Creating an outstation

An outstation in opendnp3 is a component that communicates with a single master via a communication channel. It makes measurements of the physical world and then sends them to a master upon request (solicited) or on its own accord (unsolicited). Occasionally a master requests that it do something by sending it a control. Just like a master, an outstation can be attached to any communication channel that opendnp3 supports.

To add an outstation to a communication channel you call the AddOutstation method on the channel interface:

OutstationStackConfig stackConfig;

// You must specify the shape of your database and the size of the event buffers
stackConfig.dbTemplate = DatabaseTemplate::AllTypes(10);
stackConfig.outstation.eventBufferConfig = EventBufferConfig::AllTypes(10);

// you can override an default outstation parameters here
stackConfig.outstation.params.allowUnsolicited = true;

// You can override the default link layer settings here
// in this example we've changed the default link layer addressing = 10; = 1;

auto outstation = channel->AddOutstation(
  "outstation",                             // alias for logging
  SuccessCommandHandler::Create(),          // ICommandHandler (interface)
  DefaultOutstationApplication::Create(),   // IOutstationApplication (interface)
  stackConfig                               // static stack configuration



When a new measurement is read from an input or a new value is received from a downstream protocol, you need to update the corresponding value in the outstation. This is accomplished with the UpdateBuilder and corresponding Updates class.

UpdateBuilder builder;
builder.Update(Counter(state.count), 0);
builder.Update(Analog(state.value), 0);
// ... update more types and indices

// finalize the set of updates
auto updates = builder.Build();

// apply the updates to one or more outstations

The update is atomic. All of the updated values are applied to the outstation database and event buffers at the same time. The Updates instance returned from UpdateBuilder::Build() can be safely sent to any number of outstation instances. The outstation automatically decides if these updates produce events. How events are detected are defined within the DNP3 standard, and varies from type to type. Analogs and counters can use deadbands to ensure that unimportant changes are not reported.


When the outstation receives a control request, it dispatches individual commands in the message to the ICommandhandler interface supplied when the outstation was added to the channel.

class ICommandHandler : public ITransactable
    virtual CommandStatus Select(const ControlRelayOutputBlock& command, uint16_t index) = 0;
    virtual CommandStatus Operate(const ControlRelayOutputBlock& command, uint16_t index, OperateType opType) = 0;
    /// ... additional methods for the 4 types of analog outputs - Group 41Var[1-4]

You'll notice that the interface is transactable meaning that it has Start()/End() methods just like the ISOEHandler interface in the master. ASDUs can contain multiple controls in a single object header, and possibly multiple headers. The Start()/End() methods tell you when an ASDU containing commands begins and ends. Many applications probably don't care, but this knowledge is there if you need it for some reason.

The Select operation shouldn't actually perform the command. Think of it as a question along the lines of "Is this operation supported?". Select-Before-Operate (SBO) is an artifact of the days before the SCADA community really trusted CRCs. It's a 2-pass control scheme where the outstation verifies that the select/operate are identical. It was intended as an additional protection against data corruption on noisy networks.

The Operate method is called from a successful SBO sequence or from a DirectOperate or DirectOperateNoAck request. Many applications don't care how the request came in, but the OperateType optype parameter provides an enumeration that can be used to reject certain operations or to forward the same mode downstream in gateway applications.

enum class OperateType : uint8_t
  /// The outstation received a valid prior SELECT followed by OPERATE
  SelectBeforeOperate = 0x0,
  /// The outstation received a direct operate request
  DirectOperate = 0x1,
  /// The outstation received a direct operate no ack request
  DirectOperateNoAck = 0x2


You must immediately return a CommandStatus enumeration value in response to each callback. This callback should never block.

enum class CommandStatus : uint8_t
  /// command was accepted, initiated, or queued
  SUCCESS = 0,
  /// command timed out before completing
  TIMEOUT = 1,
  /// command requires being selected before operate, configuration issue
  NO_SELECT = 2,
  /// more values ...

The enumeration contains about ~18 different values, and you should refer to 1815 or the code comments for a description of each. In general, you'll be choosing SUCCESS or some kind of error code.

It's important to understand that SUCCESS doesn't imply that the command was synchronously executed. It really just means that the command was received and queued. Some devices can synchronously process a command, e.g. quickly writing to memory mapped I/O, but you'd never want to block in a gateway application to perform a downstream Modbus transaction. You'd pass the control of to another thread or queue the operation in some way for subsequent processing.