S7-312C to S7-314C-2PN/DP Migration: STEP 7 Hardware Rewire Guide

Overview

Migrating from a SIMATIC S7-300 CPU 312C to a CPU 314C-2PN/DP (MLFB 6ES7 314-6EH04-0AB0) is the typical compact-to-compact upgrade path when a plant needs additional technology channels (counting, frequency measurement, PWM), integrated analog I/O, and PROFINET connectivity without expanding the rack. The migration is more involved than a drop-in swap because the integrated digital I/O addresses move from the 124.x area to 136.x, the analog I/O defaults to the 752.x area, and the primary communication port changes from MPI/DP to PROFINET with a parallel PROFIBUS DP interface.

This reference walks through a STEP 7 V5.x migration that preserves your existing program logic, remaps every on-board I/O bit using the Rewire function, converts the HMI link from MPI to PROFINET, and verifies the new hardware end-to-end. The same procedure applies if you instead choose to bring the project into the TIA Portal using the official migration tool.

Prerequisites and Engineering Environment

Verify the following items before opening the project for editing:

Refer to the SIMATIC S7-300 CPU 31xC and CPU 31x: Technical specifications manual on Siemens Industry Online Support (entry ID 12996906) for the authoritative list of firmware versions and HSP requirements: S7-300 CPU 31xC manual.

Source and Target CPU Comparison

The CPU 312C and the CPU 314C-2PN/DP share the S7-300 compact footprint and the technology function library, but differ significantly in integrated I/O count, address area, and interface mix.

The two rows that drive the migration procedure are the integrated I/O address areas and the communication interfaces. The Rewire operation in HW Config exists precisely to redirect every symbolic and absolute reference to the new address space.

Integrated I/O Address Map (Why Rewire Is Mandatory)

The on-board I/O addresses are allocated by STEP 7 from a base area that depends on the CPU MLFB. If the old project used absolute addresses (I 124.0, Q 124.5) the compiled program still references them after the CPU swap. Without rewiring, the program runs, but every input reads 0 and every output stays off because the physical terminals are wired to a completely different byte.

The Rewire function in STEP 7 V5.x is invoked from HW Config with the new CPU selected: Options → Rewire. The dialog accepts a From / To byte range; for this migration enter 124 → 136 with a length of 2 bytes for digital I/O. STEP 7 updates every reference inside the project, including DB initial values, FC/FB parameter defaults, and the symbol table comments.

Pre-Migration Safety and Backup Procedure

1. Bring the machine to a safe state per lockout-tagout procedure before opening the cabinet.
2. Document every wired terminal of the existing CPU 312C: DI 124.0..124.7, DO 124.0..124.3, MPI cable pinout, 24 V sensor supply, and ground.
3. Connect the PG (programming device) to the existing MPI network and go online to the CPU 312C.
4. Read the project back from the PLC (PLC → Upload to PG) and save as a separate file. This captures any online-only changes.
5. Open the original STEP 7 project and immediately File → Archive. Repeat to a second location.
6. Power down, label and remove the CPU 312C, install the 314C-2PN/DP in the same slot, and verify the backplane bus connector is fully seated.
7. Power up and verify the CPU enters STOP with no SF LED. If the SF LED is on, the backplane connection is suspect; re-seat before proceeding.

Step-by-Step Migration in STEP 7 V5.x

Step 1 - Replace the CPU in HW Config

1. Open the STEP 7 project and double-click HW Config in the project tree.
2. Click the CPU 312C slot to select it, then right-click → Change Module (or Delete and drag from the catalog).
3. In the catalog tree, browse to SIMATIC 300 → CPU-300 → CPU 314C-2PN/DP → 6ES7 314-6EH04-0AB0 and drag the new CPU into slot 2.
4. When prompted, accept any Hardware Support Package (HSP) update.
5. Save the hardware configuration (Station → Save).

Step 2 - First Compile to Identify Address Conflicts

1. Choose Station → Save and Compile. Do not download.
2. Open the compile log. Capture every warning to a text file for review.
3. Expect warnings in two categories: (a) "Address area differs" warnings on integrated digital I/O, and (b) warnings on technology object assignments.

Step 3 - Rewire the On-Board Digital I/O

1. With the CPU 314C-2PN/DP selected in HW Config, choose Options → Rewire.
2. Build the substitution table:
   • Old range 124 to 136, length 2 bytes (covers DI 124.x and DO 124.x of the 312C)
3. Confirm the operation. STEP 7 prints a summary of every symbol and block reference changed.
4. Save and re-compile HW Config.

Step 4 - Reconfigure the PROFINET Interface

1. In HW Config, double-click the PROFINET interface X1 of the new CPU.
2. Open Properties → Ethernet Address → IP Address and assign a static IP, for example 192.168.0.10 with subnet mask 255.255.255.0.
3. Confirm that no PROFINET IO system has been assigned to the X1 port yet.
4. If the CPU participates in an IO system, assign the PROFINET device name (for example cpu314c-1) using S7-PCT.

Step 5 - Convert the HMI Connection from MPI to PROFINET

1. Open NetPro from HW Config (button on the toolbar).
2. Locate the S7 connection between the CPU and the HMI station.
3. Right-click the connection → Connection Properties → Interface: PROFINET (Industrial Ethernet).
4. Update the partner IP to the CPU's new IP address.
5. Save and compile NetPro. The download-ready flag must turn green.
6. Open the HMI project (WinCC flexible or TIA Portal), change the connection type from MPI/DP to Ethernet, and enter the new CPU IP.
7. If the HMI project was created in WinCC flexible, transfer it to the panel before further work.

Step 6 - Reassign Technology Object Instance DBs

1. Return to HW Config and double-click the CPU 314C-2PN/DP.
2. Open the Technology tab. The 312C had 2 counters at channels 0 and 1; the 314C exposes 4.
3. Re-enable the two original counters (Channels 0 and 1) with the same configuration (24 V, single-ended, no inversion).
4. Enable channels 2 and 3 only if the program will reference them.
5. For PWM or frequency measurement, repeat the same enable and verify the SFB 47 / SFB 48 / SFB 49 instance DBs match.

Step 7 - Configure Analog Inputs and Outputs

1. In the CPU Properties dialog, switch to the Analog Inputs tab.
2. For each of channels AI0..AI3, set the measurement type: 0..10 V, ±10 V, 0..20 mA, or 4..20 mA as wired.
3. For AI4, set to RTD (Pt100 or NI1000) or to disabled if not used.
4. Set the integration time (default 60 Hz / 50 Hz rejection).
5. Open the Analog Outputs tab and configure AO0, AO1 as 0..10 V or 0/4..20 mA.

Step 8 - Compile, Download, and Go Online

1. Save and compile HW Config one last time. Verify zero "Address area differs" warnings on the integrated channels.
2. Select the CPU in HW Config and click Download. STEP 7 will first use the MPI/DP interface to load the project; switch the PG/PC interface to TCP/IP and re-download to verify the PROFINET path.
3. Place the CPU in RUN. The SF, BF, and MAINT LEDs must be off. PROFINET port LEDs (Link and Activity) must be lit at the HMI end.

HMI and PROFINET Cutover Details

The HMI link is the most failure-prone part of the migration because the operator panel often retains its old project and the wrong connection type. Apply the following steps in order:

1. Verify the HMI panel IP and subnet match the CPU. The default Comfort panel address is 192.168.0.10; the CPU must use a different address in the same /24 subnet.
2. In the HMI project, set the connection to "SIMATIC S7 300/400 over Ethernet."
3. Enter the CPU's PROFINET IP exactly; do not rely on name resolution.
4. Compile the HMI project and transfer it to the panel.
5. On the panel, open Control Panel → Network and verify the panel IP and subnet.
6. Start the runtime. The connection icon must turn green.

If the icon stays red, ping the CPU from the PG: ping 192.168.0.10. A reply confirms Layer 3; if it fails, check the switch port VLAN, the cable, and whether the CPU's PROFINET interface has been enabled in HW Config.

Technology Function Migration

The 312C technology functions and the 314C-2PN/DP technology functions use the same SFBs (SFB 47 counting, SFB 48 frequency, SFB 49 PWM) but the channel numbering starts fresh. The migration procedure is:

Open the technology DB and confirm the symbolic I/O field assigned to LADDR matches the new base address. The 314C-2PN/DP base addresses for the technology channels are visible under CPU Properties → Addresses.

Compile Warning Catalog and Resolution

Verification Procedure

Once the project is downloaded and the CPU is in RUN, validate the migration with the following sequence:

1. Online → Accessible Nodes: confirm the 314C-2PN/DP appears with its PROFINET IP.
2. Open OB 1 in online / monitor mode and verify every formerly 124.x bit reflects the corresponding 136.x bit.
3. In the VAT table VAT_Inputs, force one input at a time at the field end and read it back at I 136.0..137.7.
4. For each output Q 136.0..Q 137.7, use Force to energise and verify the field load.
5. Run a counter test: feed a 1 kHz pulse to the first counter and verify the technology DB value increments.
6. For PWM: set a 50% duty cycle at 1 kHz on the first PWM channel and observe with an oscilloscope.
7. Apply a known analog signal (e.g., 5 V from a calibrator) to AI0 and verify PIW 752 reports 50% of the configured range.
8. Drive AO0 from the program to 50% and verify the field device responds.
9. Confirm the HMI panel shows live values for at least 10 different tags.

Troubleshooting Matrix

Migrating to TIA Portal as an Alternative Path

Many plants are in the process of moving projects from STEP 7 V5.x to the TIA Portal. The same Rewire principle applies, but the tooling is different. To bring a STEP 7 project into the TIA Portal:

1. Open the TIA Portal and choose Migrate Project from the start screen.
2. Select the STEP 7 archive (.zip) and enable the "Include HW and Network data during the migration" option. See the official guide at Creating a migration file.
3. After migration, the project will appear in the TIA Portal with the 312C hardware. Replace the CPU with the 314C-2PN/DP and accept the address remap wizard.
4. Use the TIA Portal "Rewire" function in the device view to migrate the integrated I/O addresses.
5. Recompile and download.

The official Siemens migration guide is at Migration of projects with the TIA Portal. For background on the broader Siemens migration service offering, see Migration for Automation Systems.

Commissioning Checklist

Long-Term Maintenance Notes

After the migration is in production, document the following for the maintenance crew:

• CPU MLFB and firmware version (6ES7 314-6EH04-0AB0, firmware V3.3.x or later).
• Updated I/O address map: digital at 136.x, analog at 752.x.
• Static IP address of the PROFINET interface.
• Device name of the CPU on the PROFINET IO system.
• Mapping table for every tag renamed by the Rewire (124.x → 136.x).
• Backup archive name and location of the migrated STEP 7 project.
• HMI project archive (if migrated).

FAQ