S7-1500 to ControlLogix Ethernet Communication Without a Gateway

Overview

Direct peer-to-peer Ethernet communication between a Siemens SIMATIC S7-1500 and an Allen-Bradley (Rockwell Automation) ControlLogix L6x/L7x/L8x processor is technically possible without any third-party protocol gateway. Three production-grade paths exist on a standard 10/100/1000 Mbit/s TCP/IP Ethernet network:

1. EtherNet/IP scanner/adapter on the S7-1500 (firmware V2.5 and later for scanner, V2.6 for adapter in TIA Portal V15.1 and later) talking to a ControlLogix EtherNet/IP adapter on the 1756-EN2T or 1756-EN2TR module, or to the on-board port of an L8xE processor.
2. Open User Communication (OUC) over ISO-on-TCP (RFC 1006), TCP, or UDP using the S7-1500 TSEND/TUSEND, TRCV/TURCV and TMAIL instructions on one side and a MSG instruction or socket API on the ControlLogix side.
3. Modbus TCP using the S7-1500 MB_CLIENT/MB_SERVER instructions paired with a Modbus TCP-capable AOI or instruction on the ControlLogix side.

This reference covers each path with hardware prerequisites, TIA Portal and Studio 5000 configuration blocks, register/tag mapping, data-size limits, error codes, and a field-tested troubleshooting matrix.

Communication Path Comparison

Path 1 — EtherNet/IP Scanner on S7-1500

The S7-1500 CPU family gained native EtherNet/IP connectivity through PROFINET/Ethernet-IP linkage. The integrated PROFINET interface of an S7-1511, S7-1513, S7-1515, S7-1516, S7-1517, S7-1518 or the rack-mounted S7-1505-S (CPU 1505 SP) supports the CIP (Common Industrial Protocol) layer and presents itself as a Class 1 EtherNet/IP scanner in TIA Portal V15.1 and later. Firmware prerequisites:

• S7-1500 CPU firmware V2.5 or higher for EIP_SCANNER.
• S7-1500 CPU firmware V2.6 or higher and TIA Portal V15.1 or higher for EIP_ADAPTER (so that an AB scanner can consume S7-1500 tags).
• 1756-EN2T firmware V5.007 or higher recommended for stable Class 1 connections to third-party scanners.

Library blocks used from the "EtherNet/IP Scanner / Adapter" library in TIA Portal:

Reference Siemens entry 109739495 — Connecting S7-1200 / S7-1500 to a third-party EtherNet/IP device (Entry ID 108740380) for the canonical configuration walk-through and connection parameter spreadsheet.

Step-by-step: S7-1500 as Scanner to ControlLogix

1. In TIA Portal, open Devices & Networks, add the S7-1500 CPU, drag the EtherNet/IP scanner from the catalog onto the PROFINET interface. Assign the scanner IP (e.g. 192.168.0.10/24).
2. Add a ControlLogix station entry with IP 192.168.0.20/24, vendor ID 0x0001 (Rockwell Automation/Allen-Bradley), device type 0x000C (generic CIP adapter) or 0x000E (drive). For a 1756-EN2T, choose vendor 0x0001, device type 0x000E or the generic adapter type and supply the assembly instance numbers from the Studio 5000 project (Input 100, Output 101, Configuration 102, Heartbeat 199 are typical for explicit messaging only).
3. Configure Connection parameters: RPI ≥ 5 ms (per practical field experience with non-Rockwell scanners), Output assembly (T→O) instance 101 length = number of bytes to write, Input assembly (O→T) instance 100 length = number of bytes to read, Configuration assembly 102 length 0, timeout multiplier 4, transport class 1.
4. Map the assemblies to PLC tags of matching byte length (max 480 bytes per direction per CIP connection — the S7-1500 implementation limit observed on tested firmware V2.6 and V2.9). For larger payloads, instantiate multiple connections.
5. Call EIP_SCANNER from OB1 (or a cyclic OB with the same scan time). Wire REQ to a one-shot trigger, supply CONNECTION_HANDLE from EIP_CONNECT, supply source/target buffers as VARIANT pointers (e.g. P#DB100.DBX0.0 BYTE 480).
6. Download the project and observe the scanner status word (STATUS = 16#0000 when connected, non-zero CIP error otherwise).

Step-by-step: S7-1500 as Adapter to an AB Scanner

1. Add the S7-1500 CPU and on the same PROFINET interface, switch the role to EtherNet/IP adapter. TIA Portal will create a default CIP assembly template with instances 100 (input), 101 (output), 102 (configuration).
2. Resize the assembly lengths to match the application. The library enforces a 480-byte ceiling per direction.
3. Export the EDS file (or vendor ID + device type + instance numbers) and import it into Studio 5000 via EDS Hardware Installation Tool.
4. In Studio 5000, add a new Ethernet Module under the 1756-EN2T or L8xE backplane. Pick the imported EDS, set Connection Parameters: Input = instance 101 (data S7 sends), Output = instance 100 (data S7 receives), Configuration = instance 102, RPI = 50 ms starting point.
5. Use the generated I/O tags (e.g. I_Scanner:I, O_Scanner:O) in ladder or AOI logic.

Path 2 — Open User Communication (OUC) over ISO-on-TCP / TCP / UDP

Open User Communication is the legacy Siemens term for socket-style communication that does not require an additional CP module on the S7-1500 side. The on-board PN interface of every S7-1500 CPU (and S7-1200 V4.0+) supports it natively. The blocks involved are summarised below:

On the ControlLogix side, a MSG instruction with Service = SOCKET_CREATE / SOCKET_ACCEPT / SOCKET_SEND / SOCKET_RECV / SOCKET_CLOSE (CIP Sync, requires 1756-EN2T firmware V5.011+ for full SOCKET services) handles the equivalent function. For older 1756-EN2T firmware, write your own socket AOI using the Socket_Open instance or use third-party AOI bundles such as the public Rockwell sample "Socket Messaging AOI".

TIA Portal OUC Configuration

1. In Devices & Networks > S7-1500 > Properties > Ethernet addresses, enable Use router only if the AB device is on a different subnet.
2. Insert the desired instruction (TSEND_C + TRCV_C) into a function block such as FB100 "OUC_AB_Comms".
3. Right-click on the CONNECT pin → Connection properties → Partner. Specify Partner IP (e.g. 192.168.0.20), Partner port (e.g. 2000), Local port (e.g. 2001), Connection type = ISO-on-TCP for parity with legacy AB integration, or TCP if the AB side uses CIP socket or a TCP-capable AOI.
4. Set TSAPs: Local TSAP = 10.00 hex is the S7 default; for AB use the equivalent CIP path or AOI convention (the TSAP bytes are sent but AB MSG-based sockets ignore them).
5. Map send/receive buffers to DBs. Example: P#DB101.DBX0.0 BYTE 256.
6. Run TSEND_C in OB1 (rising edge on REQ to send). Place TRCV_C in OB1 with EN_R = TRUE for continuous receive.

Studio 5000 OUC Configuration

1. Confirm the 1756-EN2T module is in the chassis, double-click → Connection tab → verify IP Address matches the partner IP entered on the S7 side.
2. Create an AOI or a routine containing ladder logic for the MSG instruction with the SOCKET service class. A skeleton:

MSG(SocketService := SOCKET_CREATE,
      SocketInstance := SocketInst,
      Server := 0,                // 0 = client mode
      RemotePort := 2001,
      RemoteAddress := '192.168.0.10',
      LocalPort := 2000,
      Status => MsgStatus);

3. On a successful create, perform SOCKET_SEND with the byte payload as the SourceBuffer. For each poll cycle, perform SOCKET_RECV and copy the received buffer into a tag of matching length. Close the socket only on shutdown.
4. If your 1756-EN2T firmware does not expose SOCKET services, use the CIP Generic MSG against a CIP path of the AB-side Ethernet module with service 0x4C (CIP encapsulation socket open) — see EN2T user manual Rev. 1756-UM540.

Reference OUC Example for HMI Integration

The AutomationDirect C-more HMI setup guide for Siemens Ethernet ISO over TCP/IP (S7-1200 or S7-1500) documents the equivalent connection parameters (port 102, TSAP 10.00 / 10.01) and is useful for sanity-checking the TIA side of any ISO-on-TCP socket between an S7-1500 and a third-party device.

Path 3 — Modbus TCP

Modbus TCP is the lowest-friction option when both vendors are accepted but the firmware revision of the ControlLogix is not known. Siemens S7-1500 provides the MB_CLIENT and MB_SERVER instructions (under Communication > Modbus TCP) natively on every firmware V1.7+ CPU. The ControlLogix side requires either a Rockwell-published AOI ("Modbus TCP AOI") or a vendor AOI from Prosoft, HMS, or Red Lion. For systems where neither AOI can be added, fall back to OUC.

1. Add MB_CLIENT to the S7-1500 program. MB_Unit_ID = 255 for TCP, REQ pulsed, MB_DB_ADDR = the Modbus function code (16#03 = read holding registers, 16#06 = write single, 16#10 = write multiple).
2. Set Connect_ID unique (1, 2, 3...), IP address 192.168.0.20, port 502.
3. Map MB_DATA_PTR to a global DB of WORD registers. Each Modbus register consumes one WORD in the Siemens DB, while each AB-side SINT/INT/DINT needs explicit byte-swap consideration (Modbus is big-endian, ControlLogix is little-endian).
4. Verify with the standard Modbus diagnostic DONE, BUSY, and ERROR outputs.

Hardware & Firmware Prerequisites

Data Layout & Byte Order

Every byte transferred between the two controllers requires attention to two distinct ordering conventions:

• Siemens S7-1500: Big-endian at the bit and byte level, but the on-board PROFINET stack is little-endian for CIP and ISO-on-TCP payloads. Use SWAP instructions or WORD_TO_INT for explicit conversion.
• Allen-Bradley ControlLogix: Little-endian throughout. Tags of type SINT[], INT[], and DINT[] consume memory in low-to-high byte order.

When mapping a Siemens WORD to an AB INT, a single SWAP on the Siemens side or a SWAP_INT AOI on the AB side normalises the value. When mapping a Siemens REAL (DWORD) to an AB REAL (32-bit float), the same SWAP operation is required because the IEEE 754 byte order differs.

Performance & Throughput

Verification Steps

1. Layer 1 / 2: confirm physical link (green/yellow LEDs on both PN ports).
2. Layer 3: ping both controllers from a maintenance laptop. On the S7-1500 use Online & Diagnostics > Functions > Ping; on the ControlLogix use Studio 5000 > RSLinx Classic > Ethernet Devices > Browse.
3. Layer 4: from a maintenance laptop run telnet <ControlLogix IP> 2000 to verify the AB-side socket port is reachable (only valid for OUC path).
4. Application: insert a 4-byte counter in both projects. Force a known value on the source, observe it on the destination within one RPI/poll cycle.
5. Diagnostics: S7-1500 TSEND_C.STATUS = 16#0000, DONE = TRUE pulse on success. ControlLogix MSG instruction .ER = 0, .DN = 1 on completion. For EtherNet/IP scanner, watch the STATUS output of EIP_SCANNER: 16#0000 = running, 16#FF01 = not connected, 16#FF02 = connection timeout.

Troubleshooting Matrix

Safety-Critical Considerations

Recommended Path by Application

• High-speed I/O mirror (≤ 480 bytes, ≤ 10 ms): EtherNet/IP scanner/adapter, S7-1500 FW V2.6+.
• Large block data exchange, recipe transfer, fault logs: ISO-on-TCP OUC with TSEND_C / TRCV_C.
• Mixed fleet, brownfield, unfamiliar firmware: Modbus TCP with vendor AOIs.
• Best-effort telemetry, heartbeats, HMI traffic: UDP TSEND/TURCV.

FAQ