PROFIBUS vs PROFINET Data Limits: Frame Size and Throughput Reference

Field engineers designing a distributed I/O network routinely ask two questions: how many bytes can a single device move per cycle, and how much total traffic will a controller push across the bus. The official answers live in the IEC 61158 / IEC 61784 family, in PI (PROFIBUS & PROFINET International) specifications, and in each controller's technical datasheet. This reference consolidates those values, separates the per-frame payload from the per-cycle throughput, and shows where MB- and GB-class transfers genuinely belong versus where the I/O bus stops being the right tool.

1. Overview of the Two Bus Families

PROFIBUS (IEC 61158 Type 3) and PROFINET (IEC 61158 Type 10) are I/O bus protocols — they connect PLCs / controllers to distributed I/O racks, drives, valve manifolds, and sensors in real time. They are not the right protocol for moving bulk record archives or video; that is the role of IT-grade TCP/IP on the same Ethernet cable. The 240- and 1440-byte values frequently cited are the per-frame process-data payload, not a hard ceiling on a PROFINET cable as a whole.

PROFIBUS DP runs on RS-485 (copper) or fibre (plastic/glass) at 9.6 kbit/s to 12 Mbit/s. PROFINET runs on 100 Mbit/s or 1 Gbit/s full-duplex switched Ethernet. The physical-layer change is the main reason PROFINET's per-frame payload is roughly six times larger than PROFIBUS DP V0's.

2. PROFIBUS DP Frame Capacity by Version

PROFIBUS DP is documented in IEC 61158 Type 3. The user-data payload per telegram depends on the DP version and on the services the device supports.

2.1 DP V0 — Cyclic I/O Only

DP V0 defines one cyclic I/O message per device per cycle. The standard telegram is structured as:

The widely cited value of 240 bytes is the practical DP V0 cyclic I/O limit per device. The theoretical maximum, defined by the IEC 61158 Type 3 protocol data unit, is 244 bytes. Slaves are configured with input / output slot sizes in their GSD file; most DP V0 slaves expose 1 to 32 bytes by default, and only a few drives or complex I/O stations reach the 240-byte ceiling.

2.2 DP V1 — Acyclic Parameter Data Added

DP V1 keeps the 240-byte cyclic I/O ceiling and adds acyclic services (Read / Write Record, MS1 and MS2 channels) for parameterisation, diagnostics, and device identification. The acyclic payload is again bounded by the 244-byte PDU size, but the same device can run one cyclic frame and multiple acyclic frames per cycle, so a DP V1 device can in total move more bytes per second than a DP V0 device — not because the per-frame size grows, but because the per-cycle transaction count grows.

2.3 DP V2 — Isochronous and Slave-to-Slave

DP V2 introduces isochronous mode (clock-synchronised I/O for motion, e.g. PROFIdrive class 4) and direct slave-to-slave (publisher / subscriber) communication using a Sub-Telegram multiplexer (DXB, Data Exchange Broadcast). The 240-byte cyclic limit is unchanged; the change is timing (jitter < 1 µs with isochronous) and topology (slave-to-slave without a master round-trip).

3. PROFINET Frame Capacity and Conformance Classes

PROFINET is documented in IEC 61158 Type 10 and IEC 61784-2. The protocol data unit rides inside a standard IEEE 802.3 Ethernet frame, so it benefits from the same 1,518-byte MTU as office Ethernet.

3.1 Standard PROFINET Frame (RT and IRT)

For real-time (RT) and isochronous real-time (IRT) I/O, the PROFINET specification reserves a maximum of 1,440 bytes of process data per device per cycle. This value is the total of input + output + status + diagnostic data in a single AR (Application Relationship) / CR (Communication Relationship) for that device.

Conformance Classes (CC-A, CC-B, CC-C, CC-D) describe the minimum capability a device must provide; the 1,440-byte payload is the RT / IRT maximum defined in the PROFINET specification and is not a property of conformance class. A typical ET200S / ET200SP module moves 4 to 32 bytes of process data per slot; the 1,440-byte limit is reached only by complex stations such as drives with PROFIdrive, robot controllers, or large I/O racks with many subslots.

3.2 PROFINET RT vs IRT Cycle Time

PROFINET RT and IRT differ in cycle time, not in payload size. Both share the 1,440-byte per-frame limit.

3.3 Why 1,440 Bytes?

1,440 bytes is the largest payload that fits inside a single 1,518-byte Ethernet frame while leaving room for two VLAN tags, an Ethernet header, and the PROFINET frame ID. PROFINET deliberately does not fragment process data — a cyclic frame must always fit in a single Ethernet frame, so the controller / switch scheduling remains deterministic.

4. Frame-Size Comparison Table

5. Controller and CPU Address Limits

The 240 / 1,440 value is the device-side ceiling. The network-side ceiling is the controller's address area. Every CPU datasheet lists:

• Maximum number of PROFIBUS DP slaves on a line (typical: 32 stations per segment, 126 with repeaters).
• Maximum number of PROFINET devices (typical S7-1500: 256 IO devices; S7-1200: 16; ET200SP CPUs vary).
• Maximum size of the I/O address area (typical S7-1500: 32 KB process image per side).
• Maximum number of acyclic connections for DP V1 / PROFINET records.

Quick reference for the Siemens SIMATIC family (always verify against the current datasheet, firmware V being the most common baseline in 2024–2026 deployments):

Verify the exact limit against the SIMATIC S7-1500 System Manual and the specific CPU's technical data sheet for the firmware you are commissioning — the values above are typical, not guaranteed.

The binding rule for a network sizing calculation is:

Sum over all devices of (max input bytes + max output bytes) 
   <= controller process image size
   <= sum of per-device ceilings (240 B PROFIBUS, 1,440 B PROFINET)

If the per-device ceiling is the binding constraint, you are on PROFIBUS DP V0 and the slave itself is full. If the controller process image is the binding constraint, you need a bigger CPU or a second PROFINET IO controller / PROFIBUS DP master.

6. PROFIBUS PA for Process Automation

PROFIBUS PA is a variant of PROFIBUS DP for process automation, particularly in the chemical and petrochemical industries. PROFIBUS PA combines supply and data transmission in a single shielded two-wire cable, and it is the layer that drives the field-mounted instruments (pressure, temperature, flow, level) that today still dominate tank farms, refineries, and pharmaceutical skids.

Key parameters to keep straight when sizing a PA segment:

PA devices still respect the 240-byte per-telegram limit. Throughput is lower only because the bus runs at 31.25 kbit/s, not because the per-frame payload shrinks. Detailed segment design rules are in the PI document "PROFINET — The Solution Platform for Process Automation" referenced at the bottom.

7. Non-I/O Data Transfer on PROFINET

The 1,440-byte limit is the I/O ceiling. PROFINET also defines channels that are not I/O:

• PROFINET CBA / RPC over UDP — acyclic component-based communication, 1,472 B typical UDP payload.
• Standard TCP/IP — full Ethernet MTU, 1,500 B per TCP segment, file transfer, web server, OPC UA.
• OPC UA over TCP — large payloads, typically tens of KB per read, used for diagnostics and recipe transfer.
• PROFINET video / camera profile — uses UDP on the same cable; not real-time deterministic.

This is the answer to the original question "how much can a device handle" when the application is not cyclic I/O. A PROFINET device that exposes a TCP socket can in principle accept GB-class file transfers, subject to the device's own buffer size, the controller's file system, and the line rate of the 100 Mbit/s link (~12 MB/s theoretical, ~10 MB/s real-world with TCP overhead).

8. Practical Sizing Rules

1. Per-device I/O ceiling: 240 B on PROFIBUS DP, 1,440 B on PROFINET. Anything larger is not real-time I/O.
2. Per-controller ceiling: sum the I/O sizes of all configured devices and compare to the controller's process image size from the CPU datasheet.
3. Throughput ceiling: at PROFIBUS 1.5 Mbit/s, 1 ms cycle, 240 B per device, the bus can carry roughly 240 devices' worth of full-length I/O per second before saturating. PROFINET at 100 Mbit/s with 1 ms cycle, 1,440 B, is one to two orders of magnitude higher.
4. Cycle-time budget: the cycle time you set in the IO controller must be larger than the per-frame transmission time and the I/O-update processing time of every connected device. Watch the controller's Web server "Cycle time" and "I/O update time" diagnostics.
5. Bulk data: route large records, images, or video over standard TCP/IP, not the I/O cycle.

8.1 Worked Calculation — PROFINET 1 ms Cycle

Frame size (PROFINET)            = 1,440 B process data
Preamble + SFD                   =    8 B
Ethernet header                  =   14 B
PROFINET RT header               =    4 B
VLAN tag (optional)              =    4 B
FCS                              =    4 B
IFG                              =   12 B
------------------------------------------------------------
Total on-wire per frame          = 1,486 B  (1,490 B with VLAN)

On-wire time @ 100 Mbit/s        = 1,490 * 8 / 100,000,000
                                 = 119 µs per frame

Per-device throughput @ 1 ms     = 1,440 B / 0.001 s = 1.44 MB/s
Number of full-size devices      = 1,000 µs / 119 µs  ≈  8
  that fit a 1 ms cycle on a
  single port (no switching)

8.2 Worked Calculation — PROFIBUS DP 1.5 Mbit/s

Frame size (DP V0)               =   11 B overhead + 240 B user data = 251 B
On-wire time @ 1.5 Mbit/s        =   251 * 8 / 1,500,000 = 1.34 ms per full-length device
  -> 1 ms cycle is impossible at 240 B/device

That is why most DP V0 stations with 240-byte payloads run at 3 to 12 Mbit/s, where the same frame fits inside a 1 ms cycle.

9. Specifications Summary

10. Verification and Field Checks

After commissioning, verify the design assumption in three places:

1. Controller Web diagnostics (e.g. TIA Portal "Online > Diagnostics > Cycle time / I/O update time): confirm that the I/O update time is less than the configured send clock. If not, the I/O process image is saturating the cycle and the bus cannot keep up with the configured update rate.
2. Per-device slot / subslot size in the GSDML / GSD: open the device's GSDML in TIA Portal and confirm that the configured input / output length for each slot matches the actual user data on the wire. Many devices reserve up to 32 bytes per slot even when only 4 are used — that overhead counts against the 1,440-byte per-device limit on PROFINET.
3. Bus statistics on the IO controller: watch for "Port statistics: Discarded frames", "Lost frames", "CRC errors", and "Late frames". Late frames on PROFINET IRT are an immediate sign that the cycle budget is exceeded.

Authoritative references: IEC 61158 / IEC 61784 standards (Type 3 for PROFIBUS, Type 10 for PROFINET), PI (PROFIBUS & PROFINET International) specifications and Conformance Class documents, and the SIMATIC S7-1200 / S7-1500 system manuals. For a process-automation perspective, see PI's PROFINET — The Solution Platform for Process Automation white paper. For a working introduction to the two protocols and how they differ from a control-system perspective, see What is PROFIBUS/PROFINET? | Siemens I/O Protocols Explained.