CP 343-1 Lean Maximum Bytes Per Telegram: S7-300 SCADA Guide

Overview

The SIMATIC CP 343-1 Lean (6GK7343-1CX10-0XE0) is the entry-level Industrial Ethernet / PROFINET communications processor for the S7-300 platform. It is frequently deployed as a low-cost gateway between an S7-300 CPU (for example 315-2 PN/DP) and a third-party SCADA package such as Wonderware InTouch, System Platform, or AVEVA Edge. The two hard constraints that dominate every integration project are:

1. Maximum 208 bytes of user data per S7 telegram (PDU) when the S7-300 is the server of an S7 Put/Get or Fetch/Write request.
2. Backplane throughput of the S7-300 rack, which is shared between the CPU, the CP, and any other CPs/IMs installed in the same station.

Both limits are imposed by the S7-300 architecture and by the S7 communication protocol stack. They are not bugs in the CP 343-1 Lean firmware and they cannot be raised by configuration. Any tag whose effective size exceeds 208 bytes will fail at runtime with a communication error.

CP 343-1 Lean Hardware Variants and Order Numbers

All variants ship with one RJ45 port operating at 10/100 Mbit/s auto-negotiation, half- or full-duplex. The CP supports S7 communication (server + client), TCP, UDP, ISO-on-TCP (RFC 1006), and PROFINET IO Controller/Device (firmware-dependent). PROFINET IO has no influence on the S7 PDU limit discussed in this article.

S7-300 PDU Size and the 208-Byte Limit

S7 communication between a CP 343-1 Lean (server) and a SCADA station (client) uses the S7 protocol on top of ISO-on-TCP (port 102, TSAP-based addressing). Every S7 request/response is wrapped in a Protocol Data Unit (PDU). The maximum PDU length for the S7-300 family is fixed at 240 bytes, of which the S7 header consumes 28 bytes, leaving approximately 208 bytes of payload for the user data area.

Consequence: any single Put/Get/Fetch/Write operation that asks for more than 208 bytes of process data will be rejected. The CP 343-1 Lean logs error 0x031E / 0x031F in the diagnostic buffer; the SCADA side typically surfaces a WSAECONNRESET, 0x80040154 (class not registered / OPC quality BAD), or a tag value of ??? with quality OPCNOTINUSE.

Tag Sizing Rules for Wonderware / Third-Party SCADA

When configuring tags in Wonderware DASIDirect, DAServer for SIMATIC S7 MPI/TCP, or the ArchestrA S7 TCP IO server, observe the following rules:

• Boolean (1 bit): 1 byte is reserved per item in the request; group up to 208 booleans per read.
• Integer (16 bit): 1 item = 2 bytes; up to ~104 items per PDU.
• Real (32 bit): 1 item = 4 bytes; up to 52 floats per PDU.
• String (default 254-byte DB string): exceeds the PDU. Use a length prefix of <= 200 characters in the data block.
• Raw data block slices: stay below 208 bytes per slice.

For Wonderware InTouch, the AVEVA Connectivity Channel (formerly Wonderware) implements an automatic segmentation algorithm that splits a large tag into multiple 208-byte reads. This works but increases scan time linearly with the number of segments. For latency-sensitive applications, manually segment the DB into multiple smaller tags.

Backplane Throughput: The Real Bottleneck

The S7-300 backplane is a parallel bus with a peak throughput of approximately 2 Mbyte/s in practice (theoretical max ~6 Mbyte/s for the central rack). Every read/write from the SCADA traverses the following path:

SCADA  <-- 10/100 Mbit Ethernet -->  CP 343-1 Lean  <-- S7-300 backplane -->  CPU 315-2 PN/DP  <-- backplane -->  SM/DM/CP modules

Every Put/Get requires a request/response handshake. A typical 208-byte read takes 8-15 ms measured at the SCADA side, of which roughly 60% is backplane transit. At a 1-second scan rate with 50 tags (each forced to its own 208-byte PDU), the backplane load is ~50% which is already at the edge of stability. If the CPU is also running PROFINET IO or a slow OB1 cycle, dropped requests become common.

Number of Simultaneous S7 Connections

A single Wonderware IO server consumes one S7 connection per logical device. If the SCADA opens redundant IO servers, an HMI panel, and a programming PG, you can exhaust the 4-connection limit of a V1.0 CP within a single commissioning session. Upgrade to firmware V2.x or V3.x and configure at least 8 connections for any production SCADA project.

Recommended DB Layout for SCADA

Lay out the data block used by Wonderware in fixed 200-byte slices to leave headroom for the S7 header and to keep each slice well inside the PDU limit:

DB1000  // SCADA interface, 5 slices x 200 bytes = 1000 bytes total
  Slice_0  : BYTE 0..199   // Process values block A
  Slice_1  : BYTE 200..399 // Process values block B
  Slice_2  : BYTE 400..599 // Process values block C
  Slice_3  : BYTE 600..799 // Process values block D
  Slice_4  : BYTE 800..999 // Process values block E
END_VAR

Each slice is accessed by one Put/Get request (200 < 208) and yields a clean, predictable scan time. Avoid ANY pointers that span more than 208 bytes; the S7-300 CPU will return W#16#8090 / W#16#80A1 at the AG_SEND / AG_RECV block if the length is invalid.

Configuration in STEP 7 (Classic) and TIA Portal

For STEP 7 V5.x:

1. Insert the CP 343-1 Lean in HW Config next to the CPU. The CP occupies one slot in the S7-300 rack.
2. Open the CP properties > Connection tab. Enable S7 communication (server).
3. Configure the IP address and subnet mask. Default router is optional.
4. Set the maximum number of S7 connections to the licensed value (4 / 8 / 16).
5. Download the HW config. Cycle power is not required.

For TIA Portal (V14 or later):

1. Add the CP 343-1 Lean from the hardware catalog > SIMATIC S7-300 > Communication modules.
2. In the device properties, open Ethernet addresses and assign the IP.
3. Open Ethernet interface > S7 communication and tick Permit Put/Get communication. This is the master switch for any third-party Put/Get including Wonderware.
4. Optionally activate the OPC UA server function (FW 3.0+ only).

Wonderware (AVEVA) IO Server Setup

1. Install the DAServer for SIMATIC S7 MPI/TCP (formerly Wonderware DAServer, part of the Connectivity pack).
2. Create a new Siemens TCP/IP port. Enter the CP IP address and the TSAP 03.01 for the CPU slot 1, or 03.02 for slot 2 (where the CP is installed in the rack).
3. In the Device Group, define a device with a primary address that matches the CPU slot (1 or 2).
4. Create an Address for every DB slice: e.g. DB1000, BYTE 0, LENGTH 200, TYPE USINT.
5. Map the address to an InTouch tag of type Memory Discrete or Memory Real as appropriate.
6. Set the scan rate to 1000 ms minimum. 500 ms works on a quiet S7-300; below 200 ms risks request pile-up.

Verification and Diagnostic Procedure

1. Verify the PDU limit at runtime. From a Windows PC with the S7-300 OPC Scout or a simple Put/Get test tool, request 209 bytes. The request must fail with 0x03 / 0x01 "Object does not exist" or similar. Request 208 bytes: it must succeed. This proves the 208-byte ceiling.
2. Verify backplane load. Open STEP 7 > CPU > Module Information > Diagnostics. Check the Communication tab. The "S7 communication load" should remain < 50% under steady-state SCADA traffic.
3. Verify connection count. In the CP diagnostics buffer, count active S7 connections with Diagnostic > Connections. The value must stay <= the licensed maximum.
4. Verify tag quality in InTouch. In the dbview tool, every tag should show quality Good and a timestamp that updates each scan. A ??? or stale timestamp indicates a PDU overflow or connection drop.

Troubleshooting Matrix

Migration Path: When to Move Off the CP 343-1 Lean

If the SCADA tag count exceeds roughly 500 tags or the project requires redundant acquisition, the CP 343-1 Lean is the wrong product. Migrate to:

• CP 343-1 (full version, 6GK7343-1EX30-0XE0): same 208-byte PDU but more connections (32), integrated 4-port switch in some variants, and faster backplane handling.
• CP 343-1 Advanced (6GK7343-1GX30-0XE0): 32 connections, FTP, web diagnostics, security functions.
• S7-1500 with onboard PROFINET (e.g. CPU 1515-2 PN): PDU size grows to 960 bytes in S7-1500, backplane is far faster, OPC UA server is built in. This is the recommended target for any greenfield Wonderware / AVEVA System Platform deployment in 2024 and later.

Field-Commissioning Checklist

1. Confirm CP 343-1 Lean firmware version with Module Information > Identification. Record FW, MLFB, and serial number.
2. Verify the IP address with Ping from the SCADA node. A ping works even when S7 communication is misconfigured, so do not stop here.
3. Run OPC Scout on the SCADA PC and add the CP as a server. Browse for DB1000. If DB1000 does not appear, the Put/Get permission is missing.
4. Read 208 bytes from DB1000 BYTE 0. Verify the read completes in < 50 ms on a quiet network.
5. Read 209 bytes from the same address. Verify the diagnostic buffer records a rejected PDU.
6. Start the Wonderware IO Server and bind to the CP. Watch the IO Server log for "Connected" / "Disconnected" transitions over a 10-minute burn-in.
7. Stress test with 100% tag polling for 30 minutes. Watch the CPU's S7 communication load; it should plateau < 60%.
8. Document the final DB layout, TSAP, IP, and connection count in the project handover.

Key Constraints Recap

• Maximum user data per S7 telegram: 208 bytes (PDU envelope 240 bytes).
• Maximum S7 connections: 4 / 8 / 16 depending on CP 343-1 Lean firmware.
• Backplane: S7-300 central rack only; the CP cannot be placed behind an IM 360/361 expansion rack for fast Put/Get traffic.
• Ethernet: 10/100 Mbit/s, half/full duplex, RJ45.
• PROFINET IO: supported on FW V2.0+; counts against the same connection pool.
• OPC UA server: supported on FW V3.0+; still subject to the 208-byte PDU when reading classic S7 areas.