Configuring S7-300 MPI Network with TP177A Panels and PC Adapter
This reference documents the practical wiring, connector selection, addressing, and commissioning sequence for an S7-300 CPU connected to two TP177A operator panels and a programming PC over the MPI port. Because the Multi-Point Interface (MPI) shares the RS-485 physical layer with PROFIBUS-DP, you can reuse PROFIBUS-DP cables, connectors, and termination practice for an MPI segment without violating either protocol. The approach below avoids burning the PROFIBUS-DP interface for HMI traffic so it remains free for higher-bandwidth fieldbus devices.
1. Problem Definition
The target topology is:
- One SIMATIC S7-300 CPU (for example CPU 315-2 DP, 6ES7315-2AH14-0AB0) on its MPI/DP combined port operating as MPI master.
- Two SIMATIC TP177A 6" touch panels (6AV6642-0BA01-1AX0) on MPI as slaves.
- One engineering PC using a Siemens PC Adapter USB (6ES7972-0CB20-0XA0) plugged in only during commissioning or for runtime diagnostics.
- PROFIBUS-DP interface of the S7-300 left untouched for a separate DP segment.
The constraint is that no PROFIBUS-DP traffic may run on the MPI segment, and no DP cable may need to be borrowed from the DP master. The hardware-level answer is yes: MPI runs over RS-485 and uses the same PROFIBUS cable and 9-pin D-sub connectors.
2. Prerequisites
| Item | Specification | Notes |
|---|---|---|
| S7-300 CPU with MPI/DP port | Any CPU 31x with X1 MPI/DP combined connector | Set X1 selector switch to MPI for this segment |
| TP177A panel | 6AV6642-0BA01-1AX0 (MPI/RS-485 variant) | Confirm the IF1B RS-485 port on the bottom |
| PC Adapter USB | 6ES7972-0CB20-0XA0 | USB to MPI/DP, 12 Mbps max |
| PROFIBUS cable | 6XV1830-0EH10 (violet, 2-core, shielded, twisted) | Loop impedance ~135-165 Ω/km, capacitance ~30 nF/km |
| PROFIBUS connectors, with piggyback | 6ES7972-0BB50-0XA0 | Two required: one at CPU end, one at far end |
| PROFIBUS connectors, without piggyback | 6ES7972-0BA50-0XA0 | Used on intermediate nodes |
| Termination | On connector, switchable 390 Ω / 220 Ω / 390 Ω bias network | ON only at segment ends |
| Engineering software | STEP 7 V5.5 SPx or TIA Portal V13+ | TP177A projects require WinCC flexible 2008 SP5 or TIA Portal |
3. MPI Physical Layer and Protocol
MPI is a Siemens-proprietary, RS-485-based, token-passing, half-duplex master-slave network. The relevant physical-layer numbers are:
| Parameter | MPI value | Reference |
|---|---|---|
| Physical layer | RS-485, 9-pin D-sub, pin 3 = B, pin 8 = A | SIMATIC S7-300 CPU 31xC and CPU 31x Manual, section 4.2 |
| Default baud rate | 187.5 kbit/s | STEP 7 hardware catalog default for MPI |
| Optional baud rates | 19.2 / 187.5 kbit/s (default); 1.5 / 3 / 6 / 12 Mbit/s only with S7-300/400 + CP at 12 Mbps | PC Adapter USB supports up to 12 Mbit/s |
| Max nodes per MPI segment | 32 addresses (0-31), address 0 reserved for PG, address 1 reserved default | STEP 7 "MPI Address Overview" report |
| Repeaters | One RS-485 repeater (6ES7972-0AA01-0XA0) extends segment to 32 + 31 additional nodes and doubles segment length | Each repeater counts as one node load |
| Cable length, single segment, 187.5 kbit/s | 50 m max without repeater | Pro-face SIMATIC S7 MPI Direct Driver Manual, cable-length note |
| Cable length, full speed 12 Mbit/s | 100 m max without repeater | PROFIBUS IEC 61158-2 distance table |
4. Cable and Connector Selection
MPI has no dedicated Siemens cable catalog number, but the PROFIBUS cable 6XV1830-0EH10 is the universal RS-485 cable for any Siemens 9-pin D-sub port. The 9-pin pinout used by both MPI and DP at the S7-300 is:
| Pin | Signal | Function |
|---|---|---|
| 1 | nc | — |
| 2 | M24V | 24 V return (ground) |
| 3 | RxD/TxD-P (B) | Data line B |
| 4 | RTS | Request to send |
| 5 | M5V | 5 V return (data ground) |
| 6 | P5V | +5 V, terminated only at end node |
| 7 | P24V | +24 V, fused in connector |
| 8 | RxD/TxD-N (A) | Data line A |
| 9 | nc | — |
Connector ordering (full ordering data per Siemens catalog 6GK1972-0BA50-0XA0, 6GK1972-0BB50-0XA0 product info):
| MLFB / Part Number | Description | Use |
|---|---|---|
| 6ES7972-0BA50-0XA0 | PROFIBUS connector, axial cable entry, 35° outgoing, without PG socket | Intermediate TPs and any node you will not plug the PC Adapter into |
| 6ES7972-0BB50-0XA0 | PROFIBUS connector, axial cable entry, 35° outgoing, with PG socket (piggyback) | The CPU end and the panel end where you will connect the PC Adapter during commissioning |
| 6ES7972-0CB20-0XA0 | PC Adapter USB A2 | The PC connection; powered via USB |
5. Topology and Wiring
The MPI segment is a linear, daisy-chained RS-485 bus. The S7-300 CPU's MPI/DP port X1 occupies one end. Each TP177A mounts one PROFIBUS connector on its IF1B port. The second TP177A forms the far end of the segment.
The PC Adapter is plugged into the piggyback socket of the connector mounted on TP177A #1 or on the CPU, not into a free node. Plugging the adapter creates a temporary 33rd bus load that is removed when the PC disconnects.
5.1 Termination rules
- Activate the termination switch on the connector at each physical end of the segment only.
- For the topology above, ON at the CPU connector and ON at TP177A #2. All intermediate nodes OFF.
- The bias network (390 Ω pull-up on pin 8, 390 Ω pull-down on pin 3, 220 Ω between them) draws ~10 mA on the +5 V supply. Only the two end nodes must supply it; intermediate terminations disturb the impedance.
- Termination power comes from the node's pin 6 (P5V). If the CPU is powered down, only TP177A #2 is the active terminator; if both end nodes are unpowered, the bus floats and you see SF/BF faults on the panels.
6. MPI Addressing
Each device on an MPI segment must have a unique address in 1..31. Address 0 is reserved for programming devices in STEP 7 (PC/PG default), but a PC Adapter does not consume an address; it joins the token ring only while online.
| Device | MPI address | Highest MPI address | How set |
|---|---|---|---|
| S7-300 CPU | 2 | 31 | STEP 7 → Hardware → CPU properties → MPI/DP interface |
| TP177A #1 | 3 | 31 | WinCC flexible / TIA Portal → Devices & Networks → MPI/DP Interface → Address |
| TP177A #2 | 4 | 31 | Same as #1 |
| PC Adapter (transient) | 0 (default) or auto | n/a | Set in PC Adapter's MPI dialog; do not assign an address to it |
7. STEP 7 / TIA Portal Configuration
7.1 CPU side (STEP 7 V5.5)
- Open the S7 project, select the CPU in HW Config, double-click the MPI/DP port.
- Set Interface: MPI.
- Address:
2, Highest MPI address:31. - Subnet:
MPI(1)187.5 kbit/s. - Confirm. Save and download to the CPU.
7.2 TP177A side (WinCC flexible 2008 SP5)
- Insert TP177A device from catalog → 6" Touch → TP177A.
- Open Connections → right-click → Add Connection → SIMATIC S7-300/400 → MPI/DP.
- Station address =
3for TP #1,4for TP #2. - Slot =
2(CPU slot). - Set the network to
187.5 kbit/s. - Compile, transfer, and set the HMI's Control Panel → S7-Connection to the same address on the panel itself (Start → Settings → MPI/DP).
7.3 PC Adapter side
- Install PC Adapter USB driver (part of STEP 7 / TIA Portal setup).
- In the PC Adapter's MPI dialog set Address =
0and Baud =187.5 kbit/s. - In STEP 7 choose Set PG/PC Interface → PC Adapter (MPI) and click Diagnostics → Test to read the online list.
8. Verification
- With only the CPU powered, measure ~110 Ω across pins 3-8 at any unpowered connector (two 220 Ω terminators in parallel = 110 Ω; with bias network in series the DC value is lower but the AC impedance is correct). Open circuit or 220 Ω indicates only one end terminated.
- Power the CPU; SF and BF LEDs should extinguish within 3-5 seconds.
- Power TP177A #1; the panel should boot to the configured screen.
- Power TP177A #2; both panels should remain online.
- Plug the PC Adapter into the piggyback of the CPU connector. In STEP 7 run Accessible Nodes; you should see CPU at 2 and the two panels at 3 and 4.
- From WinCC flexible, use Online → Start Runtime on each panel; status bar should show
Connection active.
9. Diagnostics
| Symptom | Most likely cause | Action |
|---|---|---|
| SF on CPU + BF steady on | No termination at one end, or cable short | Verify termination switches; check for 220 Ω between A and B at each end connector with the node unpowered |
| One panel never connects, the other is fine | Duplicate MPI address or bad connector pins | Power only the CPU and the failing panel; if it connects, suspect termination; if not, swap connectors and retry |
| PC Adapter "Accessible Nodes" empty | Baud mismatch or termination off at CPU end | Force PC Adapter baud to 187.5 kbit/s; re-check the X1 selector switch on the CPU (MPI vs DP) |
| Intermittent timeouts at runtime | Cable near VFD output or unshielded | Re-route cable >200 mm from VFD power, ground shield at both ends through PROFIBUS connector |
| Panels reboot together with CPU | CPU is supplying 24 V on pin 7; connector fuse on CPU blown | Inspect CPU MPI/DP connector; replace if pins damaged |
| TP177A shows "Connection interrupted - 33" | Address mismatch between panel and project | Match panel Control Panel address with WinCC project |
10. Power and Fusing
The PROFIBUS connector on the S7-300 is internally fused at ~200 mA on the +24 V pin 7 to power the bus terminator of downstream nodes. When you plug the PC Adapter into the piggyback, the adapter receives +24 V from the host's pin 7. The PC Adapter draws <120 mA. If you are powering the adapter from the PLC end, do not exceed the cumulative rating of two terminators + one PC Adapter (~240 mA peak). If the PC is located >5 m from the CPU, use an external 24 V supply for the PC Adapter (the optional adapter power supply 6ES7972-0PB01-0XA0) instead of drawing from the connector.
11. Field-Commissioning Tips
- Wire the segment in the cabinet before energizing. Power up only the CPU, confirm PC Adapter finds it, then add panels one at a time.
- Use the same PROFIBUS cable color for both ends to make polarity obvious (violet jacket = A in green, B in red inside).
- Always strip shield back only as far as the connector clamp; do not let the shield reach the data pins.
- Label each connector with its termination state using a yellow sticker ("ON" / "OFF").
- If you ever need a third HMI, you have 27 addresses left; you do not need a repeater unless the cabinet run exceeds 50 m.
12. Safety and Maintenance
- Hot-plugging the PC Adapter is allowed by design (the connector provides power sequencing), but the cable should be physically secured to avoid lateral stress on the host's D-sub.
- Inspect connectors annually: bent pin 3 or 8 is the most common failure mode after years of plug cycles.
- Replace connectors as a pair (both ends of a cable) when one end has been re-terminated more than three times; the IDC insulation-displacement contacts lose spring tension.
- The TP177A IF1B port is not isolated; ground loops between cabinets must be broken by running the shield at one end only, or by using a fiber-optic PROFIBUS OLM for galvanic isolation.
Can I run MPI and PROFIBUS-DP on the same connector at the same time?
No. The S7-300's X1 MPI/DP combined port is either MPI or DP, selected by a physical switch under the door or by STEP 7 hardware configuration. Use X1 for MPI and X2 (if present) for DP if you need both buses.
Do I need a special "Y cable" to attach the PC Adapter?
No. The piggyback PROFIBUS connector 6ES7972-0BB50-0XA0 exposes a second female DB-9 on the back of the connector so you can plug the PC Adapter without interrupting the segment. A true Y cable is unnecessary and not supported for MPI/DP.
What is the maximum cable length for an MPI segment at 187.5 kbit/s?
50 m per segment without repeater; 100 m total is reachable with one RS-485 repeater (6ES7972-0AA01-0XA0). For longer runs, switch to PROFIBUS cable rated for 12 Mbit/s and use a repeater chain, or migrate to PROFINET.
Why does my TP177A show "MPI connection interrupted" even though wiring is correct?
Most often the panel's local MPI address (Control Panel → MPI/DP) does not match the address configured in WinCC flexible. Set the panel to address 3 (TP #1) or 4 (TP #2) and reload the project.
Can I substitute CAT5 cable for PROFIBUS cable on MPI?
Not recommended. PROFIBUS cable has 150 Ω characteristic impedance, defined shielding, and a defined core diameter; CAT5 is 100 Ω and unbalanced for RS-485 use. You will get reflections and intermittent faults over any distance beyond a few meters.