Connecting Multiple LOGO! to WinCC Flexible: Limits and Setup

Connecting Multiple Siemens LOGO! to WinCC Flexible: Connection Limits and Architecture

Engineers commissioning small-to-medium LOGO! fleets routinely need to visualise eight or more LOGO! 8 base modules from a single supervisory PC running WinCC Flexible 2008 Runtime. The runtime enforces a hard ceiling on the number of simultaneous S7 connections, which prevents a flat one-to-one topology once that ceiling is reached. This reference documents the ceiling, the LOGO! 8 communication capabilities available to bypass it, three production-grade architectures, and the configuration and verification steps for each.

1. Problem Definition: The 8-Connection Ceiling

WinCC Flexible PC Runtime allocates one S7 connection slot per configured PLC partner. Once the runtime reaches its maximum, additional connection objects in the project are reported by the configuration tool and the runtime will not start the extra channel. For most PC Runtime installations this limit is 8 active S7 connections, which is also the number of remote partners most engineers report as the practical limit when configuring LOGO!-to-WinCC projects.

Consequences for a fleet of 10 LOGO! PLCs (or even 8 with spare capacity for a safety or energy partner):

• Direct 1:1 connection from the PC to each LOGO! is impossible once the ceiling is reached.
• Tag updates slow down because every polled point competes for one of the limited channels.
• Adding a new machine in the field forces a redesign instead of a small change.

Three architectural patterns resolve the conflict without abandoning WinCC Flexible or the LOGO! 8 fleet.

2. WinCC Flexible PC Runtime: Connection Architecture

WinCC Flexible distinguishes between logical connections (channel objects in the project tree) and active connections (channels opened by the running runtime against remote partners). The runtime enforces both a hard cap and per-connection point limits.

For the typical engineer, the binding constraint is the row labelled "Maximum S7 connections". The project will configure cleanly until the ninth connection object is added, at which point the editor warns and the runtime refuses to bring up the extra channel.

3. LOGO! 8 Communication Capabilities

LOGO! 8 base modules ship with a 10/100 Mbit Ethernet port on the bottom of the BM. This single port is used for programming, HMI, and peer-to-peer traffic.

The critical capability for the patterns in this article is the S7 client role: a LOGO! 8 can act as an active S7 partner and pull or push data from another LOGO! 8 on the same network using the network input/output blocks in LOGO!Soft Comfort. This makes the LOGO! itself a valid data concentrator.

4. Architecture Options for 8+ LOGO! PLCs

Patterns A and C are the most common for brownfield LOGO! installations because they reuse the LOGO! 8 itself and avoid new licensing. Pattern B is preferred when the cabinets are already on isolated VLANs. Pattern D is a transitional architecture that pays off when migrating to TIA Portal WinCC.

5. Option A: LOGO! Data Concentrator Pattern

In this pattern one or two "gateway" LOGO! 8 modules act as data concentrators. Each remote LOGO! pushes its critical tags to the concentrator using S7 PUT/GET blocks programmed in LOGO!Soft Comfort. WinCC Flexible connects only to the concentrator(s) and reads the merged process image.

5.1 Sizing the Concentrator

A LOGO! 8 BM exposes 24 network inputs (NI) and 20 network outputs (NO) per peer direction. A practical cluster size is 4–6 peers per concentrator because each peer typically needs 4–8 NI/NQ words for status, setpoints, and counters. Two concentrators comfortably cover a 10–12 LOGO! fleet and stay well under the runtime's 8-connection cap if you choose to also connect the second concentrator.

5.2 What Belongs in the Concentrator and What Does Not

6. Option B: Multi-Subnet Architecture with WinCC Routing

When the cabinet layout already separates LOGO! PLCs across VLANs or physical switches, use a managed switch with IP routing or a small SCALANCE router to expose each subnet to the PC. WinCC Flexible then opens a connection to a router address with a TSAP- or IP-routing extension in the connection properties.

Use this pattern when:

• Cabinets are physically remote and already on their own switches.
• Tag update latency must match a single WinCC cycle (no extra hop).
• You have at most 8 partners per subnet (one WinCC connection per partner).

7. Option C: Modbus TCP Polling

LOGO! 8.1 and later expose a Modbus TCP server on the same Ethernet port as the S7 server. WinCC Flexible supports Modbus TCP as a native channel since WinCC Flexible 2008. Polling all peers through a single Modbus TCP channel uses one connection object in WinCC, leaving the rest of the connection budget free for alarms or a non-LOGO! partner.

7.1 Modbus Address Planning

The LOGO! Modbus mapping is documented in the LOGO! 8 system manual. A practical subset for fleet visualisation:

Mapping is firmware-dependent. Always cross-check against the active system manual entry for your BA-level firmware on the Siemens support portal.

7.2 Polling Strategy

WinCC Flexible issues one read per polling cycle. To poll N peers, either:

• Use a slow scan group (e.g. 1000 ms) and cycle through peers using a script, or
• Use a separate connection object per peer with a fast cycle, accepting the connection budget hit.

8. Step-by-Step: Configuring S7 PUT/GET Between LOGO! 8 Peers

The data concentrator pattern requires network input (NI) and network output (NQ) blocks in LOGO!Soft Comfort. The example below wires one peer (LOGO! Peer A at 192.168.0.20) into a concentrator (192.168.0.10).

1. In LOGO!Soft Comfort open the concentrator project. From the toolbar choose Tools → Ethernet Connections (or right-click the project node and add an S7 connection).
2. Add a new S7 connection: Connection Type = S7, Remote IP = 192.168.0.20, Local TSAP = 03.00, Remote TSAP = 03.00. Save the connection as PeerA.
3. Insert a Network Input (NI) block in the Ladder/FBD diagram. Configure:

   Block: NI
   Connection: PeerA
   Read Start Address: VB0   (peer process image start)
   Length: 8 bytes
   Read Cycle: 500 ms
   

4. Insert a Network Output (NQ) block to send setpoints back:

   Block: NQ
   Connection: PeerA
   Write Start Address: VB100 (peer setpoint region)
   Length: 4 bytes
   Trigger: rising edge of M50
   

5. Compile and download the program to the concentrator. Verify with Online → Test that NI flags are updating.
6. Repeat for each peer, using a unique S7 connection object and unique NI/NQ blocks. Use the same local TSAP across connections if the concentrator is acting as a single server (S7 allows multiple server connections on the same TSAP).

9. Step-by-Step: Configuring the WinCC Flexible Connection to the Concentrator

1. In the WinCC Flexible project tree, right-click Connections → Add Connection.
2. Set the communication driver to SIMATIC S7-200 / LOGO! for a direct LOGO! 8 partner, or SIMATIC S7-300/400 if the concentrator is an S7-300 used as gateway.
3. Configure the partner:

   Partner IP: 192.168.0.10
   Partner TSAP: 03.00
   Local TSAP: 01.01
   Connection Type: S7 (ISO-on-TCP)
   

4. Under Area Pointers, enable the process-image areas you actually need (e.g. DB 1, Byte 0..31 for the merged peer image).
5. Add tags mapped to the area pointer offsets. Keep tag names structured (e.g. PeerA_Status, PeerB_Counter) to make the project readable when a new peer is added.
6. Repeat for the second concentrator (if used). WinCC Flexible will report a connection-budget warning if the total exceeds 8.

10. Network Topology and IP Plan

Lay out the IP plan in writing before connecting cables. The 192.168.0.0/24 range below is an example; adapt to your site standard.

11. Verification and Diagnostics

After the project is loaded, run the following checks before handing the system over to operations.

12. Troubleshooting Matrix

13. Migration Path: TIA Portal WinCC

If a migration to TIA Portal is on the roadmap, the connection budget changes. WinCC Runtime Advanced (TIA Portal) supports more S7 connections depending on the licence panel, and the connection editor exposes the budget explicitly. Migration of a concentrator project is largely a port of the WinCC Flexible area pointers into TIA Portal tag tables; the LOGO!Soft Comfort peer program is unchanged.

Practical steps for a staged migration:

1. Document the existing area pointer map in a spreadsheet.
2. Import the WinCC Flexible project into TIA Portal using the migration tool (TIA Portal V16+ supports WinCC Flexible 2008 SP5 import).
3. Re-add any connections that the migration tool flags as needing reconfiguration.
4. Validate tag-by-tag against the running system before decommissioning WinCC Flexible.

For further background on the LOGO! 8 system and S7 communication refer to the Siemens Industry Online Support portal, specifically the LOGO! 8 system manual and the LOGO!Soft Comfort online help. For WinCC Flexible configuration refer to the WinCC Flexible 2008 Communication manual set.

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