Configuring a TP177A to Read Tags from Multiple S7-300 PLCs on a Shared Profibus DP Network

1. Problem Statement and Architecture

You have two Siemens S7-300 CPU 313C-2DP controllers deployed on the same physical Profibus DP segment, each at a unique PROFIBUS address, and a single TP177A touch panel connected to PLC 1. You want to:

• Display live process tags from PLC 1 on the TP177A.
• Display live process tags from PLC 2 on the same TP177A.
• Avoid implementing a peer-to-peer data copy between the two CPUs if the only consumer is the HMI.

The 313C-2DP has a single integrated DP interface (MPI/DP combined) and limited peer-to-peer capability when both CPUs are configured as DP master. There is no straightforward "direct data exchange" (DX) configuration between two S7-300 DP masters without an additional coupler, and configuring a global data exchange (GD) circle is generally a non-starter on this hardware. The most reliable, supported solution is to put the HMI in charge of the multi-drop traffic by configuring multiple PLC connections inside the WinCC flexible project for the TP177A.

2. Architecture Options Compared

For the canonical "two 313C-2DP + one TP177A" configuration, Option A is the engineering default. Option D is the fallback when the second PLC's DP port is already consumed as a slave on another master.

3. Prerequisites

4. STEP 7 Project: Build the Combined DP Subnet

Place PLC 1, PLC 2, and the TP177A into the same STEP 7 project. This guarantees STEP 7 builds a consistent PROFIBUS topology with one subnet, one master system, and correct bus parameters.

1. Insert PLC 1. In SIMATIC Manager choose Insert → Station → SIMATIC 300 Station, insert a SIMATIC 300 → RACK-300 rail, slot 2 = PS 307, slot 3 = CPU 313C-2DP (6ES7 313-6CF03-0AB0 or later). Open HW Config.
2. Configure the DP port of PLC 1 as a DP master on a new PROFIBUS subnet. Set the address to 2. Keep "DP master" selected.
3. Insert PLC 2. Same procedure, place on the same PROFIBUS subnet as PLC 1. Set address to 3. Mode: DP master (for Option A) — this puts both CPUs as masters on the same physical bus with the TP177A bridging them.
4. Insert TP177A. Add a SIMATIC HMI Station, open the HMI device configuration in WinCC flexible later, but first wire it on the bus: in HW Config of the HMI station, double-click the DP interface and assign it to the existing subnet. Set its PROFIBUS address (for example 4). Save and compile.
5. Cross-reference check. In SIMATIC Manager, Options → Cross-reference → Display. Every device (PLC 1, PLC 2, TP177A) must list the same PROFIBUS subnet. If PLC 2 created a new subnet, delete its second DP master system and re-attach the port to the shared subnet.
6. Download the HW Config to all three devices.

5. WinCC Flexible: Configure Two PLC Connections

Open the TP177A project in WinCC flexible. The key configuration lives in Project → Connections (or the Connections editor in the project tree). You will define one connection per PLC.

5.1 Connection #1 — PLC 1 (S7-300/400, Profibus)

5.2 Connection #2 — PLC 2 (S7-300/400, Profibus)

5.3 Bind Tags to the Correct Connection

For every tag in the HMI tags editor, set the Connection field to either PLC1_Connection or PLC2_Connection. The address (DB number, byte/bit offset) refers to the corresponding PLC's data block.

• Example: A motor current display in PLC 1 DB100.DBD0 → tag Motor1_Current with Connection = PLC1_Connection, Address = DB 100 DBD 0.
• Example: A motor current display in PLC 2 DB200.DBD0 → tag Motor2_Current with Connection = PLC2_Connection, Address = DB 200 DBD 0.

Tags in the HMI project are global — the same screen element can read from one connection, and another element from the second connection. There is no concept of "current connection" inside the panel for tag addressing; the connection is a property of the tag.

5.4 Area Pointer and Screen Wiring

Configure the standard area pointers (Connection → Area Pointers):

• Coordination — required, points to a DB/bit in each PLC for life-bit and operator LED feedback.
• Date/Time — optional; if used, must be assigned to one PLC only.
• Project ID — assign to PLC 1 to drive the project version page.

Wire your screens in the Screens editor: each I/O field, bar, or numeric display references a tag; that tag already carries the correct connection. The runtime will poll the corresponding CPU on PROFIBUS.

6. Option B — DP/DP Coupler

Use a Siemens DP/DP Coupler (6ES7 158-0AD01-0XA0) when the two PLCs must remain in physically isolated PROFIBUS segments (different baud rates, different grounding, different plant areas) but still need to share data.

6.1 Wiring

1. Build two distinct PROFIBUS subnets in STEP 7, each with its own master and bus terminators.
2. Connect PLC 1's subnet to channel X1 of the DP/DP coupler; PLC 2's subnet to channel X2.
3. On the coupler DIP switches, set the maximum number of input/output bytes per direction (typical: 32 bytes in, 32 bytes out each side).
4. Power the coupler from a 24 V DC supply. Status LEDs: SF (group fault), BF (bus fault per channel), ON.

6.2 Configuration in STEP 7

1. Install the GSD file: HW Config → Options → Install GSD. The GSD for the DP/DP coupler ships on the Siemens support site (search order number 6ES7 158-0AD01-0XA0).
2. In PLC 1's HW Config, drag the DP/DP coupler from the catalog onto PLC 1's PROFIBUS master system. Configure I/O slots: pick the input/output byte counts that match the DIP switches.
3. Repeat for PLC 2, mirroring the byte layout.
4. The bytes the coupler exchanges appear in the I/O image of each PLC. Move them into a DB with L PIB / T DB... instructions, or use them directly as process I/O.

6.3 Diagnostic LED Meanings

7. Option C — MPI Network for PLC-to-PLC PUT/GET

If PLC 1 must actively use a value from PLC 2 in its own logic (not just display it), configure an MPI subnet between the two CPUs and use S7 communication PUT / GET blocks (FB15 / FB14, or library blocks). The TP177A is not involved.

1. In NetPro of the STEP 7 project, configure an MPI subnet on each CPU's MPI/DP port. Use unique MPI addresses: PLC 1 = 2, PLC 2 = 3.
2. Insert an S7 connection from PLC 1 to PLC 2. Local ID is assigned by STEP 7.
3. Call FB15 PUT in PLC 1 to write data to a DB in PLC 2; call FB14 GET to read. Both blocks live in the standard library under Communication Blocks → S7 Communication.
4. Pass the connection ID, parameters REQ, ID, ADDR_1 (any pointer, e.g. P#DB100.DBX0.0 BYTE 20), SD_1 (source), and monitor DONE, ERROR, STATUS.

8. Verification

After download, perform these checks in order:

1. Bus diagnostics in STEP 7: PLC → PROFIBUS → Diagnose. All nodes (PLC 1 master, PLC 2 master, TP177A) should appear with green status and no "Station failure" entries.
2. TP177A connection status: on the panel, open System → Transfer → Status, or watch the connection icons in the WinCC flexible runtime. Both connections should show "Connected".
3. Tag test: in the WinCC flexible runtime, use the tag simulator or a temporary screen with an I/O field bound to a tag from each connection. Confirm live update from both PLCs.
4. LED inspection on every node: no SF, no BF, no flashing STOP on either CPU.
5. SF indicator on TP177A: if lit, an active diagnostic buffer entry exists; view on the panel under System → Settings → OP Properties → Diagnostics.

9. Troubleshooting Matrix

10. Field-Proven Recommendations

• One project, one bus. Never split the two PLCs into separate STEP 7 projects when they share a physical PROFIBUS segment. STEP 7 must own the bus parameter generation or the two masters will fight over Tslot values.
• Terminate the bus physically. With two masters and one panel on a single cable, the panel often ends up at the far end of the cable. The connector at PLC 1 and the connector at the panel must have terminator switches ON, the one in the middle OFF.
• Do not write tag data into PLC 1 from the HMI as a side effect. If the user thinks "display PLC 2 values" requires "copy into PLC 1 first," the tag editor is doing the right job. Copying data PLC 2 → PLC 1 via the panel is unnecessary load on the bus.
• Tag naming convention. Use a connection prefix in the tag name (e.g. PLC1_, PLC2_) so screen designers cannot accidentally wire a tag from PLC 1 thinking it is PLC 2.
• Update rate on slow-changing tags. WinCC flexible polls on a default cycle. For non-critical tags, lengthen the acquisition cycle to 1 s or 2 s to reduce PROFIBUS load when both PLCs and the panel are on the same bus.
• Keep the panel's role consistent. On a shared multi-master bus, the TP177A is a DP slave to both masters. Do not configure it as a master on top — the TP177A is not a master on a 313C multi-master segment, it is a slave to each master that polls it. The HMI driver still speaks S7 over the slave connection.