Stahl ET-208 to Siemens S7-400 via MPI: Adapter, Wiring, and TwinRepeater Integration
1. Overview
The R. STAHL ET-208 is a compact, ruggedised HMI designed for operation in Zone 1 / Zone 2 hazardous areas. The unit is part of the SHARK device platform and is typically deployed where an intrinsically safe operator interface is required close to the process. In its standard configuration, the ET-208 does not carry a native MPI interface; instead, it exposes an RS-422 point-to-point port that speaks the Siemens RK512 protocol family (a 3964R / RK512 framing). To integrate the panel with a SIMATIC S7-300 or S7-400 CPU over the MPI bus, R. STAHL specifies a dedicated adapter - the SSW7-RK512-RS-422 - that converts the RK512 traffic to MPI/PROFIBUS on the bus side.
This guide documents the two real-world deployment patterns that the source projects require:
- Project 1: One ET-208 connected to the CPU's MPI port through a single SSW7-RK512-RS-422.
- Project 2: Two ET-208 panels connected to the same CPU MPI port, each terminated with its own SSW7-RK512-RS-422, both branches aggregated through a Helmholz FLEXtra twinRepeater (700-972-2AA02).
2. Why the SSW7-RK512-RS-422 Adapter is Required
SIMATIC S7-300 and S7-400 CPUs communicate with engineering stations, HMI panels, and other CPUs over the Multi-Point Interface (MPI) - a Siemens-proprietary bus based on the PROFIBUS physical layer (RS-485, 9-pin D-sub, asynchronous serial). WinCC Runtime uses MPI as one of its native HMI connection drivers, as documented in the TIA Portal help for Communication via MPI (Basic Panels, Panels, Comfort Panels, RT Advanced, RT Professional).
The Stahl ET-208 firmware does not speak the MPI token-passing protocol directly. Its COM port is wired for RS-422 and runs the Siemens RK512/3964R transport. The SSW7-RK512-RS-422 closes the gap by:
- Accepting the panel-side RS-422 3964R/RK512 frames.
- Mapping the read/write requests to MPI/PROFIBUS data access (PUT/GET-style or DB/PI/MI access depending on the Stahl configuration profile).
- Terminating the segment on the RS-485 side with the correct baud rate and station address.
The adapter's bus side is therefore a fully-fledged MPI/PROFIBUS slave that can sit on the same segment as the CPU's MPI socket, an HMI, or another repeater branch.
3. Hardware Bill of Materials
| Pos. | Component | Manufacturer | Function |
|---|---|---|---|
| 1 | SIMATIC S7-400 CPU (e.g. CPU 414, 416, 417) | Siemens | Provides the native MPI port (X1) that the HMI segments attach to. |
| 2 | ET-208 operator panel | R. STAHL | Ex-rated HMI for hazardous-area process visualisation. |
| 3 | SSW7-RK512-RS-422 MPI adapter | R. STAHL | RK512 ↔ MPI/PROFIBUS protocol converter; RS-422 port to the panel, RS-485 port to the bus. |
| 4 | FLEXtra twinRepeater, order code 700-972-2AA02 | Helmholz | Two-segment PROFIBUS/MPI repeater with galvanic isolation; merges the two SSW7-RK512 branches into the CPU MPI port (Project 2 only). |
| 5 | PROFIBUS cable (e.g. 6XV1830-0EH10) for MPI segments | Siemens / 3rd party | Twisted pair, shielded, characteristic impedance 150 Ω, 22 AWG typical. |
| 6 | 9-pin D-sub PROFIBUS connectors with bus termination | Siemens (6ES7972-0BA42) or equivalent | Segment termination, integrated termination resistor (220 Ω / 390 Ω). |
| 7 | RS-422 shielded twisted pair (e.g. LiYCY 4 x 0.25 mm²) | Various | Link from ET-208 to the SSW7-RK512-RS-422; pinout per Stahl wiring diagram. |
4. MPI Physical Layer, Bus Parameters, and Cabling Rules
Although S7-400 CPUs expose a 9-pin D-sub labelled "MPI", the underlying transport is identical to PROFIBUS DP at the electrical level. The relevant bus parameters are summarised below.
| Parameter | Value / Rule | Notes |
|---|---|---|
| Physical layer | EIA-485 (RS-485), 2-wire, half-duplex | Same transceiver as PROFIBUS DP. RS-485 is the differential descendant of RS-422; the panel-side RS-422 port on the SSW7-RK512 is converted to RS-485 on the bus side. |
| Baud rate supported by MPI | 19.2 kbit/s and 187.5 kbit/s | 12 Mbit/s is PROFIBUS DP only and is not a valid MPI rate. All nodes on the same segment must use the same baud rate. |
| Default MPI baud rate | 187.5 kbit/s | Recommended for new installations. |
| Maximum stations per MPI segment | 32 (addresses 0-31) | CPU, PG, OP, and S7-200/300/400 stations. Each repeater segment counts separately. |
| Maximum segment length at 187.5 kbit/s | 1,000 m | Per PROFIBUS DP cable spec. Use repeaters to extend beyond this distance. |
| Connector | 9-pin D-sub (male on the bus side) | Pin 3 = Data B (green core), Pin 8 = Data A (red core), Pin 5 = shield/drain, Pin 6 = VP (+5V isolated, optional). |
| Bus termination | Active at both physical ends of the segment | 220 Ω pull-up to +5V on pin 6, 390 Ω pull-down on pin 5, 220 Ω between pins 3 and 8 - built into Siemens 6ES7972 connectors. |
The PROFIBUS cable is a shielded, twisted-pair design with the following key properties:
- Characteristic impedance Z0 = 150 Ω ± 15 Ω at 3-20 MHz.
- Loop resistance ≤ 110 Ω/km.
- Capacitance ≤ 30 pF/m.
- Bending radius ≥ 10 x cable diameter (static), 20 x (dynamic).
5. MPI Addressing Plan
Every node on an MPI segment needs a unique address. The default assignments on a SIMATIC S7-400 system are well known and should be respected to avoid collisions.
| Address | Default Role | Notes |
|---|---|---|
| 0 | PG (engineering station) | Reserved for programming device. Do not assign to the HMI. |
| 1 | OP / HMI (default) | WinCC panel default MPI address. Avoid using 1 for the ET-208 if a WinCC panel shares the segment. |
| 2 | CPU (default MPI) | S7-400 CPU default MPI station address. Reserved. |
| 3 - 31 | Free for HMI, additional CPUs, SSW7-RK512 slaves | Plan deliberately; do not duplicate. |
For the projects in this guide, a workable address map is:
| Address | Project 1 (1 panel) | Project 2 (2 panels) |
|---|---|---|
| 0 | PG | PG |
| 1 | (free, for future OP) | (free, for future OP) |
| 2 | S7-400 CPU | S7-400 CPU |
| 3 | SSW7-RK512 #1 ↔ ET-208 #1 | SSW7-RK512 #1 ↔ ET-208 #1 |
| 4 | - | SSW7-RK512 #2 ↔ ET-208 #2 |
Each address must be set both on the S7-400 side (STEP 7 / TIA Portal) and on the SSW7-RK512 adapter (DIP switch or software tool, depending on the Stahl firmware revision - consult the Stahl manual for the exact procedure).
6. Single-Panel Configuration (Project 1)
The wiring for one ET-208 is a straight point-to-point-to-bus cascade:
- Run an RS-422 shielded cable from the ET-208's COM port to the panel-side connector on the SSW7-RK512-RS-422 adapter. Pinout must follow the Stahl wiring diagram shipped with the unit; a typical assignment uses the four RS-422 signals T+, T-, R+, R- plus signal ground.
- Mount a Siemens PROFIBUS D-sub connector on the bus-side of the SSW7-RK512 and wire it to the S7-400 CPU's MPI socket (X1) using PROFIBUS cable.
- Terminate the segment at both physical ends: the SSW7-RK512 (if it is the bus terminator) and the CPU MPI port (the CPU itself is not internally terminated - use a connector with switched termination).
- Set the MPI baud rate on the SSW7-RK512 to 187.5 kbit/s and configure its station address to 3.
7. Two-Panel Configuration with twinRepeater (Project 2)
For two ET-208 panels on the same CPU MPI socket, the topology becomes a star-like arrangement that needs a PROFIBUS/MPI repeater. The Helmholz FLEXtra twinRepeater (700-972-2AA02) integrates two electrically isolated segments into a single 35 mm DIN-rail module, which is ideal for cabinet-side installation close to the CPU.
Topology (logical view):
Key points for the twinRepeater topology:
- Connect the CPU's MPI socket to one repeater port (e.g. B1).
- Connect SSW7-RK512 #1 to port A1 and SSW7-RK512 #2 to port A2.
- Each repeater segment must be terminated at its physical end - the SSW7-RK512 itself or its connector - and must not be terminated at the repeater side, because the repeater regenerates the signal and isolates the segments.
- Set the repeater segment detection to "automatic" (default on the 700-972-2AA02) so it adapts to the 187.5 kbit/s rate used on the MPI bus.
- All three segments (CPU side, branch 1, branch 2) must run at the same baud rate.
8. Wiring, Pinout, and Shielding
Correct pin assignment is the single most common cause of intermittent MPI faults. Always cross-check against the device manuals; the values below are reference defaults.
8.1 MPI 9-Pin D-Sub (S7-400 CPU side and SSW7-RK512 bus side)
| Pin | Signal | Wire colour (Siemens 6XV1830) |
|---|---|---|
| 1 | n.c. | - |
| 2 | n.c. (M24V in some CPUs) | - |
| 3 | RxD/TxD-P (Data B) | Green |
| 4 | RTS (CNTR-P) | - |
| 5 | DGND (5V ground) | - |
| 6 | VP (+5V bus termination supply) | - |
| 7 | P24V (24V supply, optional) | - |
| 8 | RxD/TxD-N (Data A) | Red |
| 9 | n.c. | - |
| Shell | Cable shield / drain | Bare / drain wire |
8.2 ET-208 to SSW7-RK512-RS-422 (panel side)
The Stahl manual shipped with the SSW7-RK512 defines the exact pinout for the panel-side connector. The link is a 4-wire RS-422 full-duplex channel plus signal ground:
- T+ (TXD+ from panel), T- (TXD-), R+ (RXD+ to panel), R- (RXD-), and SGND.
- Use a shielded, twisted-pair cable with 120 Ω nominal differential impedance; shield bonded at the adapter end only when the panel is in a separate cabinet, or at both ends if both ends share a common ground bar.
- Maximum length is dictated by RS-422 driver capability (typically 1,200 m at 9.6 kbit/s; well below that for the higher RK512 rates the ET-208 uses - check Stahl's manual for the figure).
8.3 Termination
Termination must be applied at exactly two points: the two physical ends of each electrical segment. The PROFIBUS connectors with switchable termination (Siemens 6ES7972-0BA42 or equivalent) provide the standard 220 Ω / 390 Ω network. Intermediate nodes must be left unterminated.
9. Software Configuration
9.1 STEP 7 (Classic) / TIA Portal - CPU Side
- Open the S7-400 station in STEP 7 V5.x or in TIA Portal (V15 and later). For TIA Portal, follow the procedure documented under Communication via MPI in the TIA Portal help for Basic Panels, Panels, Comfort Panels, RT Advanced, and RT Professional (see the TIA Portal V21 manual).
- Confirm the CPU's MPI interface is enabled, with address 2 and baud rate 187.5 kbit/s. In TIA Portal: Devices & Networks → CPU → Properties → MPI address.
- Define a logical connection "ET208_MPI" of type S7 connection, with the local endpoint set to the CPU MPI interface and the partner endpoint set to unspecified (since the SSW7-RK512 is a third-party slave, not a Siemens HMI that you can browse).
- Set the connection resource to one of the CPU's free S7 connection resources. Each S7-400 CPU has a finite number (e.g. CPU 414-3: 16, CPU 416-3: 32, CPU 417-4: 64 - refer to the CPU's datasheet for the exact count).
9.2 Stahl SSW7-RK512-RS-422 Adapter - Parameters
Configuration is normally performed with a Stahl configuration tool (Windows) that is downloaded to the adapter over its service USB or RS-232 port. The required fields are:
| Field | Project 1 Value | Project 2 Value |
|---|---|---|
| Bus protocol | MPI slave | MPI slave |
| Bus baud rate | 187.5 kbit/s | 187.5 kbit/s |
| MPI station address | 3 | 3 (SSW #1) and 4 (SSW #2) |
| Highest station address (HSA) | 31 | 31 |
| Panel-side protocol | RK512 / 3964R | RK512 / 3964R |
| Panel-side baud rate | Per Stahl default (commonly 9.6 / 19.2 / 38.4 kbit/s) | Same; both adapters must match the panel |
| Data area mapping | DB / PI / PA / M (per Stahl DB) | Same; two adapters must not overlap data areas |
9.3 HMI Tag Configuration in WinCC
- Open the WinCC project for the ET-208 (or for a WinCC Runtime that is logically attached to the project). Create a new HMI connection of type SIMATIC S7 300/400 and select the protocol MPI.
- Enter the CPU's MPI parameters: address 2, baud 187.5 kbit/s. The connection path goes through the SSW7-RK512 slave; the WinCC driver does not need to be aware of the adapter's address 3 - it talks MPI directly to the CPU and the adapter is transparent at the bus layer.
- Define tags with the correct data-block addresses on the CPU side. Avoid using bit-level access to large DBs - aggregate into 16-bit or 32-bit variables where the panel allows it.
10. Verification and Commissioning Procedure
- Power check. Verify 24 VDC at the SSW7-RK512 and at the ET-208, and 120/230 VAC or 24 VDC at the S7-400 PS407/PS405 power supply, as applicable.
- Wiring continuity. With the bus powered down, measure the DC resistance between pins 3 and 8 at each connector. A properly terminated segment should read ~220 Ω. An unterminated segment should read several kΩ (the on-board input impedance of the transceivers).
- Address uniqueness. In STEP 7 / TIA Portal, run Target system → Display accessible nodes (STEP 7) or Online → Accessible nodes (TIA). All configured stations (CPU 2, SSW 3, SSW 4, any PG) must be listed exactly once.
- Adapter diagnostics. Open the Stahl configuration tool and read the adapter's diagnostic buffer. No "bus fault", "station failure", or "address collision" entries should be present.
- Cyclic test. From WinCC, read and write a known tag (e.g. a counter in DB100) at 1-second interval for 10 minutes. The S7-400 diagnostic buffer should remain free of MPI communication-error entries.
- Panel-side loopback. On the ET-208, trigger a screen change that is driven by a CPU tag. If the screen updates, the entire stack (CPU → MPI → SSW7-RK512 → RS-422 → ET-208) is healthy.
11. Troubleshooting Matrix
| Symptom | Likely Cause | Check / Fix |
|---|---|---|
| SSW7-RK512 not visible in "accessible nodes" | Wrong baud rate; missing termination; MPI cable crossed (A/B swapped) | Verify all nodes at 187.5 kbit/s; check pin 3 = green, pin 8 = red at every connector; confirm two terminations per segment. |
| ET-208 shows "no PLC link" although SSW7-RK512 is online | RS-422 wiring fault, RK512 parameter mismatch, or panel baud wrong | Loopback-test the RS-422 cable; verify RK512 baud and station address match the panel project; check the Stahl adapter's "panel status" LED. |
| Intermittent MPI faults at shift change / motor start | EMC coupling into the PROFIBUS cable; shield not bonded at gland | Re-bond the cable shield to the backshell of the D-sub connector, not to the conductor pin; reroute the cable away from VFD power cables; check that the cabinet ground bar is at the same potential as the field junction box. |
| Both ET-208 panels drop out simultaneously (Project 2) | twimRepeater lost power, or its CPU-side port is unterminated while both branch ends are terminated | Verify 24 V to the Helmholz repeater; enable termination on the segment between the repeater and the CPU; check that no more than two terminations are powered per segment. |
| Address collision in STEP 7 "accessible nodes" | Two devices configured to the same MPI address | Re-plan the address map from Section 5; assign unique addresses to the SSW7-RK512 adapters; download the configuration. |
| WinCC reports "partner not reachable" only after warm restart | CPU comes up faster than the SSW7-RK512; first token round missed | Add a 2-3 second delay in the S7-400 startup OB100 before enabling cyclic MPI traffic, or enable the adapter's "retries on startup" option in the Stahl tool. |
| Tags update on WinCC but not on the ET-208 | Two S7 connections with overlapping data areas; one adapter shadows the other | Split the CPU data into disjoint DBs (e.g. DB100 for panel 1, DB200 for panel 2); verify each adapter's data area mapping is exclusive. |
12. Migration Note: Newer STAHL Panel Family
R. STAHL has since extended the SHARK platform with the ET-438 and MT-438 panel PCs, which include native Ethernet and are designed for OPC UA / PROFINET integration rather than MPI. New projects that do not have a hard requirement for the ET-208 and its RS-422 / RK512 stack should evaluate the 438 series, as described in the R. STAHL ET 438 & MT 438 panel PCs announcement. The SSW7-RK512-RS-422 path documented here remains the correct approach for installations that must reuse the ET-208 or for which the 438 series is not yet certified in the target hazardous area.
13. Frequently Asked Questions
Does the Stahl ET-208 have a native MPI port?
No. The ET-208 exposes an RS-422 port running 3964R/RK512. An external adapter - the R. STAHL SSW7-RK512-RS-422 - is required to translate RK512 to MPI/PROFIBUS and to terminate the segment on the bus side.
Can I connect two ET-208 panels to a single S7-400 MPI socket?
Yes, by using one SSW7-RK512-RS-422 per panel and aggregating the two PROFIBUS branches through a Helmholz FLEXtra twinRepeater (700-972-2AA02). The twinRepeater provides two galvanically isolated segments so that both adapters can run at 187.5 kbit/s on the same CPU MPI port without contention.
What MPI baud rate must I use?
Use 187.5 kbit/s unless an existing segment is locked at 19.2 kbit/s. All nodes on the same electrical segment, including the SSW7-RK512 adapters and the S7-400 CPU, must be set to the same baud rate. 12 Mbit/s is a PROFIBUS DP rate and is not valid for MPI.
How many stations can I put on one MPI segment?
Up to 32 stations (MPI addresses 0-31). Reserved defaults are 0 for the PG, 1 for the OP/HMI, and 2 for the S7 CPU. Plan the SSW7-RK512 adapter addresses around these defaults; the two-panel example in this guide uses addresses 3 and 4.
Where can I find the official TIA Portal procedure for MPI on S7-300/400?
The configuration of an MPI HMI connection for SIMATIC S7-300/400 in WinCC Runtime is documented in the TIA Portal help under Communicating with PLCs → SIMATIC S7 300/400 → Communication via MPI, available in the TIA Portal documentation portal (e.g. TIA Portal V21: Communication via MPI).
What is the maximum segment length at 187.5 kbit/s?
1,000 m per PROFIBUS DP cable specification, end-to-end including the repeater chain. Use additional repeaters to extend the network, with up to 10 repeaters in series between any two stations and a maximum of 32 stations per segment.