP; the S7-1200 has no CP slot). The added PROFINET interface can host an additional 128 IO devices. The two PROFINET interfaces (CPU and CP) operate as independent subnets, so an additional managed switch is required.
2. Use a Scalance SCALANCE W / XC switch to mirror the PROFINET subnet to a second sub-network for partitioning. Each subnet retains its own 128-device budget.
3. Substitute S7-1215C with S7-1515-2 PN (256 IO devices) or S7-1511-1 PN (128 IO devices) for new installations. Both support PROFINET IRT and OPC UA natively.
4. Substitute S7-300 with S7-1500 (CPU 1516-3 PN supports 512 IO devices; CPU 1518-4 PN supports 1024). The PROFINET VFDs and GSDMLs are the same; only the controller changes.
5. Add a PROFINET/PROFIBUS gateway (e.g., Siemens PN/PN Coupler, Scalance XC-200) so a PROFIBUS DP segment of VFDs (Sinamics G120D with DP option) appears as a single PROFINET node to the controller. This is a lossy compression (one PN node replaces N DP nodes) and is only acceptable when the per-drive visibility is not required by the user program.

Field Application: Five-Job Reference Architecture

Mapping the original application scenario to the published numbers:

Each S7-1215C application has comfortable headroom. If a future scope expansion adds five more VFDs to Job 3, the total becomes 10 VFDs + 2 I/O islands = 12 PN IO devices, still inside the 16-device budget. Going to 17 PN IO devices requires either an architectural change (PN/PN coupler to a second subnet) or a controller change to the S7-1500 line.

PUT/GET and S7 Communication With Drives

PUT/GET is the legacy cross-CPU communication primitive, used for S7-300/S7-400 to S7-1200/S7-1500 peer messaging. It is not typically used to talk to a VFD (VFDs use PROFIdrive over PROFINET, not PUT/GET). However, PUT/GET is relevant for HMI panels, peer controllers, and S7 routing.

For S7-1200 PUT/GET, the S7 connection must be enabled in the CPU properties under "Connection mechanisms → Permit access with PUT/GET communication partner." When this is unchecked, the CPU rejects PUT/GET requests with a 0x80A7 error (resource limit reached or service rejected). The Siemens S7 communication PDF explains the configuration sequence in detail.

Reference: "S7 Communication with PUT/GET — Support (PDF)," Siemens Industry Online Support, document support.industry.siemens.com.

For third-party controllers (e.g., Delta Motion RMC) using S7-300 / S7-1200 as PROFINET masters, the controller acts as a PROFINET IO Device and the Siemens CPU acts as IO Controller; the S7 protocol itself is not used for drive control.

Reference: "Using Siemens S7 PLCs via PROFINET — Delta Motion" — deltamotion.com.

Standards and Reference Documents

Engineers verifying any of the device-count numbers should consult:

• PROFIBUS Nutzerorganisation e.V. (PI) — PROFIdrive Profile v4.2 (or current revision) for telegram definitions.
• IEC 61784-2 — Industrial communication networks — Profiles — Part 2: Additional fieldbus profiles for real-time networks (covers PROFINET conformance classes).
• IEC 61158 — Industrial communication networks — Fieldbus specifications (PROFINET is mapped to Type 10).
• Siemens S7-300 Automation System, Module Data Manual (entry ID 8859629 in Industry Online Support).
• Siemens S7-1200 Programmable Controller System Manual (entry ID 109825007).