Advanced: Exposing a PLC as JSON Web Services (using PLC4X)

Find the complete source code on GitHub



This example shows how to connect to a Siemens S7 PLC through Ethernet (e.g. using a CP 243-1), and expose its functionality as JSON webservices.

Depending on what model you are using, a different connection string will be needed, and at the very least you will need to correct the IP address used in the example code, in the PlcServer class:

static final String connectionString = "s7://";

s7 refers to the Siemens S7 protocol. The IP Address is that of your ‘Communication Processor’. The two next components, 0 and 2, are the Rack and Slot address of your siemens PLC (this part will be different for other PLC types). This will depend on the type and configuration of your PLC, but some common values are:

  • S7 200: Rack 0, Slot 0
  • S7 300: Rack 0, Slot 0
  • S7 400: Depending on the configuration
  • S7 1200: Rack 0, Slot 0 or Slot 1
  • S7 1500: Rack 0, Slot 0 or Slot 1

The key-value pairs after the ? are additional parameters that are passed to the PLC4X driver. In this case, controller-type=S7_300 is a hint the driver needs because it fails to autodetect my PLC (The 200 and 300 series are equivatent for the PLC4X Driver).

Refer to the PLC4X documentation for more information about connection strings. Also, the Sharp7 project page has some good information about the settings for various types of S7 CPUs.

The example assumes that:

  • a ‘Control Box’, containing 4 buttons, is connected to the inputs I1.0 through I1.3 (as seen in the picture below)
  • a stack of 3 status lights will be connected to outputs Q0.0 through Q0.2 (missing in the picture)

example setup

But it will work without either: If no switches are connected to the inputs, their state will always be “low”. If no lights are connected to the outut, the state of the lights can still be seen on the S7 PLC’s diagnostic lights.

The Mappings: PLC to Java Object, then Object to JSON

The example maps individual ‘fields’ from the PLC to fields in a simple Java class (a POJO, ‘Plain Old Java Object’) using annotations. Data is read from the PLC into instances of this class, and made available via HTTP, serialised in a JSON document (or the other way around: Received via HTTP, serialised into an instance of the POJO, then sent to the PLC). The MyControlBox class, for example, models the state of the gray control box with 4 buttons seen in the above picture:

import java.util.Objects;

public class MyControlBox {

    public boolean high;

    public boolean start;

    public boolean stop;

    public boolean emergency_stop;


In this example, the emergency_stop field will be mapped to the state of the input 3 in the block 1, and is mapped to a bolean, meaning it’s true when the emergency stop button is pressed, false otherwise.

This same object will then be serialized using JSON when accessed through the web service (e.g. by pointing your browser to http://localhost:8080/controlbox), and the result will look like this:

  emergency_stop: true,
  high: false,
  start: false,
  stop: false

In this case, all buttons are in their resting position, but the emergency stop button is pressed.

Code walk-through

The entire web service is realised in the PlcServer class, in about 75 lines of code. The main class, as in the other examples, creates the Akka ActorSystem and Materializer but also an Akka HTTP server (①).

This time, the flow is created by Akka HTTP using Routes (②), which are defined in a separate method (explained next). The Akka HTTP server is started using these flows (③).

Note that all threads are daemons at this point: If the main method terminates, the server is shut down. For this reason, we need to block indefinitely (④).

    public static void main(String[] args) throws InterruptedException {
        ActorSystem system = ActorSystem.create("plc-server");
        Materializer mat = ActorMaterializer.create(system);
      Http http = Http.get(system);

      final Flow<HttpRequest, HttpResponse, NotUsed> routeFlow = new PlcServer().createRoute().flow(system, mat);
      final CompletionStage<ServerBinding> binding = http.bindAndHandle(routeFlow,
                ConnectHttp.toHost("localhost", 8080), mat);


The routes are defined in the createRoute method. Setting up routes is described in details in the Akka HTTP documentation. For this example, let’s just observe that we set up two routes: - one on the path /controlbox accepts GET requests (①), and serves them by executing the getControlBoxState() method: This will query the state of our control box, and return it as JSON object. - another, on the path /status, accepts POST requests (②), and serves them using the method postStatusLights(json). This will set the state of the 3 status lights.

private Route createRoute() {
    return concat(
          path("controlbox", () -> get( 
                    () -> getControlBoxState())
          path("status", () -> post(
                    () -> entity(Jackson.unmarshaller(MyStatusLights.class), this::postStatusLights))

The getControlBoxState() methods execute PLC4X’s method asynchronously (①), and return a Future (②) that will complete successfully and yield an HTTP response 200 if and when the read method completes successfully:

private Route getControlBoxState() {
    CompletionStage<MyControlBox> future = CompletableFuture.supplyAsync(new Supplier<MyControlBox>() {
        public MyControlBox get() {
            try {
              return, connectionString);
            } catch (OPMException e) {
                throw new RuntimeException(e);

    return onSuccess(future, done ->
          completeOKWithFuture(future, Jackson.marshaller())

The postStatusLights(state) methods work similarly, using PLC4X’s EntityManager.write(...) method (①), and return a Future (②) that will yield an HTTP 200 response, containing the current state of the output as a JSON object (②):

private Route postStatusLights(MyStatusLights state) {
    CompletionStage<MyStatusLights> future = CompletableFuture.supplyAsync(new Supplier<MyStatusLights>() {
        public MyStatusLights get() {
            try {
              return entityManager.write(MyStatusLights.class, connectionString, state);
            } catch (OPMException e) {
                throw new RuntimeException(e);

    return onSuccess(future, done ->
          completeOKWithFuture(future, Jackson.marshaller())

If the execution throws a RuntimeException instead, the server will reply with an HTTP 500. The OPMException raised by PLC4X must be caught: It wouldn’t be identified by Akka HTTP as a non-critical exception, and would cause the server to restart.

Mappting annotations: PLC to Java object

The @PlcEntity marks this class as being instantiable by PLC4X from a PLC’s state. The 4 fields, high, start, stop, and emergency_stop, will contain the state of the black speed switch, the green button, the red button, and the twist-to-reset red emergency stop button. These are connected to the inputs I1.0 through I1.3 on the PLC, and can only have two states: open or closed. This is indicated by the @PlcField annotation, which specifies the field and type to use. For S7 PLCs, the fields can be:

  • %I for Input fields, followed by the port number (that should be written on your PLC, besides the input terminal), e.g. “%I1.0” as above for the first bit of the second input block, or “%0” for the entire byte resulting from the 8 bits of the first input block
  • %Q for Output fields (not O!), followed by the port number (similarly, written on the PLC, besides the output terminal), e.g. “%Q0” for the 8 output bits of your S200 CPU, or “Q124” for the first output of your S300 CPU (provided it has outputs).
  • %F for Flags used when programming the PLC
  • %DB for Data Block, followed by the block number, a period, and the offset from which you want to read, e.g. “%DB05.10” for the tenth byte in data block 5 (won’t work on an S200, which only has 1 block), or “%DB01.0” for the first byte of the first block (works on an S200).
  • %V for data from Data Block 1, e.g. “V100” for the hundredth byte in an S200’s memory, or the hundredth byte in the S300’s data block 1.

The type is specified after a colon (:). For S7 PLCs, these can be:

  • BOOL (1 bit input as java boolean)
  • BYTE (1 byte as 8 bits)
  • WORD (2 byte as 16 bits)
  • DWORD (4 byte as 32 bits)
  • LWORD (8 byte as 64 bits)
  • SINT (8 bits as signed Byte)
  • USINT (8 bits as unsigned Short)
  • INT (16 bits as signed Short)
  • UINT (16 bits as unsigned Integer)
  • DINT (32 bits as signed Integer)
  • UDINT (32 bits as unsigned Long)
  • LINT (64 bits as signed Long)
  • ULINT (64 bits as unsigned BigInteger)
  • REAL (as java Float)
  • LREAL (as java Double)
  • CHAR (fixed-length UTF-8 data as String)
  • WCHAR (fixed-length UTF-16 data as String)
  • STRING (variable-length UTF-8 data as String)
  • WSTRING (variable-length UTF-16 data as String)
  • DATE_AND_TIME (date/time as Java LocalDateTime)
  • TIME_OF_DAY (time as Java LocalTime)
  • DATE (date as Java LocalDate)

In the example above, the emergency_stop field is the state of the input 3 in the block 1, and is mapped to a bolean, meaning it’s true when the emergency stop button is pressed, false otherwise.

Mappting annotations: Java object to JSON

This example uses Jackson as the JSON serializer. Any public member will be serialised in the JSON response, and no annotation is strictly necessary. In fact, the MyControlBox class doesn’t contain any JSON Annotations.

The MyStatusLights must be instantiated by Jackson, and JSON fields must map to it, To do so, a Constructor was annotated with Jackson annotations, containing the field mapping information (①). The fields themselves were mapped to the PLC’s output field, using PLC4X’s annotations (②).

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;

public class MyStatusLights {

    public MyStatusLights() {

    public MyStatusLights(@JsonProperty("red") boolean red,
                   @JsonProperty("yellow") boolean yellow,
                 @JsonProperty("green") boolean green) { = red;
        this.yellow = yellow; = green;

    public boolean red;

    public boolean yellow;

    public boolean green;


Combining both annotations allows the mapping to a JSON object and to the PLCs fields to change independently of each other.

Testing the web services

To test the controlbox service, simply point your browser to http://localhost:8080/controlbox, you should see the following result:

  emergency_stop: true,
  high: false,
  start: false,
  stop: false

To control the lights on the light tower (or to just toggle the outputs on your S7), use an HTTP test client such as Postman, and send a POST request to http://localhost:8080/status with the following raw payload, of type `application/json’:

  "red": false,
  "yellow": true,
  "green": false

Or use the Curl program from a terminal window:

curl -H "Content-Type: application/json" -X POST -d '{ "red": false, "yellow": true, "green": false }' http://localhost:8080/status