E-Ink Display Spec

Device-side specification for the wifi e-ink displays (MEDORO panels, 800x480, black/white/red/yellow). Source: vendor OpenAPI 3.1 schema (eink-openapi-3.1.yaml). See Server E-Ink Integration for how SafeCall serves these devices.

Polling model

Each display is configured with a server URL. It calls it periodically:

GET https://<server>/eink/display?voltage=1.66&mac=A0:F2:62:46:F8:38

• mac — the display's MAC address (its identity)
• voltage — current battery voltage

The response body must be the layout JSON (no envelope), encoded as Windows-1250 (Central European, required for Czech diacritics). The firmware compares the version field with the version of the layout it last drew; if it differs, it redraws.

Layout document

{
  "rotation": "0",
  "validity": "30",
  "version": "2",
  "objects": [ ... ]
}

Field	Meaning
rotation	Display rotation: 0–3 (quarter turns; 0 = landscape)
validity	Seconds between device polls ("how often it checks whether the UI changed")
version	Layout version, max 99, then restart from 1. The device redraws only when this changes
objects	Drawing primitives, rendered in array order (later objects paint on top)

All numeric values are integers encoded as strings ("50", not 50).

Drawing primitives

type	Fields	Description
text	text, x, y, font, color	Text; (x, y) is the baseline-left position
fillrect	x, y, w, h, color	Filled rectangle
drawrect	x, y, w, h, color	Rectangle outline
fillroundrect	x, y, w, h, r, color	Filled rounded rectangle
drawroundrect	x, y, w, h, r, color	Rounded rectangle outline
drawline	x0, y0, x1, y1, color	Line segment
fillcircle	x, y, r, color	Filled circle (center + radius)
drawcircle	x, y, r, color	Circle outline

Colors

Id	Color
0	Black (GxEPD_BLACK, default)
1	White (GxEPD_WHITE)
2	Red (GxEPD_RED)
3	Yellow (GxEPD_YELLOW)

Fonts

Firmware font ids (Verdana, regular and bold):

Id	Font	Id	Font
0	Verdana 10pt (default)	6	Verdana 32pt
1	Verdana Bold 24pt	7	Verdana Bold 10pt
2	Verdana 10pt	8	Verdana Bold 12pt
3	Verdana 12pt	9	Verdana Bold 16pt
4	Verdana 16pt	10	Verdana Bold 24pt
5	Verdana 24pt	11	Verdana Bold 32pt

Encoding rules

• The JSON must be served as Windows-1250 bytes (Content-Type: application/json; charset=windows-1250). UTF-8 responses render Czech characters incorrectly.
• Characters outside Windows-1250 cannot be displayed; SafeCall substitutes ?.
• Although the vendor schema describes the response as ISO-8859-2, deployed panels render byte 0xA9 as ©. Windows-1250 sends Š as 0x8A, which matches the firmware's Central European font table.

Constraints summary

• version wraps: 1 → 99 → 1. Never exceed 99.
• Keep validity reasonable — every poll wakes the radio and an e-ink redraw is slow and flashes the panel.
• Display resolution of the deployed panels: 800x480 (coordinates up to 799/479 in landscape).