Modbus Bridge: optimize execution of Tasks

[sfeilmeier]

In its core this is a rewrite of the 'ModbusWorker', that schedules the execution of modbus Tasks. See readme below for details on the new version:

Modbus

Modbus is a widely used standard for fieldbus communications. It is used by all kinds of hardware devices like photovoltaics inverters, electric meters, and so on.

Modbus/TCP

https://github.com/OpenEMS/openems/blob/develop/io.openems.edge.bridge.modbus/src/io/openems/edge/bridge/modbus/BridgeModbusTcpImpl.java[Bridge Modbus/RTU Serial] for fieldbus communication via TCP/IP network.

Modbus/RTU

https://github.com/OpenEMS/openems/blob/develop/io.openems.edge.bridge.modbus/src/io/openems/edge/bridge/modbus/BridgeModbusSerialImpl.java[Bridge Modbus/TCP] for fieldbus communication via RS485 serial bus.

Implementation details

OpenEMS Components that use Modbus communication, must implement the ModbusComponent interface and provide a ModbusProtocol. A protocol uses the notion of a Task to define an individual Modbus Read or Write request that can cover multiple Modbus Registers or Coils depending on the Modbus function code. It is possible to add or remove tasks to/from a protocol at runtime or to change the execution Priority. The Modbus Bridge (Bridge Modbus/RTU Serial or Bridge Modbus/TCP) collects all protocols and manages the execution of Tasks.

Execution of Modbus Tasks

Execution of Modbus Tasks is managed by the ModbusWorker. It...

• executes Write-Tasks as early as possible (directly after the EXECUTE_WRITE event)
• executes Read-Tasks as late as possible to have values available exactly when they are needed (i.e. just before the BEFORE_PROCESS_IMAGE event). To achieve this, the ModbusWorker evaluates all execution times and 'learns' an ideal delay time, that is applied on every Cycle - the 'CycleDelay'
• handles defective ModbusComponents (i.e. ones where tasks have repeatedly failed) and delays reading from/writing to those components in order to avoid having defective components block the entire communication bus. Maximum delay is 5 minutes for read from defective components. ModbusComponents can trigger a retry from a defective Component by calling the retryModbusCommunication() method.

Priority

Read-Tasks can have two different priorities, that are defined in the ModbusProtocol definition:

• HIGH: the task is executed once every Cycle
• LOW: only one task of all defined LOW priority tasks of all components registered on the same bridge is executed per Cycle
  Write-Tasks always have HIGH priority, i.e. a set-point is always executed as-soon-as-possible - as long as the Component is not marked as defective

Channels

Each Modbus Bridge provides Channels for more detailed information:

• CycleTimeIsTooShort: the configured global Cycle-Time is too short to execute all planned tasks in one Cycle
• CycleDelay: see 'CycleDelay' in the 'ModbusWorker' description above

Logging

Often it is useful to print detailed logging information on the Console for debugging purposes. Logging can be enabled on Task level in the definition of the ModbusProtocol by adding .debug() or globally per Modbus Bridge via the LogVerbosity configuration parameter:

• NONE: Show no logs
• DEBUG_LOG: Shows basic logging information via the Controller.Debug.Log
• READS_AND_WRITES: Show logs for all read and write requests
• READS_AND_WRITES_VERBOSE: Show logs for all read and write requests, including actual hex or binary values of request and response
• READS_AND_WRITES_DURATION: Show logs for all read and write requests, including actual duration time per request
• READS_AND_WRITES_DURATION_TRACE_EVENTS: Show logs for all read and write requests, including actual duration time per request & trace the internal Event-based State-Machine

The log level via configuration parameter may be changed at any time during runtime without side-effects on the communication.

---

See also the discussion in OpenEMS Community: https://community.openems.io/t/zeitlicher-ablauf-der-modbus-tasks/734

[sfeilmeier]

TODO: consider introducing an IMMEDIATE priority for Registers (or via special Write-Channels). See discussion at https://community.openems.io/t/power-distribution-between-battery-and-pv-inverter/1184/6

Update: In fact writing as early as possible after EXECUTE_WRITE is very close to an IMMEDIATE priority, so this has not been explicitely implemented.

[sfeilmeier]

Planned State-Machine for CycleTasksManager:

graph TD
ON_BEFORE_PROCESS_IMAGE>ON_BEFORE_PROCESS_IMAGE]
ON_EXECUTE_WRITE>ON_EXECUTE_WRITE]

ON_EXECUTE_WRITE ==> WRITE
INITIAL_WAIT -->|sleep| READ_BEFORE_WRITE
INITIAL_WAIT -.- ON_EXECUTE_WRITE
READ_BEFORE_WRITE -->|read finished early| WAIT_FOR_WRITE
READ_BEFORE_WRITE -.- ON_EXECUTE_WRITE
WAIT_FOR_WRITE -.- ON_EXECUTE_WRITE

WRITE -->|write finished| WAIT_BEFORE_READ
WAIT_BEFORE_READ -->|sleep| READ_AFTER_WRITE
READ_AFTER_WRITE -->|read finished| FINISHED

ON_BEFORE_PROCESS_IMAGE ==> INITIAL_WAIT
FINISHED -.-o ON_BEFORE_PROCESS_IMAGE

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[sfeilmeier]

@sebastianasen, @huseyinsaht Please test this implementation on your test systems if possible. From my side it's very close to being released.

[sfeilmeier]

Backreference to OpenEMS Community: https://community.openems.io/t/asking-for-feedback-new-modbusworker-implementation/1747

[sfeilmeier]

Followup refactoring & cleanup for Modbus Elements: #2273

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