v2.4.1

Date: 2026-10-11
Previous release: v2.4.0

Summary

Patch release: adopts go-modbus v1.3.0, which hardens the client against lost, slow and out-of-step connections. No changes to commands, flags or JSON output.

Changes

• go-modbus: Requires github.com/otfabric/go-modbus v1.3.0 (was v1.2.1).
• No stale answers after a timeout (TCP/TLS): After a request times out or a response is malformed, the connection is replaced before the next request. A late response to the timed-out request can no longer be taken for the answer to the next one, which matters most for client poll, client record and client scan against slow devices.
• Reconnects by itself: A long-running client poll or client record whose connection drops fails that one sample and continues on a new connection with the next.
• Ctrl-C and --timeout take effect immediately: A request in flight is interrupted instead of running to its timeout.

v2.4.0

Date: 2026-10-09
Previous release: v2.3.0

Summary

Closes the capability gap with classic poll-and-write tools. modbusctl now speaks Modbus RTU and ASCII on serial lines, Modbus UDP, and RTU/ASCII framing through serial-to-Ethernet bridges; it can write coils and holding registers (client write) and poll continuously at an interval without recording (client poll). Every existing client command works over the new transports. No breaking changes to flags or JSON output.

Highlights

• Transports (all client commands): --url now accepts rtu://<device>, ascii://<device>, udp://host:port, rtuovertcp://, rtuoverudp:// and asciiovertcp:// in addition to tcp://. Serial devices are written as rtu:///dev/ttyUSB0 (Linux/macOS) or rtu://COM3 (Windows).
• Serial settings: New --baud, --data-bits, --parity (none/even/odd) and --stop-bits flags on every client command, with MODBUSCTL_BAUD, MODBUSCTL_DATA_BITS, MODBUSCTL_PARITY and MODBUSCTL_STOP_BITS. Defaults: 19200 baud, 8 data bits for RTU and 7 for ASCII, no parity, 2 stop bits without parity or 1 with. They are ignored for network URLs.
• client write (new): Writes coils (FC05, FC15) and holding registers (FC06, FC16). --values takes one value or a comma-separated list; with --function omitted, one register uses FC06 and several use FC16. --codec encodes typed values (floats, 32/64-bit integers, strings, IP addresses, timestamps) with the same codec registry as client decode. --start is mandatory, everything is validated before anything is sent, and a write is sent exactly once (never retried).
• client poll (new): Reads the same block of coils, discrete inputs or registers every --interval ms and prints each sample as it is read — one line (text), one JSON object per line (json) or one row (table) — until --polls samples or Ctrl-C. Optional --codec decoding. Failed polls are reported in-stream and polling continues (reconnecting when needed); exit code 7 when any poll failed. Nothing is written to disk; client record remains the MCAP capture command.
• Serial buses: On rtu:// and ascii://, identify and reportserverid ignore --parallel and query units one after the other, since a serial line carries one request at a time.
• MCAP: Captures made over a serial line store the device path in the header ip field with port 0. The format is unchanged.
• Completion: Tab completion for --parity and for client write --function (5, 6, 15, 16).
• go-modbus: Requires github.com/otfabric/go-modbus v1.2.0, which adds the Modbus ASCII transport and corrects function probing.

Fixes

• Silent empty result after a dropped connection: client read, decode and scan could report success with no data when the connection was reset during the request: the reconnect succeeded and its result overwrote the read error. The read error is now reported (and counted as a transport failure by scan).
• client scan --algo safe (the default) lost registers: After finding a block that followed unreadable addresses, the scan skipped one register (for example the last register of a short block), and could read one register below --start. Both are fixed.
• client scan with --end 65535 (the default): --algo smart and --algo deep never finished building their work list and ran out of memory, and --algo safe could wrap around instead of finishing. Position arithmetic no longer overflows at the top of the address space.
• client scan --algo linear: No longer loops forever when the range reaches address 65535, and no longer issues zero-register reads for the full range 0–65535.
• client scan --algo boundary: Finds the left edge of blocks of any size (it used to stop after the first failed read), never reads more than 125 registers at once, and handles blocks and seeds that reach address 65535.
• client fingerprint and client sunspec probe: A function is reported as supported only when the device implements it. Previously any exception counted, so a device answering Illegal Function — or a gateway answering for an absent unit — appeared to support every probed function; and a device that correctly echoed FC08 was reported as not supporting diagnostics. (Fixed in go-modbus v1.2.0.)
• MCAP files cut short: A capture truncated exactly between two fields of a record was read as complete, silently dropping the damaged record. Every mcap command now reports it as an unexpected end of file. A record too large for the format is rejected instead of corrupting the file.
• server static / server replay with coils or discrete inputs: A request for more than one coil returned wrong values for every coil after the first. Multi-coil reads now return the recorded bits.
• server replay: Fixed a data race between the startup message and the replay loop.
• client sunspec commands: --timeout now bounds every read. Previously a device that did not answer blocked for the whole operation budget (10 s or more) on the first base address.
• client identify / client reportserverid with several units: Ctrl-C now stops at the next unit. Previously every remaining unit still waited for its full timeout.
• Error classification: A connection closed by the device is reported as a transport error (exit code 4), not an internal error (10).
• --format table: The rule line under the header no longer ends in trailing spaces.
• Validation message: An --output path whose parent is a file reported %!w(<nil>); it now says the output directory is not a directory.
• Error text: Connection failures are reported as connection error: … instead of TCP connection error: …, since the transport may be a serial port. The JSON kind (transport) and code (transport_connect_failed) are unchanged.
• Documentation: float32/layout:1234 was described as big-endian in examples; it is little-endian. Examples now use float32/layout:4321 (big-endian). client read reads MODBUSCTL_ADDRESS (not MODBUSCTL_START) for --start; the help example and README now say so.

Quality

• Coverage: Test coverage raised from 51% to 96% of the whole module, with a floor of 85% enforced by make coverage (COVERAGE_MIN) and a Codecov project target; CI now tests the cmd packages too. A Code Coverage badge replaces the retired Go Report Card badge.
• Hardware-free tests: New internal/mbtest in-memory Modbus device served over TCP, UDP, RTU/ASCII over TCP, and RTU/ASCII on a pseudo-terminal, so serial transports are exercised end to end in CI.
• Makefile: Sets GOWORK=off so a parent go.work cannot interfere.

Upgrading

• Scripts matching error text: Replace matches on TCP connection error with the JSON code (transport_connect_failed) or the exit code (4).
• Completion users: Regenerate and re-source your shell completion script to pick up the new commands and flags.
• Float examples: If you copied float32/layout:1234 from earlier documentation expecting big-endian values, switch to float32/layout:4321. Codec behaviour itself has not changed.

Release workflow

• Unchanged from v2.3.0: reusable go-binary-release workflow and link-time version metadata.