Modbus Pulse Coils with Spring Boot project

Question

Have to implement call to Digital IO with Modbus protocol on Spring Boot project build by Maven.

It should be a pulsed call with some duration at the body (like 5 seconds, etc.)

Here is a snippet from the specification:

And more about Response and Errors:

Seems at this call application should behave like a master. And Web Relay should be like a slave in this case.

I suppose that taking some library for integrating with Modbus will be much more preferable than writing it from zero.

Which approach will be preferable to adopt this functionality with Spring Boot?

Here are Web Relay manuals.

---

UPDATE:

Tried answer by KevinHerron

Here is the code part:

@Override
public void callDigitalIO(String ipAddress, String relay) {
    log.debug("call Digital IO by MODBUS");
    checkNotEmpty(ipAddress, relay);

    ModbusTcpMasterConfig config = new ModbusTcpMasterConfig.Builder(ipAddress).build();
    ModbusTcpMaster master = new ModbusTcpMaster(config);
    master.connect();

    ByteBuf buffer = Unpooled.buffer();
    buffer.writeInt(0x00003F00);
    buffer.writeInt(0x000040A0);

    int relayNum = Integer.parseInt(relay);
    WriteMultipleRegistersRequest request = new WriteMultipleRegistersRequest(
            relayNum,
            0x02 * relayNum,
            buffer
    );

    log.debug("MODBUS_REQUEST: {}", request);

    master.sendRequest(request, 0xFF)
            .whenCompleteAsync((response, ex) -> {
                if (response != null) {
                    log.info("MODBUS_RESPONSE: {}", response);
                } else {
                    log.error("EXECUTION_FAILED: {}", ex.getMessage(), ex);
                }
            });

    master.disconnect();
}

Tried below snippet with a real device, the output is following:

2021-02-25 22-07-03.955 [carpark-ex-4] DEBUG c.s.s.dio.ModbusDigitalIoServiceImpl - MODBUS_REQUEST: WriteMultipleRegistersRequest(UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 8, cap: 256))
2021-02-25 22-07-03.970 [ForkJoinPool.commonPool-worker-5] ERROR c.s.s.dio.ModbusDigitalIoServiceImpl - EXECUTION_FAILED: not connected
java.lang.Exception: not connected
    at com.digitalpetri.netty.fsm.ChannelFsmFactory.lambda$null$2(ChannelFsmFactory.java:115)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
    at java.base/java.lang.Thread.run(Thread.java:832)

Only when I did:

master.sendRequest(request, 0xFF).get()

it started to work.

Jordison

Answer 1

This should be easy to accomplish using an existing library that supports the WriteMultipleRegisters function.

Try https://github.com/digitalpetri/modbus

---

An example of how to construct a WriteMultipleRegisters request based on that screenshot of documentation:

class Scratch {

  public static void main(String[] args) throws ExecutionException, InterruptedException {
    ModbusTcpMasterConfig config = new ModbusTcpMasterConfig.Builder("ip_or_hostname").build();
    ModbusTcpMaster master = new ModbusTcpMaster(config);

    master.connect().get();

    ByteBuf buffer = Unpooled.buffer();
    buffer.writeInt(0x00003F00);
    buffer.writeInt(0x000040A0);

    WriteMultipleRegistersRequest request = new WriteMultipleRegistersRequest(
        0x16,
        0x04,
        buffer
    );

    master.sendRequest(request, 0xFF).get();
  }

}

Joan

Could you explain more details about this call? Have it should be done, is it possible to set timeout to it – Jordison

Updated. Time for you to start experimenting. Write code and try it out. – Joan

could you help to understand meaning of unitId? – Jordison

It's in the spec. It's called slave id when it's Modbus over serial. Your device documentation mentions just using 0xFF, which is commonly used when talking directly to the server and not to a slave behind a gateway. See modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf – Joan

could you clarify one more place - Pulse Duration (4 bytes, per relay): 0x3DCCCCCC – 0x47A8C000 (Little-endian Format: 0xCCCC3DCC – 0xC00047A8) - how exactly this duration should be set depends on relay number? – Jordison

Those docs suggest you can write to one or more contiguous relays, and you would include one or more 4-byte pulse durations to match. – Joan

tried this approach with real device and it failed. Updated the question with stacktrace. – Jordison

Ah, you have to call connect() first on the master. – Joan

surrounded sending request with connect() and disconnect() with whenComplete() it didn't work, but with get() works fine. Why did this happen? – Jordison

connect() and disconnect() are async. They return a CompletableFuture that you should call get() on to block until the action is complete. Otherwise you would need to do the next set of actions in a callback instead. – Joan

You should also not disconnect() until after your response has been received. – Joan

Let us continue this discussion in chat. – Jordison