---
type: concept
tags: [parking, printer, device, monitoring, reliability]
sources: []
updated: 2026-06-14
---

# Printer status monitoring

The booth must know a printer is in trouble **before** a driver presses the entry button and no
ticket comes out. So the system polls each printer's live status (paper out, cover open, cutter
jam, off-line) and pushes changes to the operator UI. A reliability control, like
[[printer-roles-failover]] — not a threat-model one.

## Where the status comes from (the safe-decode decision)

The raw print socket (TCP 9100) is write-only for us — it returns no paper/cover feedback. ESC/POS
printers expose status via real-time queries (`DLE EOT n`). On the [[rongta-printer]] clone we
probed, **`DLE EOT` replies do NOT follow the canonical ESC/POS bit layout** (the spec's fixed
validation bits were wrong, verified on hardware 2026-06-14). Decoding those bits ourselves risked
a **false-healthy** — reporting "paper OK" when it's empty — which is the dangerous direction for
an entry lane.

Instead we scrape the device's **own status web page** (`http://<host>/prn_stat.htm`). The board
decodes the bits itself into labelled Yes/No rows (Cover Is Open, Cutter Error, Paper End, Paper
Near End, Printer Off-Line). We trust the device's decode over hand-decoding an undocumented clone.

This is captured as a device capability: `MonitorableDevice.readStatus(): PrinterStatus` in
`packages/devices`. The Rongta driver implements it; the monitor is device-agnostic via
`isMonitorable()`. A future printer with a different status mechanism just implements the same
interface.

## Status mapping (fail safe)

`readStatus()` maps to `ready | degraded | offline`:

- status page unreachable / times out → **offline** (same signal as a dead printer; never throws),
- page reachable but a recognised field is missing → **degraded** ("unexpected status page") —
  we do NOT claim "ready" off a page we didn't fully parse,
- any fault flag true (paper end, cover open, cutter error, off-line) → **degraded** + a detail
  string ("paper out", …),
- all five clear → **ready**.

## The monitor (server)

`PrinterMonitor` (`apps/server/src/printer-monitor.ts`):

- reloads the monitored set from `lane_devices` each tick (so a newly-assigned printer is picked
  up without a restart), keeping only enabled, monitorable printers;
- polls every `PRINTER_POLL_MS` (default 5000ms), never overlapping ticks;
- caches the latest status per device id;
- emits a `printer-status` event on the device bus **only when status changes** (deduped).

## API / live UI

- `GET /api/printers/status` — cached snapshot of all printers (no device round-trip).
- `GET /api/printers/status/stream` — **Server-Sent Events**: full snapshot on connect, then one
  event per change. The booth SPA subscribes for real-time paper-out / offline indicators.
- Any authenticated role may read (operational, not a setup action).

## Verified on hardware (2026-06-14)

`readStatus()` against 10.0.10.6 → `ready` (all flags false); against an unreachable host →
`offline` with "status page timeout" (no throw); bus emits on change and suppresses unchanged
reads. Full repo typechecks.

## Open / not yet done

- **Fault-state capture**: we've only observed the all-clear page. The exact label text for an
  active fault (e.g. does "Paper End" flip to "Yes"?) should be confirmed by physically removing
  paper / opening the cover, to be 100% sure the scrape catches it. The parser is built to match
  Yes/No and degrade on anything unexpected, so this is a confidence check, not a blocker.
- Tying a `degraded`/`offline` entry-dispenser into [[printer-roles-failover]] failover and the
  (not-yet-built) entry flow's all-printers-down policy ([[device-input-flow]]).