Configuring S7-300 Ethernet with CP 343-1 in STEP 7 V5.x

Overview

The SIMATIC S7-300 family does not have an integrated Ethernet port on the standard CPU. To participate in industrial Ethernet, a S7-300 station needs a CP 343-1 communications processor mounted in the central rack (or, for distributed I/O, in an ET 200M). The CP is configured inside the STEP 7 V5.x engineering suite using HW Config for the hardware slot and NetPro for the logical connection. This reference walks through a complete first-time commissioning: from part-number selection and rack planning, through IP address assignment, all the way to the call interface in the user program (LAD/FBD/STL).

The article is written for an engineer who is new to the S7-300 platform but already familiar with industrial control and basic TCP/IP networking. The default engineering tool is STEP 7 V5.5 + SP2 (or newer SP/HF) with the optional NCM S7 add-on, which is normally installed alongside STEP 7 from the same media. TIA Portal also supports the S7-300, but the workflow, catalog paths, and block names differ; the V5.x path is documented here because the source material targets STEP 7.

Prerequisites

CP 343-1 Hardware Family and Selection

The CP 343-1 family has several variants; choosing the right one avoids over-spending and avoids missing features the application requires.

For a beginner doing first-time S7-300 Ethernet, the CP 343-1 Lean (6GK7343-1EX30-0XE0) is the lowest-cost option and is fully supported by STEP 7 V5.5. If you need to connect a remote S7 station through this CP (S7 routing), the standard CP 343-1 is required; the Lean variant does not route S7 connections across subnets.

Step 1 — Create the Station in STEP 7

1. Launch SIMATIC Manager. File → New → Project. Name the project (e.g. PlantLine1_S7-300).
2. Right-click the project, choose Insert New Object → SIMATIC 300 Station. A new station icon appears in the project tree.
3. Open the station. Double-click Hardware to launch HW Config.
4. Insert a rail (RACK-300, article 6ES7 390-1AF30-0AA0) from the catalog: drag RACK-300 onto the empty rack frame.
5. Slot 1: PS 307 (e.g. 6ES7 307-1EA01-0AA0, 5 A).
6. Slot 2: CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0) or any CPU 31x. When using a CPU 31x-2 PN/DP the second PROFINET interface is on the CPU, not the CP; the CP is a separate network path.
7. Slot 4: CP 343-1 Lean (6GK7 343-1EX30-0XE0). Drag from SIMATIC 300 → CP-300 → Industrial Ethernet → CP 343-1 Lean.
8. Double-click the CP to open its properties. Set the IP address (e.g. 192.168.0.10), Subnet mask (255.255.255.0), and optional Router address if the plant has a backbone.
9. Save and compile the station. Station → Save and Compile. Address assignment is written to the CP at download time.

Step 2 — Set the CP as a PROFINET/IE Node

The CP 343-1 (Ethernet) appears in HW Config as an Ethernet node. In the Properties dialog, switch to the Properties → Ethernet Interface → General tab and verify the following:

The CP can be assigned its IP address from three sources, in order of priority: (1) configuration in STEP 7, (2) DHCP, (3) factory default (0.0.0.0). For a fixed plant, always assign from STEP 7 so the configuration is reproducible.

Step 3 — Build the Logical Connection in NetPro

After hardware compile, open NetPro (menu Options → NetPro). The S7-300 station appears with its CP 343-1 as an Ethernet node. To add a partner:

1. Insert a second S7 station, an HMI station, or a third-party node that uses the same Ethernet subnet.
2. Select the CP 343-1 of the S7-300, right-click the connection table, Insert New Connection.
3. In the dialog choose the partner node (e.g. an S7-1200 CPU on the same subnet, or a CPU 317 in another S7-300 station).
4. Select the connection type:

After confirming, NetPro shows the connection with a green/yellow line. Save and compile NetPro (Network → Save and Compile) so the connection ID, partner IP, and local/remote TSAP/port are written into the S7-300 station data.

Step 4 — Download the Configuration to the CPU and CP

1. In HW Config, click Download to Target → Selected Target Device. The download dialog appears.
2. If this is the first download, Select Target System → Accessible Nodes must find the CPU on MPI/Profibus. If you only have Ethernet, plug your PC into the same switch as the CP and target the CP's IP directly; the CP will then route the download to the CPU via the backplane.
3. Select the interface: Ethernet → TCP/IP → Network card of PG/PC.
4. Enter the CP's IP (192.168.0.10) and press Display. The CP replies with its MAC.
5. Confirm download. STEP 7 writes the hardware configuration first, then the connection data, then the user program (blocks).

Step 5 — Call the Communication Blocks in the User Program

The connection made in NetPro exposes an ID (local connection ID) used by the CP blocks in the user program. The three most common S7-300 user-program patterns are listed below.

5.1 S7 Communication: PUT / GET (with partner S7)

Use FB 15 (PUT) and FB 14 (GET) from the Standard Library. These are integrated in every S7-300 CPU and do not require a CP — the CP is just a transport. They are easiest when both sides are Siemens S7.

// STL excerpt: read 10 bytes from partner DB20 starting at byte 0
      CALL  "PUT"
           REQ    :=M0.0        // rising edge triggers send
           ID     :=W#16#0001   // must match NetPro connection ID
           DONE   :=M10.0
           ERROR  :=M10.1
           STATUS :=MW12
           ADDR_1 :=P#DB20.DBX0.0 BYTE 10
           SD_1   :=P#M100.0 BYTE 10
           LEN    :=10

5.2 ISO-on-TCP / TCP with CP 343-1 (lean variants)

The CP requires its own AG_SEND / AG_RECV function blocks (FC 5 / FC 6) for open TCP/ISO transport when no S7 partner is involved. Both are part of the SIMATIC_NET_CP library that ships with STEP 7.

// STL: send 50 bytes to a partner socket
      CALL  "AG_SEND"
           AG    :=W#16#0001   // CP 343-1 logical address from HW Config
           ID    :=1            // connection ID from NetPro
           LEN   :=50
           DONE  :=M20.0
           ERROR :=M20.1
           STATUS:=MW22
           SEND  :=P#DB100.DBX0.0 BYTE 50

On the receive side, AG_RECV (FC 6) pulls bytes from the receive buffer. Always allocate the receive buffer with adequate length (a few hundred bytes is safe for short protocols).

5.3 UDP with CP 343-1

UDP uses AG_SEND (FC 5) and AG_RECV (FC 6) with a connection configured as UDP in NetPro. UDP has no DONE handshake; the receive is edge-driven by the LEN output of AG_RECV transitioning from 0 to a non-zero value.

Step 6 — Configure Time Synchronization (Optional)

The CP 343-1 supports SIMATIC time-of-day and NTP (NTP only on the Advanced variant). To enable NTP on the Advanced CP:

1. Open HW Config and double-click the CP 343-1 Advanced.
2. Properties → Time-of-Day Synchronization → NTP mode.
3. Enter the NTP server address (e.g. 192.168.0.250) and the synchronization interval (default 10 minutes, range 1-1440).
4. Set the time zone offset. Save and download.

Step 7 — Verify the Connection

LED Diagnosis and Buffer Entries

The CP 343-1 has four status LEDs. Their meaning is identical across the Lean/standard/Advanced variants.

If SF is on, open the S7-300 diagnostic buffer (online) and look for CP entries. Common diagnostic event IDs:

Common STATUS Words for AG_SEND / AG_RECV

Troubleshooting Matrix

Security and Modern Considerations

Firmware on the CP 343-1 family is mature; do not expose an S7-300 CP directly to the public internet or a hostile subnet. Mitigations to apply at commissioning:

• Use a dedicated plant VLAN with private RFC 1918 addressing.
• Disable unused services on the CP (e.g. FTP, web server) in HW Config → Properties → Services.
• Activate the Access Protection password so unauthorized S7 partners cannot be defined.
• On the Advanced CP, enable the integrated firewall with allow-list of partner IPs.
• Use S7-1200/1500 or SCALANCE SFPs at the plant boundary; do not terminate the S7-300 CP at the WAN edge.

Field-Commissioning Short List

1. Verify rack order, slot occupancy, and PS 307 sizing (5 A standard, 10 A for many I/O + CP).
2. Confirm CP MAC sticker matches HW Config.
3. Pre-stage IP plan; reserve DHCP exclusions for the CP block.
4. Compile HW Config → Compile NetPro → Download both.
5. Check the four CP LEDs, then read CPU diagnostic buffer for any new entries.
6. Run a test S7 connection (e.g. an HMI tag polling) and watch NetPro Connection Status.
7. Document the connection IDs, partner IPs, and TSAPs in the cabinet drawing.
8. Hand off to operations with a network diagram, including switch, VLAN, and the CP's IP.