PLC with Ethernet vs. Retrofitting an S7-200 PLC: A Cost Comparison That Surprised Me

I've spent the past six years managing the electrical parts budget at a mid-sized industrial maintenance company. Our annual spend on PLC-related controls runs around $120,000—maybe $118,000, I'd have to check the system. I've negotiated with a dozen or so vendors over that time and documented every order in our cost tracking system since 2019. This article isn't a generic comparison of PLC brands. It's a practical look at two options we've actually priced: keeping an existing S7-200 PLC alive with converters, or replacing it with a modern PLC with Ethernet. If you're researching industrial PLC programming or you're stuck trying to decide between a legacy S7-200 PLC and something newer, this is the cost breakdown I wish someone had given me.

The Comparison Framework

I'm not a controls engineer, so I can't speak to every programming nuance. What I can tell you from a procurement perspective is how the PLC electrical work showed up in quotes, invoices, and change orders. That's where the real difference lives.

We compared two paths on a CNC machine PLC control project:

• Path A: Keep the S7-200 PLC and add the serial-to-Ethernet components needed for remote access and AC frequency changer integration.
• Path B: Replace the S7-200 PLC with a PLC with Ethernet that communicates directly with the VFDs and HMIs.

The comparison standard was total cost of ownership over three years, not the first quote. I looked at hardware, programming, commissioning, maintenance, and the likelihood of hidden extras.

Dimension 1: The Hardware Quote Almost Fooled Me

On paper, Path A was cheaper. The legacy S7-200 PLC CPU was quoted at $1,150, and the serial-to-Ethernet converter was $380. Total hardware: $1,530. Path B was quoted at $2,700 for a PLC with Ethernet, but that figure included the CPU, network connection, and all cables. At first glance, the difference was $1,170.

Then I asked, what's NOT included? That changed everything.

The Path A quote didn't include the communication board for the AC frequency changer. That was $310. It didn't include a programming cable for the technician's laptop. That was $120. And the vendor charged $200 for a legacy compatibility check that wasn't in the original email. Path A's real hardware total was $2,160. Path B's real hardware total was still $2,700. The gap shrank to $540.

I've learned to ask what's NOT included before what's the price. The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. That's been true in every electrical project I've audited.

Dimension 2: Industrial PLC Programming and Commissioning

This is where Path A lost me completely. The CNC machine PLC control had been modified by three different technicians over the years. The program was a mess, and the old S7-200 PLC used a serial port that didn't want to connect to modern laptops. Our controls contractor quoted 12 hours for industrial PLC programming on Path A and 7 hours on Path B.

Why? Because the PLC with Ethernet had faster uploads, cleaner documentation templates, and standard function blocks for the AC frequency changer. The technician didn't have to babysit a serial connection every time he wanted to go online with the machine.

At $95/hour, that 5-hour difference was $475. It wiped out most of the remaining hardware gap.

I should add that Path A required an extra site visit because the programmer forgot the special RS-232 adapter. That wasn't in the quote. It was $200 for travel. The PLC electrical panel on Path B was also easier to verify, because the network connection gave us remote diagnostics from the shop floor PC.

Dimension 3: The S7-200 PLC Spare Parts Problem

I still kick myself for stocking a spare S7-200 PLC CPU in 2018 instead of planning a migration. That spare gave us a false sense of security for years. When one of our machines actually failed in Q2 2024, the spare CPU wasn't the issue—the AC frequency changer was. And the new VFD didn't speak the same protocol as the old PLC without an extra option card.

The S7-200 PLC is a discontinued product, according to Siemens support documentation (support.industries.siemens.com). That means the spare parts market is the only market. I want to say a used CPU was around $400 last year, but don't quote me on that exact number. The trend is clear: used S7-200 PLC modules keep getting scarcer and more expensive. A PLC with Ethernet, on the other hand, is still in production. You can price a spare from a normal distributor, order it, and know the exact cost before you need it.

From a cost-control perspective, unpredictability is a hidden cost. It doesn't show up in a quote, but it shows up in your annual budget when an emergency repair costs triple the planned amount.

Dimension 4: AC Frequency Changer and CNC Machine PLC Control Flexibility

Here's the result that surprised me: Path B was actually cheaper on the next project, not just even.

We retrofitted a conveyor system with an AC frequency changer and an HMI. The modern PLC with Ethernet handled the VFD communication natively. The programming time was 4 hours, and the total controls quote was $1,400. If we'd tried to do the same job on an S7-200 PLC, we needed a communication module, a different VFD with an option card, and extra PLC electrical wiring. The quote for that approach was $2,300.

So the PLC with Ethernet wasn't the premium option. It was $900 less than keeping the legacy platform alive. That's the opposite of what I expected when I started the comparison.

The reason is simple: the S7-200 PLC was designed before Ethernet and modern drive communication were normal. Every upgrade adds a piece of hardware. A PLC with Ethernet was designed to talk to AC frequency changers and HMIs as a standard feature. The cost is built in, not bolted on after the fact.

Which One Should You Choose?

If a machine with an S7-200 PLC is running fine and you're not changing anything around it, leave it alone. Don't spend money on a migration just because the platform is old. Start a small reserve fund for the day when something around it fails.

If you're planning new industrial PLC programming, adding an AC frequency changer, or modernizing a CNC machine PLC control, choose a PLC with Ethernet. Not because it's flashy, but because the total cost is visible. You can see the CPU cost, the programming time, the VFD integration, and the spare part strategy. There are fewer hidden line items.

This gets into network security territory, which isn't my expertise. I'd recommend talking to your controls engineer about firewalls and VLANs before you put PLCs on the plant network. But from a budget standpoint, the decision is clear to me now: a transparent quote for a PLC with Ethernet beats a vague low quote for a legacy S7-200 PLC every time. The expense you plan for is always cheaper than the one you discover later.