S7-319-3 PN/DP Communication: Recovering PC Access Without MMC

Overview

The SIMATIC S7-300 CPU 319-3 PN/DP is the highest-performance controller in the S7-300 family, designed for production-engineering applications that demand extensive distributed I/O, large user programs, and high-speed networking through PROFINET and PROFIBUS. According to the Siemens product release notice (Support ID 22782686), the unit is available in SR (Standard) and DR (Redundancy) variants and expands the S7-300 CPU series with a focus on production engineering.

One of its defining hardware characteristics - and a frequent point of confusion for technicians who receive the CPU secondhand or from a spares bin - is that the 319-3 PN/DP has no on-board non-volatile load memory. All persistent program and configuration storage is delegated to a plug-in SIMATIC Micro Memory Card (MMC) inserted into the slot behind the front door. When the MMC is missing, the CPU will not retain a user program across a power cycle, will not reach RUN, and - most importantly for the field engineer - will not accept an online connection from a programming device (PG) or PC running STEP 7 over any of its three integrated interfaces (PROFIBUS DP/MPI, PROFIBUS DP, or PROFINET).

This article explains the underlying S7-300 memory concept, details the exact reason every communication attempt is rejected without a card, and provides a deterministic procedure to bring the CPU back to an online state using a correctly-sized MMC and STEP 7 (V5.5 or TIA Portal).

The S7-300 Memory Concept Explained

Unlike an S7-400 CPU, which contains battery-backed RAM for load memory, the S7-300 family uses a fundamentally different memory architecture. Every S7-300 CPU with an MMC bay relies on a removable flash card to perform three combined functions:

1. Hold the compiled user program (OBs, FCs, FBs, DBs, SFCs, SFBs, system data).
2. Retain retentive tags, timers, counters, and DB contents across power cycles.
3. Provide a target volume that a PG can read from or write to via the online interface.

The CPU's internal RAM is split into two areas that the engineer can neither extend nor replace:

• Work memory (Anwenderspeicher) - volatile, integrated, used at runtime to execute the program. Divided into code work memory and data work memory. Not accessible to the PG as a file system.
• System memory (Systemspeicher) - volatile, integrated, holds the process image of inputs (PII), process image of outputs (PIQ), bit memory (M), timers (T), and counters (C). Not accessible to the PG as a file system.

The MMC occupies the role of load memory (Ladespeicher). During download, STEP 7 serialises the project into a S7PROG file system written to the card. During operation, the CPU copies blocks from the MMC into work memory as required. On power-up, the CPU re-loads the project from the MMC. With no MMC present, there is no load memory, no project to copy, and no non-volatile location in which the PG can deposit a new project.

Memory Architecture Diagram

Memory Areas of the CPU 319-3 PN/DP (6ES7318-3EL01-0AB0)

Why Communication Is Blocked Without an MMC

When a programming device opens an S7 online connection, it negotiates an ISO-on-TCP / S7-communication channel with the CPU and then attempts a block-directory exchange. The directory exchange uses the load memory on the MMC as the source of truth. If the MMC is absent, three distinct failures cascade in this order:

1. Link establishment fails or yields an empty directory. STEP 7 reports "Online: Cannot reach the partner", "No project on the target module", or "Connection to partner broken". Accessible Nodes (MPI) shows the CPU with a red X or with an empty module.
2. Download is rejected. STEP 7 reports "No memory card inserted" or "Cannot write to load memory". The CPU does not have a destination to which it can serialise the S7PROG container.
3. CPU diagnostics: the SF (System Fault) LED is lit, the STOP LED is lit, and the diagnostic buffer contains entries such as "Memory card missing or not formatted" and "No project; CPU stays in STOP".

Passive sniffing of the bus (e.g., with a PROFIBUS analyser or Wireshark on PROFINET) will still capture frames from the CPU - the integrated PROFINET and PROFIBUS interfaces remain electrically active for diagnostic purposes - but a read/write online session requires the MMC.

Hardware Identification and CPU Variants

Before purchasing an MMC, confirm the exact order number (MLFB / Siemens part number) of the CPU. The CPU 319-3 PN/DP exists in two functional variants that influence the firmware and the supported MMC sizes:

According to Siemens Support ID 22782686, the DR (redundancy) variant is also available for high-availability applications. All variants share the same MMC requirement: the card must be present, must be a genuine Siemens SIMATIC MMC (third-party SD adapters do not work reliably in S7-300 bays), and must be at least 512 KB in size.

MMC Selection and Sizing

The CPU 319-3 PN/DP uses the same MMC family as the rest of the S7-300. Siemens publishes the full set of supported cards in the S7-300 module data manual. The recommended part numbers for the engineer to procure are:

Step-by-Step: Restoring PC-to-CPU Communication

Prerequisites

• One CPU 319-3 PN/DP (6ES7318-3EL01-0AB0 or -0AA0) with empty MMC bay.
• One Siemens SIMATIC MMC, 512 KB minimum, 2 MB recommended.
• One PG or PC with STEP 7 V5.5 + SP2/SP3 (for classic project) or TIA Portal V13+ (for TIA project).
• Appropriate cable: PROFIBUS MPI/DP cable, PROFIBUS DP cable, or PROFINET RJ-45 patch lead.
• CP 5611 / CP 5612 / CP 5621 / NetLink PROFINET cable for the PC interface, or a built-in Ethernet port for PROFINET direct.

Procedure

1. Power down the S7-300 station. Open the front door of the CPU and confirm the MMC bay is empty. The CPU will be in STOP with the SF LED on.
2. Insert the MMC. Slide the card into the bay with the label facing the direction shown on the hinged door. The card engages with a light click. Do not force it - the connector is keyed.
3. Restore 24 VDC power to the rail. Watch the LED sequence: SF lit, STOP lit initially, RUN flashing, then either RUN steady (if an MMC with a valid project is present) or STOP steady (if the MMC is empty/unformatted).
4. Wire the PC to the CPU. For the first connection, use a direct MPI cable from the PC's CP 5611 to the CPU's X1 PROFIBUS DP/MPI port. For PROFINET, connect the PC's Ethernet port to the CPU's X3 Port 1 with a standard Cat 5e patch lead.
5. Set the PG/PC interface. In STEP 7 V5.5, open Options > Set PG/PC Interface. Select PC Internal (local) for direct Ethernet, or CP5611(MPI) for an MPI cable. In TIA Portal, use Options > PG/PC Interface.
6. Establish a live online view. In STEP 7 V5.5, choose PLC > Display Accessible Nodes (German: Erreichbare Teilnehmer anzeigen). The CPU should now appear with its MPI/PROFIBUS address (default 2) or its PROFINET device name and IP address. In TIA Portal, choose Online > Accessible Devices.
7. Download the project. Open the STEP 7 project offline, right-click the CPU, and choose PLC > Download (or Download to device in TIA Portal). STEP 7 will prompt you to select the target interface and the detected device. Confirm the "Download to target module" dialog; the project is serialised onto the MMC and copied into work memory.
8. Verify RUN state. After download, choose PLC > Operating Mode and start the CPU if it does not start automatically. Confirm RUN LED steady and SF LED off.
9. Save retentive data. Retentive M, T, C, and DB ranges declared in the CPU properties are now persisted to the MMC across the next power cycle. Confirm by reading them after a power-off/power-on test.

Communication Topology

STEP 7 Online Procedures (V5.5 and TIA Portal)

Once the MMC is in place, the PG can use the full set of online functions. The following table summarises the operation, the menu path, and what the engineer should observe on a healthy CPU.

Sample Ethernet Address Parameters in STEP 7 V5.5

Use these values as a starting point in the Properties > Interface dialog of the CPU object:

Interface X3 (PROFINET)
  IP address:   192.168.0.10
  Subnet mask:  255.255.255.0
  Router:       0.0.0.0
  PROFINET device name: cpu319-3-pndp
  Conversion:   No router used
  S7 subnet ID: PNIO_1

Verification Checklist

After every intervention on an S7-319-3 PN/DP, run the following checks before handing the station back to operations:

1. LED pattern: RUN steady green, SF off, BF (bus fault) off, MAINT off. DC5V on to confirm internal rail good.
2. Diagnostic buffer: open PLC > Diagnostic Buffer; the most recent entries should reference the most recent RUN, no "memory card" or "I/O access error" entries.
3. Cycle time: PLC > Module Information > Scan Cycle Time; cycle should be stable and well below the maximum OB1 time configured in the CPU properties.
4. Retentive values: power-cycle the station and read the retained M, T, C, and DB tags. They must match the values present before the cycle.
5. Accessible nodes: the PG can browse the CPU, read the project, and download a small test block without errors.
6. PROFINET/PROFIBUS diagnostics: the assigned slaves/devices report no diagnostic interrupt, and the CPU's PROFINET/PROFIBUS diagnostic view is clean.

Troubleshooting Matrix

Diagnostic Buffer and LED Interpretation

The CPU 319-3 PN/DP records every state change in a non-volatile diagnostic buffer that survives across power cycles only if the MMC is present. The following table maps the most common LED states to the corresponding diagnostic buffer entry, helping the engineer to triage a no-communication situation quickly.

Best Practices and Field Notes

• Always have a spare MMC. Field engineers should keep at least one 2 MB card per active S7-300 station. A 5-minute card swap is faster than a 5-day project recovery from a PC that no longer exists.
• Label every MMC. Mark the card with the station number, project version, and last firmware level. A black MMC in a parts bin is a maintenance liability.
• Power down before removing the MMC. Pulling the card on a live CPU can corrupt the file system and produce a permanent "card defective" entry in the diagnostic buffer.
• Match the card to the CPU generation. Cards from an S7-400 will not fit; SD-to-MMC adapters sold online are unreliable on the S7-300 bay.
• Read the S7-300 memory concept manual referenced as Support ID 7302326. It is the canonical Siemens explanation of why load memory lives on the card and not in the CPU.
• For high-availability DR applications, keep two identical MMCs per redundant pair; the DR variant performs card-level comparison on cold restart.
• Document the HMI/SCADA tag mapping on the card label, not in a separate binder. When the next engineer arrives with a laptop and a question, the answer is on the card.

FAQ