Letra200bSharp

C# library and mobile and desktop applications for printing images to a Dymo LetraTag 200B labelprinter, fork of letra200bsharp.

Repository contents

• Letra200bSharp contains:
  • Logic for resizing / converting an input image to a matrix representing the black and white pixels of the image
  • Protocol for communicating with the device
• Letra200bSharp.Avalonia is a cross-platform Avalonia application with a tab each for printing an image, text, or a 1D barcode label, plus a History tab that keeps the last 50 successful prints (with a thumbnail) and lets you reprint a text or barcode job with one tap. Follows the system's light/dark theme and adapts its layout to the available width, from phone to desktop
  • Letra200bSharp.Avalonia.Desktop is the Windows/Linux desktop app. Launched with no arguments it opens the GUI; launched with arguments it instead runs headless as a CLI, with one verb per tab - image, text, barcode - plus list-devices to scan for nearby printers (run with --help, or <verb> --help, for the full option list)
  • Letra200bSharp.Avalonia.Android is a Avalonia android mobile application that easily lets you select a device and an image, text or barcode label for sending to the LetraTag 200B

Screenshots

Each row shows the phone (light/dark) and desktop (light/dark) apps for that tab.

	Mobile	Desktop
Image
Text
Barcode

Used libraries

• SkiaSharp for all image processing - resizing/thresholding source images, rendering text and barcodes to a bitmap, and generating the label previews
• InTheHand.BluetoothLE for cross-platform Bluetooth LE scanning and GATT communication with the printer (Windows, Linux, Android)
• CodeGlyphX for encoding barcode data into the module pattern rendered onto the label - both 1D (Code128, Code39, Codabar, ITF, EAN/UPC, ...) and 2D (QR, Micro QR, rMQR, Data Matrix)
• Avalonia as the cross-platform UI framework behind the desktop and Android apps
• SukiUI for the app's visual theme - light/dark styling, the toast notifications, and the busy/loading overlay on the label previews
• CommunityToolkit.Mvvm for MVVM boilerplate - source-generated observable properties and relay commands in the view models
• CommandLineParser for parsing arguments in the desktop app's headless CLI mode (image/text/barcode/list-devices)
• CsWin32 for source-generated, type-safe P/Invoke - used to reattach the CLI's output to the parent console on Windows

Protocol

Dymo never published a spec for the LetraTag 200B's Bluetooth LE protocol - like the two projects credited below, this one was built by reverse-engineering it. The gist:

• The printer advertises a custom GATT service (be3dd650-2b3d-42f1-99c1-f0f749dd0678) with two characteristics under it: "print request" (...651..., write-without-response) and "print reply" (...652..., notify) - and its device name starts with Letratag.
• A print job is a byte stream split into ~300-byte packets, each prefixed with a running chunk index (skipping the value 27/0x1B, so a chunk's index byte is never mistaken for the ESC that starts every command). It always contains, in order: a small header (preamble 0xFF, flags 0xF0, magic 0x12 0x34, a little-endian body length, and a checksum), then the body itself - a "start job" command, the image data, then "form feed", "status" and "end" commands. Each command is an 0x1B escape byte followed by a command byte (0x44 for the image data command, for example).
• The label itself is sent as a single 1-bit monochrome bitmap, packed 8 pixels per byte: 32 pixels tall (the print head's full resolution), of which only the middle 30 actually print - the first and last row are physically cut off - by whatever length the content needs.
• Once the whole job has been written, the printer replies on the "print reply" characteristic with 0x1B 0x52 <status> - a single status byte this project maps to a human-readable result (0/1 = printed, 3 = printed but the battery is low, 4 = cancelled, 6 = battery too low to print, 7 = no cassette, anything else = an unrecognized failure). Status 1 ("printing"/ready for next label per some docs) is treated as a success here too - it's what the printer's firmware sends to mean "job accepted", slightly before the physical print motion actually finishes, and both the official Dymo app and other reverse-engineered clients disconnect at that point as well.
• The printer also exposes a third characteristic (...653..., "short command", write-without-response) alongside the print request/reply pair, apparently for lightweight status queries outside of a full print job - not currently used by this project.
• The protocol also supports an explicit "copies" command (0x1B 0x23 <count>) between the start-job and image-data commands, for printing multiple copies of the same label in one job - not currently sent by this project (there's no "copies" option in the UI yet), which appears to default to a single copy when omitted.

Full credit for reverse-engineering this goes to dymo-bluetooth, lt200b, homeassistant_letratag and thermal-label - this project's protocol implementation (LetraHelper.cs, LetraPrinter.cs) is built directly on what they documented.

Acknowledgements

• letra200bsharp - the original C# project this repository is forked from
• lt200b - the Python reference implementation the printing protocol is based on
• dymo-bluetooth - reverse-engineered documentation of the LetraTag 200B's Bluetooth LE service/characteristic UUIDs and status codes
• homeassistant_letratag - Home Assistant integration with its own BLE protocol reference (chunk framing, sequence numbering, manufacturer advertisement data), used to track down and fix a chunk-numbering bug that caused long labels to hang
• thermal-label - another independent BLE protocol reference (opcodes, raster packing, advertisement data), used to cross-check the chunk-numbering fix above and confirm the rest of this project's framing against a third source

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Made with ❤️ (and a Dymo LetraTag 200B)