Configuring Cross-Coupled AS-H to Single AS Communication in PCS 7 Using Industry Library

In SIMATIC PCS 7, transmitting process data between a fault-tolerant S7-400H redundant controller (e.g., CPU 417-4H 6ES7417-4HT14-0AB0) and a standard single S7-400 controller (e.g., CPU 414-3 6ES7414-3XM05-0AB0) fails when using standard APL communication blocks (Snd_AnaVal, Rcv_AnaVal) because standard APL blocks lack redundant connection failover logic. Standard APL blocks are designed strictly for symmetric single-to-single links. For asymmetric H-to-Single communication, engineers must deploy the specialized high-availability communication blocks from the SIMATIC PCS 7 Industry Library: SndH_AnaVal (FB1193) and RcvH_AnaVal (FB1194), which manage automatic sub-connection switchover between redundant H-subsystems and the single partner.

Asymmetric Controller Communication Architecture (AS-H to Single AS)

In plant modernization projects, facilities frequently possess a central high-availability redundant controller pair (S7-400H or CPU 410-5H) that must coordinate with auxiliary, non-redundant single controllers (e.g., S7-414-3 or S7-416) controlling utilities or packaging.

This creates an asymmetric communication topology:
* The S7-400H Subsystem has two physical CPUs and two Industrial Ethernet communication processors (CP 443-1).
* The Single S7-400 Subsystem has only one physical CPU and one CP 443-1.

Under this architecture, communication requires two underlying physical S7 connections (Sub-connection 1 to H-CPU 0, and Sub-connection 2 to H-CPU 1). Managing these redundant paths requires dedicated software handshakes.

Why Standard APL Snd_AnaVal Fails on Redundant Controllers

Standard PCS 7 Advanced Process Library blocks (Snd_AnaVal FB1913, Rcv_AnaVal FB1914, Snd_DigVal FB1915, Rcv_DigVal FB1916) rely on simple peer-to-peer data blocks (BSEND / BRCV):

Block LibraryCommunication Block PairRedundant H-Path FailoverTarget ArchitectureStandard APLSnd_AnaVal / Rcv_AnaValUnsupported (Tied to single static Connection ID)Single AS to Single ASStandard APL (H-H)Redundant H-ConnectionsNative (Both sides redundant S7-400H)Redundant H-AS to Redundant H-ASIndustry Library (IL)SndH_AnaVal / RcvH_AnaValSupported (Monitors sub-connections; switches dynamically)Redundant H-AS to Single AS (Asymmetric)

When an engineer configures Snd_AnaVal on an S7-400H controller, the block binds to only one sub-connection ID. If that specific CPU becomes standby or its CP cable is disrupted, communication stops completely, and data transfer halts despite the H-system remaining operational.

PCS 7 Industry Library High-Availability Blocks (SndH / RcvH)

The SIMATIC PCS 7 Industry Library (IL) provides specialized blocks engineered specifically for asymmetric H-to-Single links:

SndH_AnaVal (FB1193): Transmits analog values with Quality Codes from an S7-400H to a single S7 controller.
RcvH_AnaVal (FB1194): Receives analog values on the single controller from the S7-400H.
SndH_DigVal (FB1195) / RcvH_DigVal (FB1196): Handles digital bit arrays.

Internally, SndH and RcvH evaluate connection state words. If the active master CPU switches from Rack 0 to Rack 1, the block automatically redirects BSEND calls through the alternate connection ID without data loss or manual programming.

NetPro Connection Setup and Cross-Coupled ID Configuration

To configure the communication link in NetPro:

Open NetPro in SIMATIC Manager.
Select the Single S7-400 CPU; insert a new S7 Connection pointing to the S7-400H partner.
In connection properties, NetPro detects that the partner is fault-tolerant and creates two sub-connections:
- Connection ID 1: Single CPU CP -> H-Station Rack 0 CP.
- Connection ID 2: Single CPU CP -> H-Station Rack 1 CP.
Note the Local ID and Partner ID on both sides.
In CFC on the S7-400H, place SndH_AnaVal; parameterize both Connection IDs (ID1 and ID2).
In CFC on the single controller, place RcvH_AnaVal; parameterize the local Connection ID.

Verification

To verify H-to-Single communication:
1. In CFC on the S7-400H, connect an analog test value to SndH_AnaVal.In.
2. Open the chart on the single S7-400 controller online; verify that RcvH_AnaVal.Out.Value matches In.Value and Quality Code indicates 16#80.
3. In Hardware Configuration, place H-CPU 0 into STOP mode; verify that the S7-400H executes bumpless redundancy switchover to H-CPU 1.
4. Confirm that RcvH_AnaVal on the single controller maintains continuous valid data reception without dropping to zero or flagging connection errors.

Limitations and Operational Precautions

Asymmetric H-to-Single communication does not provide full physical redundancy at the single controller end: if the single CPU 414-3 or its single CP 443-1 fails, all cross-communication is lost. To achieve complete fault tolerance across all communication paths, both automation stations must be engineered as redundant S7-400H pairs utilizing redundant Industrial Ethernet rings.

Engineer FAQ

Can standard Snd_AnaVal communicate with an S7-400H if redundancy is ignored?

If tied to a single sub-connection, it will work only as long as that specific H-CPU and CP remain master. As soon as a redundancy switchover occurs, communication breaks.