PCS 7 AS-to-AS Communication S7 Blocks, PROFINET, and H

Overview: Communication Architecture in SIMATIC PCS 7

SIMATIC PCS 7 is Siemens' distributed control system (DCS) in which the Automation Station (AS) executes control logic and the Operator Station (OS) / Engineering Station (ES) handle visualization, archiving, and engineering. A typical PCS 7 plant contains several AS-410 / AS-410H controllers that must exchange process data continuously, redundantly, and deterministically.

CPU-to-CPU (AS-to-AS) communication in PCS 7 is implemented through three physical layers:

• Industrial Ethernet (PROFINET or TCP/IP) - the standard plant/system bus for AS-to-AS, AS-to-OS, and ES-to-AS traffic.
• PROFIBUS DP - used in legacy PCS 7 V6/V7 projects, especially for remote I/O (ET 200M/IM 153) and field-device integration through DP/PA couplers.
• Fiber-optic sync link - mandatory in H-systems (AS 410H, AS 400H) for master/standby redundancy. The two H-CPUs synchronize via dedicated sync modules over fiber; this is a separate, non-data communication channel.

Process data is moved between AS-410 controllers using the S7 Communication protocol suite, which provides both coordinated and uncoordinated data exchange with minimal configuration effort and no additional hardware beyond the standard CP 443-1 Ethernet CP.

Communication Methods Available in PCS 7 AS-410 / AS-410H

PCS 7 supports several CPU-to-CPU mechanisms. The choice depends on data volume, determinism requirements, and whether redundancy is required.

For new PCS 7 V9.0 / V9.1 / V10.0 projects, Industrial Ethernet with S7 Communication is the standard. PROFIBUS remains in use for ET 200M remote I/O rings and for AS-to-AS in older V7.1 SPx installations where the existing bus topology cannot be easily migrated to PROFINET.

Hardware Prerequisites

Before configuring AS-to-AS communication, verify the following hardware:

• AS 410 / AS 410H with firmware ≥ V8.0 (V8.1 recommended for PCS 7 V9.0; V8.2 for V9.1; V9.0 firmware for V10.0). Reference the SIMATIC PCS 7 AS 410 firmware release notes.
• CP 443-1 (6GK7 443-1EX30-0XE0 for V8.2+) or CP 443-1 Advanced (6GK7 443-1GX30-0XE0) for the plant bus connection.
• SCALANCE XC/XB/XR managed switches for the plant bus; ring redundancy via HRP/MRP is supported when using SCALANCE XC-200 or higher.
• For H-systems: two H-CPUs (e.g., 6ES7 410-5HX08-0AB0 AS 410H) and two sync modules (6ES7 960-1AA06-0XA0) with two fiber-optic sync cables (6ES7 960-1AA04-5AA0, lengths 1 m / 5 m / 10 m).
• ES (Engineering Station) with PCS 7 V9.1 + SPx and STEP 7 V5.6 or TIA Portal V17/V18 with PCS 7 V10 option package.

Configuring the S7 Connection in NetPro / HW Config

AS-to-AS communication is configured as an S7 connection. With the CP 443-1 inserted in HW Config, open NetPro (PCS 7 V9.x) or the device & networks editor (TIA V18 + PCS 7 V10) and add an S7 connection between the two AS-410 stations.

1. Insert a new S7 connection. In NetPro: right-click the CPU → Insert New Connection → S7 Connection. In TIA: drag from one CPU's S7 interface to the partner CPU's interface.
2. Set the local and partner CP/IP addresses. The connection terminates at the CP 443-1, not the CPU's PROFINET port, when the CP is in a separate subnet.
3. Activate the checkbox "Establish an active connection" on the initiating CPU side. Only one side must be active.
4. Set the connection resource (local ID). This ID is the input ID on the BSEND/BRCV/PUT/GET FBs.
5. Compile and download to both stations. The connection becomes active after a CPU STOP→RUN or after re-initialization.

For H-systems, NetPro automatically generates redundant S7 connections. The H-CPU firmware manages primary/backup switching, so user programs do not need to be modified to handle failover - the connection ID stays the same.

S7 Communication Blocks: Function Calls

The Standard Library / SIMATIC PCS 7 Library provides the IEC-conformant Function Blocks. Call them from OB1 (or OB35 if cyclic, time-synchronized) on the sender and from OB1 on the receiver.

FB12 "BSEND" - Block-Send (Sender, Coordinated)

Transmits up to 32 KB of data to one partner; receiver acknowledges. Use this for large or guaranteed-delivery transfers.

CALL "BSEND" , %DB12  // instance DB generated automatically
   REQ     := TRUE            // rising edge triggers send
   R       := FALSE
   ID      := W#16#1          // local connection ID from NetPro
   R_ID    := DW#16#1         // must match partner R_ID
   DONE    := M100.0
   ERROR   := M100.1
   STATUS  := MW102
   SD_1    := P#DB20.DBX0.0 BYTE 400  // source: 400 bytes from DB20
   LEN     := MW106            // actual length to send

FB13 "BRCV" - Block-Receive (Receiver, Coordinated)

CALL "BRCV" , %DB13
   EN_R    := TRUE
   R       := FALSE
   ID      := W#16#1
   R_ID    := DW#16#1          // must match sender R_ID
   NDR     := M110.0           // new data received
   ERROR   := M110.1
   STATUS  := MW112
   RD_1    := P#DB30.DBX0.0 BYTE 400
   LEN     := MW116            // actual length received

FB8 "USEND" / FB9 "URCV" - Uncoordinated Send/Receive

Lower overhead, no handshake. Up to 8 partners can subscribe with the same R_ID on the receive side (broadcast-like). Up to 32 KB per call. Suitable for status broadcasts (e.g., unit-mode, plant-wide setpoints).

CALL "USEND" , %DB8
   REQ     := TRUE
   ID      := W#16#2
   R_ID    := DW#16#A5
   SD_1    := P#DB40.DBX0.0 BYTE 32
   LEN     := 32

CALL "URCV" , %DB9
   EN_R    := TRUE
   ID      := W#16#2
   R_ID    := DW#16#A5
   RD_1    := P#DB41.DBX0.0 BYTE 32
   LEN     := MW132

FB14 "GET" / FB15 "PUT" - Read/Write Remote DBs

No program changes required on the partner CPU. The local CPU opens a server connection to the partner's S7 communication server.

CALL "GET" , %DB14
   REQ     := TRUE
   ID      := W#16#3
   ADDR_1  := P#DB50.DBX10.0 BYTE 20   // remote address
   RD_1    := P#DB60.DBX0.0 BYTE 20    // local target

Cyclic Triggering and Watchdog

For deterministic, time-synchronized AS-to-AS exchange, place the BSEND/PUT call in OB35 (typically 100 ms or 1 s) and the BRCV/GET in OB1. For each block instance, monitor the STATUS word against the standard S7 communication error codes:

H-System Redundancy: Master/Standby Sync Link

An AS 410H station contains two identical H-CPUs (e.g., CPU 410-5H PN/DP, 6ES7 410-5HX08-0AB0). They run in parallel and exchange:

• Redundancy state (which CPU is master, which is standby).
• Process image of the OB1-PII / OB1-PIO for fast failover.
• Non-retentive memory sections during every scan.

The two CPUs are linked through the sync modules (6ES7 960-1AA06-0XA0), each plugged into the backplane of one CPU, and joined by two fiber-optic cables. Both cables must be present; loss of one cable raises diagnostic event "Sync link 1 failed" (W#16#4522) but redundancy is maintained. Loss of both cables forces the standby CPU to STOP with event "H-synchronization failed" (W#16#4570).

Application programs on the AS 410H call BSEND/GET exactly as on a non-redundant CPU. During a master/standby switchover, S7 connections automatically re-establish on the new master; the connection ID is preserved so user programs do not need to be re-initialized.

PROFIBUS-Based AS-to-AS (Legacy Projects)

For PCS 7 V7.x projects using PROFIBUS as plant bus, AS-to-AS is configured with the CP 443-5 Extended (6GK7 443-5DX05-0XE0). In HW Config, designate one AS-410 as PROFIBUS DP master and the other as DP slave, and configure a DP master-master / slave-slave relationship. The PCS 7 V7 Standard Architectures manual documents topology limitations (max 32 nodes per segment, 1000 m with repeater).

Recommended: migrate PROFIBUS AS-to-AS links to PROFINET on new builds. PROFINET provides higher bandwidth, integrated diagnostics via SNMP/SNMPv3, and full support in PCS 7 V9.0+.

Diagnostics: Verifying Communication Health

Use the following methods to verify that AS-to-AS communication is healthy:

1. Connection status online in NetPro: right-click the S7 connection → Connection Status. Look for "Established".
2. CPU diagnostic buffer (online → CPU → Diagnostic Buffer). Filter for event IDs 0x4522 (sync link error), 0x8301 (connection abort), 0x8311 (access denied).
3. CP 443-1 Web server (firmware V3.2+): open http://<cp-ip> in a browser; check Connection Statistics → S7 connection list, error counters, dropped packets.
4. PCS 7 Maintenance Station: AS-410 connection status is automatically forwarded to the MS via the standard PCS 7 driver set; failed AS-to-AS links appear as a yellow/red alarm block in the WinCC faceplate.
5. SFC / SFB call STATUS: integrate the STATUS output of BSEND/BRCV/GET/PUT into the PCS 7 standard diagnostics (APL block CH_ACT or CTRL_PID-style alarm logic).

Troubleshooting Matrix

Best-Practice Checklist for PCS 7 AS-to-AS Projects

• Use BSEND/BRCV for critical, large, or coordinated transfers; USEND/URCV for non-critical broadcasts.
• Place BSEND/PUT in OB35 (100 ms-1 s) and BRCV/GET in OB1 - never call long-running blocks in OB1 of the sender.
• Always set a unique R_ID per logical data channel - never reuse R_ID between unrelated senders and receivers.
• Document the connection ID assignment in the project documentation. Maintain a connection-resource table per CPU.
• On H-systems, validate that NetPro generated the redundant connection automatically (look for the second connection record in the project tree).
• Always grant the S7 server access rights on every CPU that participates in GET/PUT; otherwise partner CPUs must explicitly open a connection and the local CPU must be the server side, which is rarely the case.
• Use SIMATIC PCS 7 Maintenance Station to monitor AS-410 link health instead of polling each CPU individually.

Reference Documentation (Siemens Industry Online Support)

For deeper detail, the following official documents cover the topics in this guide:

• SIMATIC PCS 7 AS 410 - Function Manual: SIOS entry 109751568
• PCS 7 Standard Architectures: SIOS entry 7764609
• PCS 7 Process Control System - Engineering Manual (V9.0): SIMATIC PCS 7 documentation overview
• S7-400 S7 Communication with STEP 7 V5.x: SIOS entry 14909396
• H-Systems (AS 410H) Synchronization Modules - Manual: SIOS portal (search "6ES7 960-1AA06")