1. Problem Overview
When commissioning or maintaining a Siemens SIMATIC S7-300 station equipped with a CPU 314 (for example 6ES7314-1AF10-0AB0 or 6ES7314-1AG14-0AB0), the engineering station running STEP 7 V5.5 / V5.6 may fail to establish an online connection and report the dialog "Online: No connection. Node does not accept the connection setup" with the diagnostic code 33:16662. The same code class is documented by Siemens as "Online: No connection" in the Siemens Industry Online Support knowledge base. The same symptom appears with the legacy SIMATIC S7 PC Adapter USB (6ES7972-0CB20-0XA0) and with later USB adapters (6GK1571-0BA00-0AA0) when the underlying MPI/PROFIBUS physical layer is broken, the node address is wrong, or the PG/PC interface is misconfigured.
Error class 33: belongs to the Online / Connection category of STEP 7 internal error messages. Code 16662 (0x4116) is a node-rejection fault generated when the S7ONLINE DLL on the engineering PC transmits a connection request to the PLC's MPI/PROFIBUS interface and the addressed CPU does not respond with a positive ACK within the configured timeout. The rejection is not a CPU fault per se — it indicates that no valid S7 partner was reached on the bus at all.
2. Error Code Reference Table
| Error Class | Decimal Code | Hex Code | STEP 7 Text | Functional Meaning |
|---|---|---|---|---|
| 33 | 16662 | 0x4116 | Online: No connection. Node does not accept the connection setup. | Connection request timed out; no S7 ACK received from target MPI/PROFIBUS node. |
| 33 | 16664 | 0x4118 | Online: No connection. Connection setup is not possible. | Bus parameter / baud rate mismatch between PG/PC interface and PLC port. |
| 33 | 16665 | 0x4119 | Online: No connection. The partner is neither a CPU nor a CP. | Reply came from a non-S7 device on the segment (incorrect bus profile). |
| 33 | 16668 | 0x411C | Online: No connection. Hardware fault on the PC adapter. | PC Adapter USB: overcurrent, no bus termination voltage, or driver fault. |
| 33 | 16670 | 0x411E | Online: No connection. CPU is in STOP / DEFECTIVE. | Target CPU exists but is in STOP-with-fault mode and rejects active links. |
The full Siemens-published mapping of the xxxx:yyyy STEP 7 error code namespace is maintained under entry ID 19139909 in the Siemens Industry Online Support portal. Always cross-check the numeric code there before treating a 33:xxxx value as the one described above — similar codes 33:16663 through 33:16670 are reported for closely related but distinct root causes.
3. Root Cause Analysis
The MPI / PROFIBUS interface on the S7-300 CPU (port X1 on the front of the CPU 314) implements an RS-485 physical layer operating at 187.5 kbit/s in the default MPI profile, 19.2 kbit/s for older MPI networks, or 9.6 kbit/s to 12 Mbit/s when the port is configured as PROFIBUS DP master or slave. The PC Adapter USB acts as a DCE device: it presents a virtual COM port to Windows and converts the serial stream to RS-485 on the 9-pin Sub-D plug.
When error 33:16662 is reported, the following five root-cause groups must be worked through in order:
- Physical layer (cable, connector, terminator). Open circuit, short circuit, missing bus terminator, or broken shield bonding. The PC Adapter requires a minimum of 5 V DC on the bus (supplied by the CPU or another active node) to power its receiver.
- Address mismatch. The MPI address configured in STEP 7 (default 2 for CPUs, 0 reserved for the PG) does not match the rotary switch on the CPU front panel. The CPU 314 ships at MPI address 2 unless the switches have been changed.
- PG/PC interface assignment. STEP 7 is set to use the wrong interface, e.g. TCP/IP or a virtual Ethernet adapter, while the physical cable is on PC Adapter (MPI).
- Baud rate / bus profile. The PC Adapter is set to Auto or to PROFIBUS 1.5 Mbit/s while the CPU is at MPI 187.5 kbit/s. Auto-detect fails on segments with only one active node and no traffic.
- CPU state or hardware fault. The CPU is in DEFECTIVE / STOP-with-fault mode, the MMC is missing or unreadable, or the front-panel MPI port is mechanically damaged.
4. Hardware Prerequisites and Identification
Before re-trying the connection, confirm the hardware IDs and ratings of every device on the bus.
| Component | Siemens Article Number | Key Specification |
|---|---|---|
| CPU 314 (24 V DC) | 6ES7314-1AG14-0AB0 | 192 KB work memory, MPI/PROFIBUS port at X1, default MPI address 2. |
| CPU 314C-2 DP | 6ES7314-6CG03-0AB0 | 96 KB work memory, integrated DI16/DO16/AI5/AO2, MPI at X1, PROFIBUS DP at X2. |
| SIMATIC S7 PC Adapter USB | 6ES7972-0CB20-0XA0 | USB 1.1/2.0, RS-485 to MPI/PROFIBUS up to 12 Mbit/s, 5 V bus-powered, ~200 mA. |
| PC Adapter USB A2 | 6GK1571-0BA00-0AA0 | USB 2.0 high speed, RS-485 to MPI/PROFIBUS, supports routing over Ethernet (TS Adapter variant). |
| PROFIBUS DP cable | 6XV1830-0EH10 | Characteristic impedance 150 Ω, loop resistance 110 Ω/km, capacitance 30 nF/km, purple PVC jacket. |
| PROFIBUS connector with PG socket | 6ES7972-0BB12-0XA0 | 35° cable outlet, integrated terminator, secondary PG socket for diagnostic connection. |
| RS-485 terminator | 6ES7972-0BA30-0XA0 | 390 Ω / 220 Ω / 390 Ω resistor network, switched via slide on the connector. |
5. STEP 7 PG/PC Interface Configuration
The PG/PC Interface dialog in Start → SIMATIC → STEP 7 → Set PG/PC Interface determines which Windows communication endpoint STEP 7 uses when Accessible Nodes is invoked.
- Open Set PG/PC Interface and select the entry that begins with PC Adapter (MPI) — for a direct MPI cable that is the correct choice, even if you ultimately want PROFIBUS, because the S7-300 CPU port defaults to MPI profile.
- If the list is empty, click Select... and install the adapter from PC Adapter > PC Adapter (MPI) > PC USB. The driver DLL is
S7EPC62X.dllfor STEP 7 V5.5 on 32-bit Windows andS7EPC64X.dllon 64-bit Windows. - Open Properties... and set the following parameters:
| Tab | Parameter | Recommended Value |
|---|---|---|
| PG/PC is the only master on the bus | checkbox | checked when no other master (HMI, DP master) is connected |
| Transmission speed | dropdown | 187.5 kbit/s (do not use Auto for first attempt) |
| Highest station address (HSA) | integer | 31 (default MPI) or 126 (PROFIBUS) |
| Timeout | integer | 10 s (default); increase to 30 s on long cables |
- Click OK and confirm the warning that the change applies globally to all STEP 7 applications.
6. PC Adapter USB Diagnostic Procedure
The PC Adapter USB enumerates as a USB device SIMATIC S7 PC Adapter USB and exposes a virtual COM port (typically COM5 and above). When the device is healthy:
- The green ACTIVE LED on the adapter body blinks during bus activity.
- The yellow TERMINATOR LED is lit only when the terminating switch is on; it does not indicate whether the bus itself is terminated.
- Windows Device Manager shows the device under Universal Serial Bus controllers with no warning triangle.
Open the PC Adapter diagnostic tool (Start → SIMATIC → STEP 7 → PC Adapter → PC Adapter Diagnostics) and read:
| Indicator | Healthy Value | Indicates 33:16662 If |
|---|---|---|
| USB status | OK, enumerated at full speed | Driver not loaded → cable suspect |
| Bus voltage | +5 V ± 10 % present on pin 6 | 0 V → terminator missing or cable broken |
| Receiver activity | Repeats on every token frame | Quiet → CPU not on the bus |
| Configured speed | 187.5 kbit/s | 1.5 Mbit/s and no traffic → speed mismatch |
If the receiver is silent even after several seconds, the CPU is not transmitting — re-check the MPI rotary switches and confirm the CPU is not in DEFECTIVE state (SF LED steady red, BF LED steady red).
7. MPI / PROFIBUS Topology and Termination
An MPI/PROFIBUS segment is a linear RS-485 bus. Every segment must have exactly two terminators: one at each physical end. The CPU 314 MPI port is not internally terminated, so the bus must be terminated at the last connector on each side of the CPU. With a CPU at the physical end of the bus, the terminator switch in the PROFIBUS connector plugged into the CPU must be set to ON; the connector at the other end (HMI panel, drive, or the PC Adapter itself) must also be terminated when the adapter is the last device.
8. CPU Address and Mode Switch Verification
On the front of the CPU 314, two rotary switches set the MPI address. With both at 0, the default address 2 applies. The factory setting is therefore address 2. A configured STEP 7 project that targets an MPI address other than 2 will produce 33:16662 even when the bus and cable are healthy.
| Switch A (tens) | Switch B (ones) | Effective MPI Address |
|---|---|---|
| 0 | 0 | 2 (default, also known as "PG-assigned") |
| 0 | 3 | 3 |
| 1 | 4 | 14 |
| 3 | 1 | 31 (typically the highest MPI station) |
The mode selector (key switch) has three positions: RUN, STOP, and MRES. A CPU in RUN accepts both passive (read) and active (write) online operations. A CPU in STOP accepts both. A CPU in MRES momentarily accepts a memory-reset sequence (described in the next section) and then returns to STOP. None of the three switch positions should, by itself, generate 33:16662; a CPU that has been left in MRES and re-powered may boot in DEFECTIVE mode (red SF LED, red BF LED) and reject connection requests until the reset procedure is completed.
9. MRES Memory Reset and Program Retention
The MRES position of the mode switch is often confused with a program-erasing operation. The behavior depends on whether a Micro Memory Card (MMC) is present. The CPU 314 (6ES7 314-1AG14) and all other 31x CPUs of the newer generation require an MMC to retain the user program across power cycles; without an MMC, the work memory is loaded from flash on every power-up only for very early 31xC-2 variants.
| Scenario | Result of MRES | Program Lost? |
|---|---|---|
| MRES with MMC inserted, MMC write-protect off | CPU performs overall reset; user program is wiped from work memory but the MMC retains a backup copy. A subsequent power cycle reloads the project from the MMC. | No, as long as the MMC is not write-protected or removed. |
| MRES with MMC inserted, MMC write-protect on | Overall reset is suppressed or partial; existing program is preserved. | No. |
| MRES without MMC (older 31x without MMC support) | Work memory is cleared; the project is lost. | Yes — re-load from STEP 7 backup required. |
| Power cycle with MMC inserted | CPU re-loads the project from the MMC automatically. | No. |
- Toggle the switch from STOP to MRES and back to STOP within 1 second. The STOP LED blinks twice, indicating the reset request was accepted.
- Toggle the switch to MRES again and hold for at least 3 seconds. The STOP LED blinks at ~2 Hz.
- Return the switch to STOP when the STOP LED is steady. The CPU is now reset and ready to receive a new online connection.
10. Alternative: TCP/IP Connection via CP 343-1 Lean / CP 343-1
When the MPI path is electrically noisy (e.g. on a shop floor with VFDs), or when the PC adapter is unavailable, configure an Ethernet path through a CP 343-1 Lean (6GK7343-1CX10-0XE0) or CP 343-1 (6GK7343-1EX30-0XE0) on the S7-300 rack. STEP 7 must be configured to allow PUT/GET access for engineering — this is enabled in the CP's properties, Connection Mechanisms.
The configuration procedure for the S7-300/400 side of a TCP/IP connection is documented in How To Configure S7-300/400 Connections in STEP 7 (PTC Knowledge Base). The PG/PC interface is then set to TCP/IP → Network card → Intel(R) PRO/1000 ... and the target IP address is entered in Accessible Nodes → Target Station → ....
For S7 communication between an S7-300 and an S7-1200, Siemens has published a complete sample project that uses the PUT (FB15) and GET (FB14) function blocks, available in the entry S7 Communication with PUT/GET (PDF). The accompanying functional description S7 Communication between SIMATIC S7-1200 and SIMATIC S7-300 (PDF) is the canonical reference for cross-family data exchange and is the right starting point whenever the S7-300 must communicate with a TIA Portal-based S7-1200/1500 controller.
11. STEP 7 Accessible Nodes Workflow
- Open the SIMATIC Manager and load or create a project that contains the S7-300 station matching the rack hardware.
- Right-click the station and choose Target System → Accessible Nodes (or press Ctrl+Alt+A).
- STEP 7 broadcasts an MPI search frame on the selected interface and lists every CPU that responds with its MPI/PROFIBUS address, rack, and slot.
- Double-click the entry to open the online view of that CPU. If the dialog re-appears with 33:16662, the search did not return any node and the cause is physical or address-related, not project-related.
12. Verification Checklist
Run the following checks in order; stop at the first failure.
| Step | Verification | Pass Criterion |
|---|---|---|
| 1 | PC Adapter USB recognised by Windows | Device Manager shows SIMATIC S7 PC Adapter USB without warning triangle. |
| 2 | Bus voltage on PC Adapter | Diagnostic tool shows +5 V. |
| 3 | Termination | Exactly two terminators, one at each end of the segment. |
| 4 | PG/PC interface assignment | Set to PC Adapter (MPI) and 187.5 kbit/s, not Auto. |
| 5 | CPU MPI address | Rotary switches (or default 2) match STEP 7 project. |
| 6 | CPU state | SF LED off, BF LED off or off-blink, RUN or STOP LED solid. |
| 7 | Accessible Nodes | CPU appears in the list within 10 s. |
| 8 | Online download | Project can be downloaded without error message. |
13. Troubleshooting Matrix
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| 33:16662 immediately, no device in Device Manager | PC Adapter USB driver not installed | Reinstall driver from STEP 7 DVD, or use Windows Update catalog. |
| 33:16662 after several seconds, device present, +5 V present | Speed or address mismatch | Set PC Adapter to 187.5 kbit/s; verify CPU MPI address rotary switches. |
| 33:16662, BF LED on CPU is solid red | Bus physically broken or CPU cannot reach next master | Re-cable the segment; check the PROFIBUS connector terminator switch. |
| 33:16662, SF + BF both solid red on CPU | CPU in DEFECTIVE state | Perform MRES procedure; re-download project from STEP 7. |
| 33:16662, PG/PC interface set to TCP/IP | STEP 7 looking on the wrong path | Change Set PG/PC Interface to PC Adapter (MPI). |
| 33:16662 only when OP is connected | OP using up the bus master token | Disable PG/PC is the only master checkbox in the PC Adapter properties. |
| 33:16662 transiently, OK after retry | EMI from VFD or welding equipment | Use shielded PROFIBUS cable with 360° bonding at the cable gland. |
14. Going Online After the Fix
Once the S7-300 station appears under Accessible Nodes with the expected MPI address, double-click it to open the online view. From this view you can read the diagnostic buffer (the canonical place to find the exact internal fault code, the time stamp, and the OB that failed), force variables, upload the program to the engineering station, or download a fresh project. The diagnostic buffer entries are written to OB100 (warm restart), OB121 (programming error), OB122 (I/O access error), and similar; their timestamps use PLC time of day (date+time in 8-byte BCD format), which the engineering station decodes automatically.
15. Field-Engineering Notes
- For an S7-300 station that has been powered down for several months, the MMC may lose its charge holder capacitor charge; symptoms include a CPU that boots into DEFECTIVE with MMC error in the diagnostic buffer. Re-seat the MMC after re-applying 24 V DC for at least 30 s.
- If Accessible Nodes lists the CPU but downloading the project fails with 33:17075 (object management error), the MMC write-protect jumper is set or the project is not present on the MMC — do a fresh download to clear the inconsistency.
- For very long MPI segments (more than 50 m at 187.5 kbit/s), drop the baud rate to 19.2 kbit/s or insert a PROFIBUS repeater (6ES7972-0AA01-0XA0). The repeater regenerates the RS-485 signal and re-isolates the two segments electrically.
- When commissioning a redundant ring topology with two CPUs and an HMI panel, only one PG may be the bus master; the PC Adapter must be set to PG/PC is the only master = off so that token passing with the HMI works.
16. Quick Reference: STEP 7 Error Class Meanings
| Class | Domain |
|---|---|
| 01:xxxx | User program errors |
| 03:xxxx | Operating system / firmware |
| 0B:xxxx | I/O access errors |
| 33:xxxx | Online / connection (this article) |
| 40:xxxx | Hardware configuration |
| 50:xxxx | PROFIBUS DP diagnostics |
17. Frequently Asked Questions
What does STEP 7 error 33:16662 actually mean on a Siemens S7-300?
Error 33:16662 means "Online: No connection. Node does not accept the connection setup." The S7ONLINE service on the engineering PC sent a connection request to the target MPI/PROFIBUS address, and no S7 CPU (or compatible CP) answered within the configured timeout. The fault is on the physical or address layer, not inside the CPU itself.
Does running MRES delete the program in the S7-300 CPU 314?
No, as long as a Micro Memory Card (MMC) is inserted in the CPU. The user program lives on the MMC; MRES clears the work-memory copy but the MMC retains a backup. On the next power-up the program is reloaded from the MMC. Without an MMC, MRES erases the program and a fresh download is required.
Which PG/PC interface should I select in STEP 7 for a PC Adapter USB on MPI?
Select PC Adapter (MPI) in the Set PG/PC Interface dialog, set the transmission speed to 187.5 kbit/s explicitly (not Auto), enable PG/PC is the only master on the bus if no HMI is connected, and click OK. Reopen the SIMATIC Manager so the new interface is loaded into the running session.
Why does the PC Adapter USB show error 33:16662 only when the HMI panel is connected?
The PC Adapter is configured as the only bus master, so the HMI cannot pass the token. Clear the PG/PC is the only master checkbox in the adapter properties to allow token passing between the PG, the HMI, and the CPU.
Can I go online with the S7-300 over Ethernet instead of MPI?
Yes, if a CP 343-1 Lean or CP 343-1 is installed in the rack. Configure the CP's IP address in STEP 7, set the PG/PC interface to TCP/IP → your network adapter, and use Accessible Nodes with the target IP. Siemens provides a complete how-to in the PTC Knowledge Base entry "How To Configure S7-300/400 Connections in STEP 7" linked in the article body.
What is the default MPI address of the S7-300 CPU 314?
The default MPI address is 2, set by leaving both rotary switches on the front of the CPU at the 0 position. A STEP 7 project that targets any other address will produce 33:16662 unless the rotary switches are changed to match.
How do I clear a DEFECTIVE S7-300 CPU that is rejecting online connections?
Set the mode switch to STOP, perform the MRES sequence (STOP → MRES within 1 s → STOP, then MRES held for 3 s → STOP), and re-download the project from STEP 7 over the PC Adapter. The diagnostic buffer (accessible from PLC → Diagnostic/Setting → Diagnostic Buffer) shows the root cause of the DEFECTIVE state.