Siemens S7-300 and S7-400 Monitoring - Native PLC Visibility Without Gateways

Somewhere on your production floor, there is a Siemens S7-300 or S7-400 running a process that simply cannot go down. A filling line. A pump station. A conveyor feeding the next stage of assembly. And chances are, nobody on the IT side is watching it the way they watch a server.

That is an odd gap once you think about it. Servers get dashboards and monitoring around the clock almost as a matter of course. The controller quietly running the actual machinery? Often it is invisible until something breaks, even though it sits squarely within OT network monitoring.

This article introduces three new native PRTG sensors built for Siemens S7-300 and S7-400 PLCs. They talk to the controller directly over its own S7 protocol, and put the results right where the rest of your infrastructure already lives: your PRTG dashboard.

Why Monitoring a Siemens S7-300 or S7-400 Has Always Been Hard

Ask anyone who has tried, and they will tell you: it was never that nobody wanted to monitor these controllers. It was that doing so used to mean a project of its own, especially for teams whose industrial IT monitoring setup was built with servers and switches in mind, not PLCs.

Two things get in the way. The Siemens S7-300 and S7-400 lines were designed decades ago, long before anyone assumed a PLC would report into the same dashboard as a Windows server. Plenty have been running the same line for fifteen or twenty years without complaint, but out of the box they hand over almost nothing a monitoring tool can use directly.

So the usual fix was to put something else in between: an OPC server, a protocol gateway, sometimes a separate monitoring product that speaks S7 and translates it into something else. That means more hardware, more licenses, and one more component that can quietly fail on a Friday afternoon - and even then, the data often lands somewhere your IT team does not normally look.

Which is why plenty of teams settled for the next best thing: pinging the PLC. Reachable? Good enough, or so it seemed. Except a Siemens S7-300 CPU can respond happily to a ping while sitting in stop mode, doing nothing at all. Ping confirms the network path. It says nothing about whether the CPU is actually running your process.

Guessing whether your Siemens S7-300 or S7-400 is really in RUN should not be part of the job. See it for yourself: download PRTG free for 30 days and add native SIMATIC monitoring in minutes.

What's New: Native S7-300 and S7-400 Monitoring in PRTG

The three new sensors connect straight to the PLC using its own S7 protocol, minus the extra hop through an OPC server or gateway. Add the PLC as a device in PRTG, add the sensors, and the data shows up next to everything else you already monitor: same dashboards, same alerts, same reports.

This builds on OT monitoring that PRTG already offers through standards such as OPC UA, Modbus, and MQTT. What is different here is depth: instead of stopping at the protocol layer, these sensors reach into the controller itself.

All three sensors are currently available as beta sensors: fully usable today, still being refined based on feedback from people running real Siemens hardware.

The Three New Sensors for Siemens S7-300 and S7-400

🧩 CPU Status: Is the Controller Actually Running?

This is the sensor to add first if you only add one. The SIMATIC S7-300/400 CPU Status sensor reports whether the CPU is in RUN, meaning your process is executing, or somewhere it should not be: DEFECTIVE or NO POWER on the serious end, STARTUP, HOLD, and various STOP mode states in between. It also tracks cycle time - how long one program loop takes - against a maximum you define, so PRTG can warn you before a controller gets pushed too close to its limit rather than after the fact.

Picture an S7-300 sequencing a bottling line. Its CPU, say a 315-2 DP, drops out of RUN into STOP mode. With this sensor watching, an alert carrying the controller's name lands in your inbox before the first call comes in from the shop floor.

🧩 CPU Restart Overview: Catching Patterns Before They Become Failures

A single restart rarely means much. A pattern of them does. The SIMATIC S7-300/400 CPU Restart Overview sensor counts every restart the S7-300 or S7-400 performs and separates them by type - cold, warm, and hot - and by whether a person triggered it or the controller restarted on its own.

That second distinction matters. A manual restart during a maintenance window is nothing to worry about. A climbing number of automatic restarts, especially overnight, often points to something like a failing power supply: a problem quietly getting worse, invisible because the controller keeps recovering by itself.

Take a PLC running a water treatment process, restarting itself three times overnight. The process recovers every time, so nobody nearby even notices. In PRTG, the restart counters keep climbing and spotting that slow trend gets a technician scheduled for a planned power supply swap instead of an emergency one.

🧩 LED Status: Reading the Control Cabinet Without Leaving Your Desk

Every Siemens S7-300 and S7-400 has a row of status LEDs on its front panel, and those lights carry more information than most people expect: power, internal and external faults, battery health, bus errors on interfaces such as Profibus DP, forced values, maintenance flags. The catch is obvious - you only see them standing in front of the cabinet. The SIMATIC S7-300/400 LED Status sensor reads those LEDs remotely instead.

This sensor reads those LEDs remotely and creates one channel for each LED your PLC model has, sorted into four categories: control, warning, error, and a handful left undefined for you to interpret. A lit battery fault LED shows up as a fault in PRTG exactly the way it would tell a technician standing right there.

Take a PLC tucked into a mechanical room nobody visits for weeks. Its battery fault LED lights up on a Tuesday night. Instead of waiting for someone to walk past the cabinet, PRTG raises the alert over Ethernet, and the battery gets swapped before it fails for good.

What's Coming Next for Siemens S7-300 and S7-400 Monitoring

Health and diagnostics are only half the picture. A Custom Process Data sensor is already in development, letting you pull actual process values from your Siemens S7-300 or S7-400 - temperatures, pressures, counters - into PRTG channels with their own thresholds and alerts. Got a specific metric or device you would like covered? The Paessler User Voice board for operational technology is the place to vote or post the idea.

The most overlooked device on your shop floor deserves a spot on your dashboard too. Download PRTG free for 30 days and start monitoring your Siemens S7-300 and S7-400 PLCs today.

Getting Started: From Commissioning to Your First Dashboard

Getting these sensors running on your Siemens S7-300 or S7-400 takes less time than you might think.

First, enable the Beta Sensors experimental feature under Setup, System Administration, Monitoring - this knowledge base article explains what that toggle does. Next, set the Siemens S7 connection settings - port, rack, and slot - on the device itself; these sensors currently support IPv4 only. Add your PLC as a device, add the three sensors, and within minutes you will see status, restarts, and LED health sitting next to the rest of your infrastructure. During commissioning, start in a test environment rather than production, since beta sensors are still being refined.