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mbus-gw-t2r
A MODBUS TCP to RTU gateway for RS-485 and RS-232 lines: several devices per line, several TCP ports per line, a transparent RAW mode.
| Version | V.01-01 (tag V01.01.0, packages 1.1.0) |
|---|---|
| Status | Released, evolving |
| Platforms | Linux: amd64, arm64, mipsel, mips64el (.deb); OpenWrt 24.10 and 25.12 (.ipk, .apk); NaPi boards (RK3568J, RK3308) |
| Licence | Apache License 2.0 |
| For whom | Industrial automation engineers, those who commission MODBUS devices, and students of industrial programming: the code can be read end to end in an evening. |
What it is
The gateway takes MODBUS TCP requests and passes them to the devices on a serial line (MODBUS RTU). The end of a frame is detected by the honest t3.5 silence of the specification, an answer is checked by its CRC16, its length and its match to the request. Line garbage is rejected: the client gets a MODBUS exception, the line is flushed, the next request is served as usual.
A single process, all the waiting is done by poll() with no background timers: an idle gateway uses practically no CPU. Several TCP ports per line, several lines per process, the RS-485 direction controlled by the kernel, the Time Stamp pseudo device, the RAW mode to pass the bytes of a line over TCP transparently. A binary of about 100 KB that needs nothing but libc. Every error is a coded message (%T2R-E-CRC16ERR, %T2R-E-NOANSWER) with its reason.
Since V.01-01 the kit has the mbus-ctl utility: SHOW CONNECTIONS (the clients with their counters of requests, answers, errors and bytes, /CONTINUOUS to keep the screen updated), SHOW LINES, SHOW GATEWAY, RESET CONNECTIONS (drop a client's session), RESTART GATEWAY (check the settings file and restart the gateway with the same PID). It works over a local Unix socket with a binary TLV protocol (StarLet AVProto) and opens nothing to the network; the commands may be abbreviated as in DCL: sho conn /cont, restart gw.
Why
An alternative to mbusd where efficiency on a small box and clear diagnostics matter. The project is also offered for teaching: the serial line discipline, poll() loops, ring buffers, configuration checks and DEC-style messages - all in a real working tool. It is developed and tested by the NaPi Lab team on NaPi industrial single-board computers.
Quick start Installing a ready-made package
sudo apt install ./mbusgw-t2r_1.1.0-1_arm64.deb # Debian, Ubuntu
sudo nano /opt/mbusgw-t2r/etc/modbus-t2r_settings.conf # lines and TCP ports
sudo systemctl enable --now mbusgw-t2r
sudo mbus-ctl sho conn /cont # who is connected, counters
opkg install ./mbusgw-t2r_1.1.0-r1_mipsel_24kc.ipk # OpenWrt 24.10
apk add --allow-untrusted ./mbusgw-t2r_1.1.0-r1_mipsel_24kc.apk # OpenWrt 25.12Source: mbus-gw-t2r/README.md; the User Guide - docs/UserGuide_EN.md and the PDF in Releases
Download
Packages .deb (amd64, arm64, mipsel, mips64el), OpenWrt .ipk and .apk (mips, mipsel, ARM, x86_64), sources and the PDF guides are in Releases on GitHub.