Modbus Library for Arduino

// 4.1.1
+ Protocol: Fix wrong error code responce on non-existent register
+ ModbusTCP: Fix potential memory leak
+ API: cbEnable/cbDisable functionality extended
+ ESP-IDF: CMakeList.txt added
+ Examples: TCP-to-RTU fixed
// 4.1.0
+ API: Raw Modbus frame processing functionality
+ ModbusRTU: Precise inter-frame interval control
+ Examples: True ModbusRTU to ModbusTCP Server bridge
+ Examples: ModbusRTU respond to multiple ID from single device
+ ModbusRTU: Add direction control pin for Stream
+ STL: Add Reg count limitation to vector limit of 4000 (for ESP8266 and ESP32)
+ Settings: Added MODBUSIP_CONNECTION_TIMEOUT (ESP32 only)
+ Settings: Set MODBUSIP_MAX_CLIENTS = 8 for ESP32
+ ModbusTCP: Make using DNS names optional feature
+ ModbusRTU: Add separate RE/DE pins control optional feature
+ API: Drop support of Ethernet library v1
+ Examples: Teknic ClearCore ArduinoWrapper examples added
+ Examples: ModbusTCP to ModbusRTU example added
+ ModbusRTU: Flush extra delay optional feature
// 4.0.0
+ Support of all Arduino boards
+ ModbusTLS: ESP8266 Client/Server and ESP32 Client
+ ModbusTCP: ModbusEthernet - WizNet W5x00, ENC28J60 Ethernet library support
+ 0x14 - Read File Records function
+ 0x15 - Write File Records function
+ Examples: FW update over Modbus fullfunctional example
+ 0x16 - Write Mask Register function+ Test: 0x16
+ 0x17 - Read/Write Registers function
+ ModbusRTU: ESP32 SoftwareSerial support
+ Build with no STL dependency (switchable)
+ API: ModbusIP => ModbusTCP
+ API: Access control callback for individual Modbus function
+ API: Master/Slave => Client/Server according to [PRESS RELEASE](https://modbus.org/docs/Client-ServerPR-07-2020-final.docx.pdf)
+ Lot of code refacting and small fixes

// 4.2.0
- API: Alternative CRC calulation (reduced memory footprint)
- ModbusRTU: Static buffer allocation
- Test: Frame accuracy to specefication
- Buffer/packet size limitation support
- Slave/Server: slavePDU use early exit by return where possible
- Master/Client: Check frame size against header data where possible
- Master/Client: Additional responce data validation
- Free global registers and callbacks on remove last Modbus instance
- Test: push/pull functions
- ModbusTCP: Refactor connect by dns name (using native implementation for ESP32 etc)
// 4.3.0
- ModbusTLS: ESP32 Server
- Test: TLS ESP32 Server
- Test: TLS ESP32 Client
- Examples: TLS Certificate test Role extension and Alt-Name
- Examples: TLS Add example explanation
- ModbusTCP: ModbusAsyncTCP
- API: Extend API to allow custom Modbus commands
- Examples: Basic file operations
- Examples: Revising