Resolving NetPro Error 276:27 When Configuring Fault-Tolerant S7 Connections to CPU 410-5H

NetPro Error 276:27 ('Connection could not be established / Protocol not supported') occurs when attempting to configure a Fault-Tolerant S7 Connection between an OS Server (equipped with CP 1623 or CP 1628) and a CPU 410-5H using the Layer-4 ISO Transport protocol. The integrated PROFINET/Ethernet interfaces of CPU 410-5H natively support TCP/IP and ISO-on-TCP (RFC 1006), but do not support pure MAC-based ISO transport. To resolve this error, configure the S7 Fault-Tolerant Connection in NetPro using TCP/IP or ISO-on-TCP addresses, verify that the PC Station network adapter is compiled with an IP assignment, and ensure SIMATIC NET S7-RedConnect is licensed.

Protocol Differences - Classic S7-400H vs CPU 410-5H

In classic SIMATIC PCS 7 systems based on S7-400H controllers (such as 6ES7414-4HM00-0AB0 or 6ES7417-4HT14-0AB0), plant bus communication historically relied on external communications processors (e.g., 6GK7443-1EX20-0XE0 / 6GK7443-1EX30-0XE0). These CP 443-1 cards supported both the MAC-address-based pure ISO Transport protocol (Layer 4) and TCP/IP.

With the introduction of the high-performance CPU 410-5H (6ES7410-5HX08-0AB0), the architecture shifted. The CPU 410-5H integrates two high-speed PROFINET/Industrial Ethernet interfaces directly on the processor module. These integrated ports are purely IP-stack based. They strictly support:
1. TCP/IP (Layer 3/4 IP routing and addressing).
2. ISO-on-TCP (RFC 1006 encapsulating ISO transport packets over IP).
3. Pure ISO (Layer 2 MAC transport) is not supported on the CPU 410-5H onboard ports.

Root Cause of NetPro Connection Error 276:27

When an engineer attempts to create an S7 Fault-Tolerant Connection from an OS Server (or redundant server pair) to a CPU 410-5H in NetPro:
* The Protocol Mismatch: If the communications processor in the PC station (e.g., CP 1623 6GK1162-3AA00 or CP 1628 6GK1162-8AA00) is assigned solely to an "ISO" industrial Ethernet network subnet, NetPro evaluates the endpoint capabilities. Because the CPU 410-5H cannot terminate pure MAC-based ISO, NetPro blocks connection synthesis and aborts with Error 276:27.
* Configuration Desynchronization: If the PC station hardware was originally configured with classic CP 443-1 associations and later reassigned to CPU 410-5H without recompiling the PC station's network objects, stale routing pointers trigger internal consistency errors during NetPro connection insertion.

Configuring Fault-Tolerant S7 Connections via TCP/IP and ISO-on-TCP

To establish the connection successfully without communication errors:

[SIMATIC Manager -> Component View]
   │
   ├─► Open HW Config of OS Server PC Station
   │     • Select CP 1623 (Slot 1) -> Properties -> Subnet: Assign to "Industrial Ethernet"
   │     • Check: "IP protocol is being used" -> Assign valid IP and Subnet Mask
   │     • Select CP 1623 (Slot 2) -> Assign second IP address on secondary subnet
   └─► Save and Compile HW Config of PC Station
         │
         ▼
[NetPro Network Configuration]
   │
   ├─► Select WinCC Application inside OS Server
   ├─► Insert New Connection -> Target: CPU 410-5H (Fault-Tolerant)
   ├─► Type: S7 connection (fault-tolerant)
   ├─► In Connection Properties:
   │     • Interface: Select TCP/IP / ISO-on-TCP (NOT MAC-based ISO)
   │     • Ensure all 4 redundant sub-routes (2x Server CPs to 2x CPU PN ports) show "active"
   └─► Execute: Network -> Save and Compile -> Check Consistency

This establishes four independent communication paths across the dual industrial Ethernet switches.

NetPro Hardware Recompilation and Station Configuration

The following matrix compares communications hardware and protocol capabilities in SIMATIC PCS 7:

Hardware ComponentMLFB / ModelSupported Protocols for S7 Fault-Tolerant ConnectionS7-RedConnect License RequirementCPU 410-5H (Onboard Ports)6ES7410-5HX08-0AB0TCP/IP, ISO-on-TCP (RFC 1006) strictlyYes (Required on WinCC Server)CP 443-1 (External AS CP)6GK7443-1EX30-0XE0Pure ISO (MAC), TCP/IP, ISO-on-TCPYes (Required on WinCC Server)CP 1623 Communications Board6GK1162-3AA00Pure ISO, ISO-on-TCP, TCP/IPHardware PCIe card for OS ServerCP 1628 (With Security/Firewall)6GK1162-8AA00Pure ISO, ISO-on-TCP, TCP/IP, IPsecHardware PCIe card with onboard VPNSIMATIC NET S7-RedConnect6GK1716-0HB14-0AC0Required software license for 4-way redundancy1 License per OS Server PC station

Procurement Notice: Validate all final license quantities, media packages, and part numbers against current Siemens Industry Mall catalogs and with an authorized Siemens distributor quotation prior to procurement.

Verification

To confirm that the S7 Fault-Tolerant Connection is functional:
1. In NetPro, select Network -> Check Intersite Consistency; verify that compilation completes with 0 errors and 0 warnings.
2. Download the network configuration to both the CPU 410-5H and the target OS Server via the Station Configuration Editor.
3. In WinCC Explorer, expand Tag Management -> SIMATIC S7 Protocol Suite -> Named Connections; confirm the connection displays a green checkmark indicating active communication.
4. Open the redundant server diagnostics utility (channel_diag.exe); verify that all 4 sub-connections to the redundant CPU 410-5H show "Connected" status.
5. Disconnect the primary Ethernet cable from CPU 410-5H PN Port 1; verify that communication smoothly fails over to Port 2 within 100ms without tag read dropouts.

Limitations and Operational Precautions

Never configure a CPU 410-5H plant bus connection with overlapping IP subnets on PN-IO Port 1 and Port 2. Both ports must reside on distinct, isolated IP subnets (for example, 192.168.1.x for Subnet A and 192.168.2.x for Subnet B) to allow deterministic path selection by SIMATIC NET S7-RedConnect. Any subnet collision will cause NetPro routing compiler errors or intermittent connection dropouts.

Engineer FAQ

Why did pure ISO work on my older S7-400H systems but fail on CPU 410-5H?

Older systems used CP 443-1 communication processors which supported native Layer-2 MAC-based ISO transport. The CPU 410-5H uses integrated PROFINET ASICs that require Layer-3 IP protocols (ISO-on-TCP or TCP/IP).

What should I do if NetPro still shows Error 276:27 after changing to TCP/IP?

Delete the CP 1623 hardware from the PC Station in HW Config, recompile the empty rack, re-insert the CP 1623 with fresh IP parameters, recompile, and re-create the S7 fault-tolerant connection.

Is SIMATIC NET S7-RedConnect required for single-client stations?

No. Standard OS Clients communicate with the OS Server via WinCC Terminal Bus. S7-RedConnect is required only on PC stations communicating directly with redundant AS controllers over the Plant Bus.

Can I use standard office network cards (Intel PRO/1000) for CPU 410-5H redundant connections?

Yes, using SOFTNET-IE S7-RedConnect, but CP 1623/1628 is strongly recommended for high tag counts (> 50,000 tags) to offload protocol processing from the OS Server CPU.