Debugging¶

Where output goes¶

Environment	`print()` output and Lua errors
Web simulator	Console panel
FrSky Suite simulator	Console panel
`ethos-tools` (`run_wasm.js`)	`node run_wasm.js log [n]`
Real radio	Serial debug over USB (see below)

When a widget is blank or a tool doesn't open, check the console first. On screen a broken script usually just looks empty.

Reading the boot log¶

Each script logs two lines when it loads:

Lua::load('SCRIPTS:/mywidget/main.lua')
Lua::compile('main.luac', strip=0)

Errors in your file's top-level code or init() appear right after these. Other messages worth knowing:

Message	Meaning
`Lua::registerSystemTool(): missing icon`	`registerSystemTool` was called without `icon`
`Lua::path is not set!`	A relative path was used from a callback; use `SCRIPTS:/<folder>/...` (details)
`Bitmap SCRIPTS:/x/icon.png loaded, RAM used: 4096 bytes`	An image was decoded (on first draw). A free per-asset memory report
`Lua has used too much RAM, it has been Killed`	Heap limit hit. See Saving RAM

Print well¶

local DEBUG = true
local function log(fmt, ...)
  if DEBUG then print(string.format("[mywidget] " .. fmt, ...)) end
end

log("source=%s value=%s", widget.source and widget.source:name() or "nil", tostring(v))

Prefix lines with your script name. The console is shared by every script.
Never print every tick in wakeup. It floods the console and costs CPU. Print on change, or throttle with os.clock().
Turn debug output off for releases (DEBUG = false), or strip it.

Catching errors in callbacks¶

An error inside a callback stops that call, and Ethos may stop calling your script. While developing, wrap risky work with pcall and log the message:

local function wakeup(widget)
  local ok, err = pcall(update, widget)
  if not ok then print("[mywidget] wakeup error: " .. tostring(err)) end
end

Remove blanket pcalls from hot paths in release builds, or keep them narrow. They hide bugs and cost a little time on every call.

On a radio¶

To see print() output from scripts running on a real radio, switch its USB connection to serial debug mode. The radio's drive unmounts and a serial port appears, carrying everything scripts print (115200 baud). Three ways to do it:

Tool	Start	Stop
FrSky Suite GUI	Lua development tools → Start debug (output shown in the panel)	Stop debug
FrSky Suite CLI	`--serial start`, then open the port (`COM15`, `/dev/ttyACM0`, ...)	`--serial stop`
`ethos_deploy.py`	`--radio --debug-only` (finds the port by USB VID/PID and prints lines)	Ctrl+C (switches back to storage)

An idle radio prints nothing, which is normal. Only your print() calls (and errors) appear. Remember to switch back to storage mode before your next deploy, otherwise the drive isn't there.

Memory and performance problems¶

Run the memory monitor task next to your script.
Check system.getInstructionsUsage() inside long loops (Saving CPU).
Read Measuring before trusting a single number.