Use Case · October 2026

Old PLC, New Tricks: Put a Unified Panel on Your S7-300/400 Without Replacing the PLC

Replacing the HMI on an S7-300 or S7-400 doesn't have to mean replacing the PLC. One small Hilscher gateway and one Ethernet cable put a new Unified panel on a veteran MPI or PROFIBUS SIMATIC. The goal: not one bit of the PLC program changes.

This one started with an urgent need. An iconic OEM had a large deployment of machines in the field, all built on SIMATIC controllers talking MPI or PROFIBUS. The PLCs were still doing their job. The HMIs needed an update, across the whole installed base.

The textbook answer was ugly: replace the control system, machine by machine. That means re-engineering, re-testing and downtime at every site. In other words, exactly the massive cost and disruption the OEM couldn't afford.

Here's the good news: the PLC isn't the problem. The panel is. And in many cases, you can replace the panel without laying a finger on the PLC.

The trick: an MPI to Ethernet gateway (think travel adapter)

An older S7-300 or S7-400 without an Ethernet port talks over MPI or PROFIBUS. A new SIMATIC HMI Unified Comfort Panel wants Ethernet. Different plugs, different languages.

The Hilscher NT 50-MPI (part 1758.111) sits in the middle and translates: PROFIBUS MPI/DP/PPI on one side, Ethernet on the other.

The panel speaks a standard Ethernet protocol, ISO on TCP (RFC1006), which the gateway was built to support for SIMATIC and third-party panels. The gateway hands the panel's requests to the MPI/PROFIBUS side, and the PLC answers on the cable it has always used.

To the panel, the PLC lives on Ethernet. To the PLC, it's business as usual. Everybody's happy.

The new panel: SIMATIC HMI MTP1200 Unified Comfort (example). Image: Siemens AG

The gateway: Hilscher NT 50-MPI (part 1758.111). Image: Hilscher

What stays where it is

This is the part your maintenance team will like best.

• The PLC. Same CPU, same I/O modules, same spot in the cabinet.
• The program. No edits, no download. Do dig out the existing STEP 7 project or the old HMI project, though: that's where the new screens start.
• The PROFIBUS cabling at the PLC. The network stays put. The only connector that moves is the one that used to plug into the old panel.
• The logic your operators trust. It keeps running the machine the way it did yesterday.

The goal: the PLC keeps its program, its hardware and its cabling. The only thing that changes is who it's talking to.

The swap, step by step

1. Unplug the existing PROFIBUS connector from the old operator panel.
2. Plug it into the Hilscher NT 50-MPI, a compact DIN-rail box that runs on 24 V DC.
3. Run one Ethernet cable from the gateway to the new Unified panel, for example a SIMATIC HMI MTP1200 Unified Comfort Panel with a 12-inch screen.
4. Configure and connect. Set up the gateway with SyCon MPI or the IBHnet driver and point the panel's connection at it. The panel talks ISO on TCP (RFC1006), and the gateway takes care of the MPI/PROFIBUS side.

That's the hardware side. The real engineering is the new screens, built in TIA Portal against the PLC you already have. That's where the design work happens.

Bonus: program it over Ethernet too

The NT 50-MPI moonlights as a programming adapter. It integrates into STEP 7 and TIA Portal as a PG adapter, so an engineer can go online with the PLC over Ethernet instead of digging for the one MPI cable that lives in somebody's truck. One box, two jobs.

A shout-out to Mike Milton at Control Source

When that OEM's urgent need landed on our desk, we went looking for a smarter way, and we called a good distributor partner.

When we told Mike Milton, our account manager at Control Source, Inc., about it, he went digging. In April 2026 he talked it through with Hilscher North America and came back with the NT 50-MPI and a refreshingly simple plan: move the PROFIBUS cable to the gateway, run Ethernet to the new panel, and leave the PLC alone.

He didn't stop at the tip. Mike arranged a Hilscher demo unit and offered to come test it with us, sent 3D models of the gateway housing, and Control Source turned around a quote with stock and lead times so we could plan real jobs around it. Thank you, Mike. This is exactly what a great distributor partner looks like.

Then Sam Priddy, one of our Think-PLC engineers, took it from idea to working hardware: in June 2026 he had it running on two panels in our shop. Nice work, Sam.

A note for the detail-minded: the MTP1200 above is an example of a new Unified panel, not a list of what was on the bench.

The new panel? We build that too.

The gateway is only half the job. The other half is the screen your operators will be looking at for the next decade, and that's home turf for us.

And the services that go with it

• New screens, built in TIA Portal. We build your operator screens in WinCC Unified against the PLC you already have, and simulate the PLC and HMI code in PLCSIM before it ever sees the machine.
• Cutover and training. We handle the swap and leave you with as-builts, backups and training.
• Field service. When you need hands on the machine, our field service team comes to you.

Want it as one tidy package? Ask us about pairing the gateway with a new HMI Station, with screens and install included.

Is this right for your machine?

Honest answer: it depends on the machine.

It's a great fit when...

• The PLC is healthy and the program works. The panel is the weak link.
• The PLC is a SIMATIC S7-300 or S7-400 on MPI or PROFIBUS.
• You need an S7-300 HMI replacement now, and you'd rather not re-prove logic that already works.
• Ethernet access for programming laptops sounds like a nice bonus.
• You'd rather plan a full migration on your own schedule than in an emergency.

A full upgrade is the better call when...

• The PLC itself is the problem: failing hardware, spares you can't find, or no room left for the changes you need.
• The machine needs more than the existing CPU and modules can deliver, such as a major expansion or new functions they don't support.
• The machine is being rebuilt anyway, so there's no better moment to do everything at once.
• You're standardizing a whole plant on one modern platform, one library and one set of screens.

One honest check before anyone promises a zero-touch swap: how was the old panel talking to the PLC? Some older panels lean on handshakes built into the PLC program, such as area pointers or alarm bits, and a Unified panel may handle those differently. And if the full upgrade wins, we handle upgrades and retrofits too.

Give your old PLC a new face

Got a tired Comfort or Basic panel on a perfectly good S7-300 or S7-400? Let's talk panel replacement. Our expert consultants will look at your PLC, your panel and your plans, and tell you straight what fits: a gateway, a complete HMI Station with new screens, or a full upgrade.

Think-PLC designs, builds, installs and services control systems from Winston-Salem, NC, with Siemens partner expertise, a UL 508A panel shop and field service that comes to you.

Talk to an engineer Call 888-407-5448

Design. Build. Install. Service.