This repository provides a lightweight MODBUS RTU implementation with STM8 eForth for "wired" control nodes, e.g. for home automation. The main target is low-cost STM8S 8bit µCs like the STM8S003F3P6 with 8K Flash and 1K RAM.

Using STM8 Forth for MODBUS has some advantages: the implementation is very compact and it gives applications access to many advanced architecture features like "I/O-locic execution in the background" or a CLI (command line interface).

The Forth compiler/interpreter is part of the binary you can literally change the code while your board is communicating with the MODBUS host!

The MODBUS RTU implementation covers basic FCs: it's a subset of MODBUS V1.1b common in simple I/O nodes. It's easy to write code for other FCs. It's also simple to turn the board in something like an independent controller for window blinds: the MODBUS host only commands "open" or "closed", not "up" and "down". Local control code can help to make home automation much more robust and reactive.

The MODBUS I/O Node implementation for the low-cost C0135 4-Relay RTU module is the default target and it serves as a demonstrator, and in GitHub Releases you'll find the ready-to-use binary (look for out/C0135/C135-forth.ihx in stm8ef-bin.zip).

You can simply transfer the ready-made binary to your board with a cheap "ST-LINK V2" dongle, or run make to flash the STM8 eForth C0135 code. After flashing you need to hold the board key S2 (the one next to the power connector) while pressing the reset key S1. The LED next to S1 flashing confirms that the Node-ID is now 1 and the baud rate 9600 baud.

After that you should be ready to test the MODBUS connection (here is an example using QModMaster:

This project doesn't just provide a better MODBUS RTU firmware for the relay board but it also turns it into something more: using a diode and a cheap USB-TTL dongle you can get a console. This means the MODBUS node is a computer, a bit like the console of a VIC20 in the old days:

Using the Forth programming language and the STM8 eForth Background Task it's very program local control features, e.g. a window blinds controller that reacts on local inputs without noticable lag and that communicates with a home-automation controller through "holding" registers.

The STM8S001J3RS485 board is a tiny MODBUS node based on the STM8S001J3M3 "Low Density Value Line" STM8S µC in a SO8 package.

The code can be built and transferred to the devide by running make -f forth.mk BOARD=STM8S001J3RS485 flash. After flashing the BUSCTRL file in the board configuration folder should be transferred using e4thcom and a 2-wire connection through PC5. After that, STM8S001J3RS485/board.fs can be transferred with #include.

It's easy to build custom targets, e.g. using the $0.80 MINDEV board, a cheap relay board, and an RS485 break-out board.

When using PB5 for RS485 direction control (-> BUSCTRL) the C0135 code can be used (refer to the C0135 STM8 eForth Wiki page).

MBSERVER contains MODBUS function plug-ins with the following function codes (FC):

A working example with Node-ID and Baud Rate stored in EEPROM is implemented in C0135/board.fs. An example that shows how to develop minimal servers with FC handlers from scratch using the Forth console is in main.fs and, for different FCs, in the folder test.

Note that there is an experimental mapping of holding registers: holding register addresses from 60000 are mapped to the EEPROM. The mapping can be changed in the future (community input on how to deal with MODBUS style register mapping is welcome).

For FC03, FC06 and FC15 the MODBUS address mapping is currently as follows:

This project uses the STM8 eForth "Modular Build" feature: make depend fetches the STM8 eForth release defined in the Makefile.

On a Linux system common dependencies are e.g. GAWK, MAKE and Python. SDCC needs to be installed. It's also possible to use tg9541/docker-sdcc in a Docker container (refer to .travis.yml for details).

The Getting Started section in the STM8 eForth Wiki provides an introduction to flashing STM8 eForth to a target µC.

Please refer to the Installation Instructions in the STM8EF-MODBUS Wiki for build instructions.

The STM8S UART is used by UARTISR for MODBUS RTU communication. The Forth console communicates through a half-duplex simulated RS232 two-wire interface on the PD1/SWIM GPIO pin. For adding a standard USB-TTL converter only a diode is needed. Other CLI communication options are easy to implment, e.g. using simulated full-duplex RxD-TxD lines (e.g. using PA1 and PA2 after removing the C0135 8MHz crystal). It's also possible to use an STM8S High Density device with two UARTs, e.g. the STM8S207RBT6.

Please refer to the STM8 eForth Wiki to learn more about half-duplex CLI communication options and preferred terminal programs.

The software architecture separates hardware abstraction and application in simple layers:

The different concerns are separeted in the code and FC handlers can be changed through the CLI without restarting the application!

The code is organized in the following execution domains:

• interrupt service routines for buffered MODBUS communication
• fixed-rate background task for I/O logic (asynchronous to MODBUS)
• foreground "idle mode" MODBUS protocol handler
• foreground command line interface (CLI) through independent COM port provided by STM8 eForth
• handlers for MODBUS I/O: mbpre for input, mbact for output actions

Please refer to the how-to in the wiki and don't hesitate to open an issue if you have questions!