Connecting Two S7 PLCs to a TP1500 Comfort via PROFIBUS DP

Problem Definition

A single Siemens SIMATIC TP1500 Comfort panel must exchange data with two S7 PLCs over PROFIBUS DP. The HMI exposes only one PROFIBUS DP interface (X2 / IF1B), and the engineering question is whether two S7 controllers can share that single physical port, or whether the network must be split.

The answer is yes — a PROFIBUS DP segment is a linear bus and the HMI is just another node on that bus. Two PLCs can be addressed by a single HMI as long as the segment topology, addresses, baud rate, and termination are respected. If the two PLCs must remain on electrically independent PROFIBUS networks, a DP/DP coupler (6ES7158-0AD01-0XA0) provides the alternative.

Prerequisites

• TP1500 Comfort (6AV2 124-1QC02 or current 6AV2 124-1(Q/M)xxxx variant) with firmware V14.0.3 or later. Comfort panels only expose PROFIBUS on the X2 (IF1B) interface. See the TP1500 Comfort manual (49313233).
• Two S7 controllers with PROFIBUS DP master or slave interface (e.g. S7-300 CPU 315-2 DP, S7-400 CPU 41x-2, S7-1200 with CM 1243-5, ET 200SP CPU 1510SP-1 PN + CM DP).
• Siemens PROFIBUS FastConnect connectors with PG socket and integrated termination resistor, e.g. 6ES7972-0BA52-0XA0 (90° cable outlet, with PG socket) or 6ES7972-0BB52-0XA0 (35° cable outlet, with PG socket).
• PROFIBUS DP cable, FastConnect, two-core, shielded, violet — 6XV1830-0EH10. Armored variant (6XV1830-0EH10-2Axx) for tray runs.
• Stripping tool 6GK1905-6AA00 and FastConnect blade 6GK1905-6AB00 for cable preparation.
• TIA Portal V16 or later, with WinCC Comfort/Advanced installed.

PROFIBUS DP Network Architecture on a Single Segment

PROFIBUS DP is implemented as a linear bus with shielded, twisted-pair cable. The TP1500 Comfort becomes a Class 1 DP master (or DP slave, depending on role assignment) coexisting with both S7 controllers. There is no "port to port" point-to-point rule — every station is wired into the bus via a T-tap inside the connector.

Topology Options

Three physical layouts satisfy the requirement. Pick the one that matches the cabinet geometry.

1. PLC1 → PLC2 → HMI — most common when the HMI is mounted in a remote operator station and the two PLCs live in the same cabinet.
2. PLC1 → HMI → PLC2 — used when the HMI is physically co-located with the PLCs.
3. DP/DP coupler in the middle — used when the two PLCs must remain on electrically isolated PROFIBUS networks (different grounding, different masters, or different baud rates).

Inline SVG — Three Topologies

Address Assignment and Baud Rate

All three stations must share the same baud rate (1.5 Mbps is the default for Comfort panels and gives 200 m of cable). Each station must have a unique PROFIBUS address in 1–125.

The HMI's PROFIBUS address is set in TIA Portal under Panel → Interfaces → IF1B and must be committed via "Download to device → PROFIBUS node address" or by the HMI control panel: Control Panel → Network and Dial-Up Connections → PROFIBUS → Node Address.

Wiring Procedure

1. Cut the PROFIBUS cable to length, strip 35 mm of outer jacket, and remove the shielding braid and inner foil using the FastConnect tool (6GK1905-6AA00) so the violet jacket and shield land in the connector backshell.
2. Crimp the two cores into the insulation-displacement terminals of the FastConnect connector. Green wire = A (negative), Red wire = B (positive). Reverse polarity on one connector is the single most common PROFIBUS failure.
3. Install the connector on the outgoing device. The PROFIBUS cable continues in a straight line — the connector has a cable-in and a cable-out terminal block; if the device is the last physical node, the outgoing terminal is left empty.
4. On the two end stations of the segment, slide the termination switch to ON. On every station in between, leave it OFF. The TP1500 Comfort is rarely an end node when placed mid-chain, so check the cabinet layout before energising.
5. Tighten the backshell until the shield makes 360° contact, then connect the grounding strap of the connector to the cabinet PE bar with a short pigtail.

TIA Portal Configuration — Two Connections on One HMI Port

The TP1500 Comfort exposes a single PROFIBUS interface, but TIA Portal allows multiple HMI connections on that one physical port. Each connection is bound to a different remote PROFIBUS address.

1. In the project tree, expand the Comfort panel and open Connections. Right-click → Add new connection. Choose the S7-300/400 driver for an S7-300/400 PLC, or S7-1200/1500 for a S7-1200 with CM 1243-5.
2. Set the connection parameters:

3. Drag two PROFIBUS-capable S7 devices from the hardware catalog into the Devices & Networks view. The CP/interface module is dropped onto the PLC, then connected to the HMI's IF1B subnet.
4. Right-click the new subnet → Properties and verify: Bus profile: DP, Highest station address: 126, Transmission speed: 1.5 Mbps (or whatever the entire segment runs at).
5. For each PLC, open Devices & Networks → PLC → Properties → PROFIBUS interface → Operating mode and decide whether the S7 is a DP master or DP slave. If both S7 controllers are masters, set the HMI to DP slave; if the HMI is the sole master, leave both PLCs as slaves and define the I/O slot mapping in the S7's DP slave properties.
6. Create HMI tags under each connection. Tags bound to Connection_1 read from PLC1, tags bound to Connection_2 read from PLC2. Tag naming must be unique across the project — duplicate names will be silently merged at runtime.
7. Compile, download the HMI project, and download both PLCs. On the HMI, Control Panel → Network → PROFIBUS should show OK with all three nodes (1, 2, 3) listed.

Alternative: DP/DP Coupler for Isolated Networks

If the two PLCs must run on different baud rates, different masters, or different grounding systems (e.g. one is battery-backed UPS, one is on a separately derived neutral), use the Siemens DP/DP coupler, order number 6ES7158-0AD01-0XA0. It is a transparent data router: it copies the configured input/output image between two PROFIBUS DP subnets, up to 244 bytes per direction.

1. Insert the coupler in the hardware catalog: PROFIBUS DP → Other field devices → DP/DP coupler.
2. Configure the left side as DP slave of PLC1's master, the right side as DP slave of PLC2's master. Each side has its own PROFIBUS address and bus profile.
3. Define the I/O length on each side. The two halves do not need to match (e.g. 16 bytes in / 32 bytes out on side A, mirrored on side B).
4. Tag mapping: in PLC1, the coupler's input/output bytes appear in the process image of PLC1's master. In PLC2, the same bytes appear mirrored. Programs in both PLCs use the same memory locations (e.g. IB 100 .. IB 115) to exchange the 16 bytes.

The HMI in this case typically still sits on one of the two PROFIBUS networks as a master and reads its own slice of the I/O from the coupler plus the local PLC tags. The TP1500 cannot be wired to both sides of the coupler — that would defeat the isolation — so the HMI talks to PLC1 directly and reaches PLC2 only through PLC1's program.

Verification

1. Bus scan from TIA Portal: Online → Accessible nodes over PROFIBUS. The three (or more) stations must appear at the assigned addresses with no "Station failure".
2. Diagnostic buffer on each S7: PLC → Online & Diagnostics → Diagnostic buffer. Look for OB82 / OB86 entries signalling bus fault; a clean buffer after a 10-minute soak indicates a stable segment.
3. HMI connection status: WinCC Comfort provides the system tag @ConnectionState_Connection_1 and @ConnectionState_Connection_2. Display these on a service screen. 0 = disconnected, 1 = connecting, 2 = connected.
4. Signal level: use a PROFIBUS tester (e.g. Softing BC-700-PB, Indu-Sol PROFI-TESTER) at the midpoint of the segment. Acceptable values: signal amplitude ≥ 2.5 V, rise time ≤ 1/8 of the bit time, no reflected pulse above 1.0 V.
5. Termination resistance: with the bus powered, measure between pins 3 and 8 at any connector. With all terminators OFF you must see > 1 kΩ; with both ends ON you must see ~110 Ω (220 Ω || 220 Ω).

Troubleshooting Matrix

Constraints and Field-Proven Caveats

• The TP1500 Comfort has one PROFIBUS interface. A second physical HMI connection over PROFIBUS is not possible without external hardware (DP/DP coupler, repeater, or migrating to PROFINET via the X1 Ethernet port).
• Maximum number of simultaneously open HMI connections on a Comfort panel is 6 (S7 + Send/Receive + OPC). Two of those can be over PROFIBUS if the panel firmware allows — for TP1500 Comfort firmware V14.0+, two S7 connections over the single IF1B port are supported.
• Both S7 controllers must have their PROFIBUS interface set to the same transmission speed. Auto-detect is not part of the DP-V0/DP-V1 master specification; configure a fixed speed in TIA Portal.
• The PROFIBUS address of the TP1500 is set through the Control Panel of the device itself. Downloading a TIA Portal project does not necessarily overwrite the address; verify it after the first download.
• If you ever add a third PLC, you will be at 4 nodes (1 HMI + 3 PLCs) — still within the 32-per-segment limit, but watch cumulative cable length and stub discipline.
• Converting the panel to PROFINET (via the X1 port) is generally easier: an Ethernet switch replaces the bus, addressing is by IP, and the Comfort panel supports up to 16 PROFINET connections. Migrate when commissioning a new network; keep PROFIBUS only for legacy expansions.

Standards Reference

• IEC 61158-6 / EN 61784 — PROFIBUS DP fieldbus standard, addressing, and station limits.
• PROFIBUS Installation Guidelines (PI, 8.022/3.502) — cable, connector, grounding, and termination rules.