Why OS Compile Wizard Insists on Creating ISO Connections with MAC Addresses in PCS 7

In SIMATIC PCS 7, the OS Compile Wizard automatically attempting to generate an ISO Named Connection (using MAC addresses) instead of a standard TCP connection occurs when the controller's communication processor (CP 443-1) or the WinCC OS station contains an Industrial Ethernet interface with ISO protocol enabled, or when NetPro lacks a compiled S7 Connection based on TCP/IP. In NetPro, PCS 7 prioritizes ISO transport protocol whenever an Industrial Ethernet subnet has the 'ISO protocol' checkbox enabled in its properties or when the OS station uses a CP 1623. To force the OS Compile Wizard to use TCP/IP connections, configure the S7 Connection in NetPro explicitly over the IP interface, ensure the CP 443-1 and WinCC Application are linked via TCP/IP, and select the IP-based connection in the OS Compile connection assignment dialog.

Executive Summary

During plant bus engineering in SIMATIC PCS 7, network administrators frequently implement routable IP networks to interconnect automation controllers (such as AS 410 or S7-400 with CP 443-1) with WinCC OS Servers. Even after explicitly configuring TCP/IP addresses and standard S7 connections in NetPro, engineers executing the OS Compile Wizard are often baffled to discover that the wizard repeatedly attempts to assign an ISO connection referencing Layer 2 MAC addresses. This causes compilation errors or prevents communication across routed subnets. This technical article analyzes PCS 7 connection prioritization rules, explains why the compiler defaults to ISO protocols, and provides the step-by-step procedure to enforce TCP/IP connections.

Architecture and Fundamental Mechanics

The communication connection assignment engine in the PCS 7 OS Compile Wizard evaluates multiple network parameters:
1. The ISO vs TCP/IP Protocol Distinction in S7 Communication:
- ISO Transport Protocol (RFC 905 / ISO 8073): Operates at OSI Layer 2 using hardware MAC addresses. Highly deterministic and non-routable. Historically favored by Siemens for high-speed plant bus communications.
- ISO-on-TCP (RFC 1006 / TCP/IP): Operates at OSI Layer 4 over IP addresses (Port 102). Fully routable across enterprise switches and VLANs.
2. Connection Hierarchy and Auto-Selection Rules:
- During OS compilation, the wizard queries the NetPro connection table for the selected AS-OS pair.
- If the AS station's communication interface (e.g., CP 443-1 or CPU onboard PN-IO) is connected to a subnet that has the ISO protocol active, or if the OS PC Station utilizes a CP 1623/1613, the wizard prioritizes the ISO connection by default.
- If both an ISO connection and a TCP connection exist in NetPro, or if the CP 443-1 parameters have 'MAC address / ISO protocol' enabled, the OS Compile Wizard pre-selects the ISO endpoint unless explicitly redirected.
3. Virtualization and Modern Plant Bus Requirements:
- In virtualized or routed environments (VMware ESXi), ISO protocols cannot traverse Layer 3 virtual switches or routers. A routable TCP connection is mandatory.

Step-by-Step Implementation Procedure

Inspect NetPro Connection Table for the AS-OS Pair:
a. In SIMATIC Manager, open NetPro for the Multiproject.
b. Click on the WinCC OS Server station.
c. In the lower connection table, inspect existing connections to the target AS station.
d. Verify whether the connection type is configured as S7 connection and note whether the interface path references ISO or TCP/IP.
e. If an old ISO-based S7 connection exists, delete it.
Configure Dedicated S7 TCP Connection in NetPro:
a. With the OS Server selected, right-click an empty row in the connection table and select Insert New Connection.
b. Select the target AS CPU (e.g., AS410 / CPU 410-5H).
c. Set 'Type' to S7 connection.
d. In the connection properties window, under 'Route', verify:
- Local endpoint: WinCC Application -> IE General or CP 1623 via IP address.
- Partner endpoint: CP 443-1 or CPU onboard PN via IP address.
  e. Assign a clear connection name (e.g., AS410_TCP).
  f. Execute Network -> Save and Compile.
Verify Subnet Properties in HW Config:
a. Open HW Config for the AS station.
b. Double-click the CP 443-1 (or CPU PN-IO interface) to open Properties.
c. In the 'General' tab, click Properties under Interface.
d. If the subnet must operate strictly on IP, verify whether 'Set MAC address / use ISO protocol' can be disabled.
e. Confirm that a valid, static IP address and subnet mask are assigned.
Assign Connection in OS Compile Wizard:
a. In SIMATIC Manager, right-click the OS Server object and select Compile.
b. Proceed to the wizard step 'Select the network connection for each S7 program'.
c. Locate the target AS controller in the list.
d. Click the 'Connection' drop-down list: do not accept the default selection.
e. Explicitly select the newly created S7 connection (TCP/IP) referencing IP addresses rather than the MAC address.
Execute OS Compilation:
a. Select 'Scope: Entire OS' (or Changes).
b. Click 'Compile' and verify that the wizard builds tags using the TCP/IP channel unit (TCP/IP under SIMATIC S7 Protocol Suite in WinCC Tag Management).

Verification

Open the WinCC project on the ES: navigate to Tag Management -> SIMATIC S7 Protocol Suite -> TCP/IP.
Confirm that the S7 connection object (AS410_TCP) appears under the TCP/IP channel unit, rather than under 'Named Connections (ISO)'.
In WinCC Runtime, open the 'Channel Diagnosis' tool: verify that the connection status reports 'Connected' and indicates IP communication.

Limitations and Operational Precautions

If the AS utilizes redundant CP 443-1 modules in an S7-400H configuration without PROFINET, Named Connections utilizing ISO transport may be required for automated fault-tolerant switchover unless redundant TCP connections are engineered.

Engineer FAQ

Why does the OS Compile Wizard default to ISO instead of TCP?

Because the Industrial Ethernet subnet or the CP 443-1 has the ISO protocol enabled in HW Config, which PCS 7 prioritizes by default.

Can ISO connections communicate across network routers?

No. ISO transport protocol operates at Layer 2 using MAC addresses and cannot cross Layer 3 IP routers or routed VLANs.

Where do I select between ISO and TCP in the OS Compile Wizard?

In the step 'Select network connections', click the connection drop-down for each AS and select the TCP/IP S7 connection.

What channel unit in WinCC handles TCP/IP connections to S7-400?

The 'TCP/IP' channel unit located within the SIMATIC S7 Protocol Suite.

Does changing from ISO to TCP require downloading NetPro to the AS?

Yes. Any changes to the S7 connection table in NetPro must be saved, compiled, and downloaded to both the AS and OS stations.

Technical References

Related Tags

OS Compile Wizard
ISO Connection
TCP/IP
NetPro
CP 443-1
Plant Bus

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Why OS Compile Wizard Insists on Creating ISO Connections with MAC Addresses in PCS 7