---
type: concept
tags: [parking, device, printer, transport, usb, escpos, provisioning]
sources: []
updated: 2026-07-06
status: settled
---

# Printer USB transport (kernel usblp, behind the ESC/POS layer)

The ESC/POS printer drivers ([[rongta-printer|rongta]], `cashino`) can deliver their byte stream
over **either a raw TCP socket (port 9100)** or a **local USB character device** (`/dev/usb/lp0`),
selected per device by `config.transport` (`"tcp-ip" | "usb"`). The original architecture always
intended one ESC/POS adapter to cover "USB **or** network" (parking-system-architecture §BOM); the
first implementation shipped TCP-only, and this closes that gap.

## The seam — render once, dispatch the transport

Every `render*()` function in `packages/devices/src/drivers/printer-escpos.ts` produces a
**transport-independent ESC/POS `Buffer`**. Only delivery differs. The transport is resolved **once**
per driver from config and every print/probe call site stays transport-blind:

- `transportFromConfig(config)` → a discriminated `Transport` (`{ kind: "tcp", host, port }` or
  `{ kind: "usb", devicePath }`). Anything other than `transport: "usb"` is TCP, so **existing
  host-only configs keep working unchanged** (no migration).
- `sendTo(t, payload, timeoutMs)` / `probeTo(t, timeoutMs)` dispatch to the TCP pair
  (`sendRaw`/`probe`) or the USB pair (`sendRawUsb`/`probeUsb`).

Adding a transport = one more arm in the dispatcher; **not a single rendered byte changes**. This is
why the CP852 map, the Code128/QR builders, roles/failover, and the receipt/ticket/voucher layouts
are all untouched by USB support.

## USB transport = the in-box `usblp` char device

A USB ESC/POS printer plugged into the appliance enumerates as a **character device** (e.g.
`/dev/usb/lp0`) via the kernel's in-box **`usblp`** driver. We just **open it `O_WRONLY` and write
the same bytes**:

- **No native dependency.** A plain `fs` write — no libusb, no CUPS, no native addon. This keeps the
  **MIT/Apache/BSD-only** dependency constraint and the **offline-first, minimal-deps appliance**
  posture (see [[technology-stack]], [[offline-first]]).
- **`usblp` is raw.** Unlike the TCP path there is **no FIN/half-close dance** (the graceful-close
  fix was a *TCP* concern — an early `destroy()` could RST-truncate the stream; see
  [[rongta-printer]]). A single open + write delivers the job; we always close the handle.
- **Bounded by a timeout.** A wedged USB printer can block the write (or the open) indefinitely; a
  stuck print must surface as a failure, not hang the entry flow. `withTimeout` rejects after
  `timeoutMs`.

## Status over USB — reachability only (honesty rule)

`probeUsb` is "does the char device exist and open writable" — the **USB analogue of the TCP connect
probe**. A present, openable `/dev/usb/lp0` means `usblp` bound a powered, enumerated printer.

- The `cashino` driver is reachability-only on **both** transports (it never had a status page).
- The `rongta` driver's rich `readStatus()` scrapes the board's **HTTP** `/prn_stat.htm` — a
  **network feature**. Over USB there is no such page, so `readStatus()` **degrades to the
  reachability floor** (ready/offline only, never a guessed paper/cover state). A USB Rongta is
  effectively a Cashino for monitoring. This preserves the standing honesty rule from
  [[printer-status-monitoring]]: never report a paper/cover verdict the transport can't actually sense.

## Threat model

The USB path is a **local character device** the booth operator (the threat model's adversary)
cannot reach over the network — narrower attack surface than the unauthenticated TCP print socket on
the VLAN. Printers are advisory output; nothing about the signed [[append-only-event-chain|ledger]]
or barrier control is touched.

## Provisioning dependency (NOT app code) — see open-questions #14

Driving a USB printer depends on the appliance image:
1. the **`usblp`** kernel module is loaded (it is in-box on Ubuntu 26.04; CUPS can claim the
   interface first — may need `usblp` to win, or CUPS masked for that device), and
2. a **udev rule** grants the server process write access to the node (e.g. a group on
   `/dev/usb/lp*`), since the appliance server does not run as root.

This is a [[appliance-provisioning]] concern, recorded as **open-questions #14** until the on-site
printer is confirmed USB and the rule is baked into the image and verified on hardware.

## Status

Built 2026-06-24 behind the existing render layer. `sendRawUsb`/`probeUsb`/`transportFromConfig`/
`sendTo`/`probeTo` in `printer-escpos.ts`; `cashino` + `rongta` resolve a `Transport` and dispatch.
The setup UI offers a **Connection** select (Network / USB) + a **USB device** path field (default
`/dev/usb/lp0`); host/port are not-required so a USB printer needs neither. Covered by
`printer-escpos.test.ts` (USB writes the exact rendered bytes; probe present/absent;
`transportFromConfig` TCP back-compat) and `printer-cashino.test.ts` (a USB-configured driver prints
to the node and reports ready/offline). The on-hardware confirmation + the udev/usblp provisioning
are pending (open-questions #14).

## Field bug — the NONBLOCK partial-write truncation (found + fixed 2026-07-06)

First on-hardware USB test (ICS XP-K200L, an ESC/POS clone): over TCP it printed + cut fine; over
USB it printed the ticket's TEXT but **no barcode and no cut**. Root cause was in OUR transport,
not the printer: `sendRawUsb` opened the node with `O_NONBLOCK` and issued ONE `write()` for the
whole job. On a non-blocking usblp fd the kernel accepts only what fits the printer's USB buffer
(~8 KB) and returns a **short write**; the old code never checked `bytesWritten`, closed the
handle, and silently dropped the tail — which is exactly where the barcode (mid-payload) and the
CUT (last bytes) live. Small jobs fit one buffer, hence "text prints fine". The regular-file test
stand-in can't short-write, so tests never caught it.

Fix: `writeAllUsb` — chunked loop (4 KB, safely under the usblp buffer) that continues after
partial writes, retries `EAGAIN`/zero-byte writes with a short pause, and fails at the caller's
deadline with a `(N/M bytes accepted)` diagnostic. Driven by fake-handle tests (short writes,
EAGAIN interleave, wedged-printer timeout, non-EAGAIN passthrough) since a real file can't
reproduce the char device's behaviour.

**Second truncation mode — close() cancels the in-flight transfer (lab bench, 2026-07-07).** The
chunked loop alone STILL truncated on hardware (test slip stopped mid-sentence, no feed, no cut —
"press the feed button to see the text"). Verified against `drivers/usb/class/usblp.c`: `write()`
returns at URB *submission* (not completion), only ONE write URB is in flight at a time (the next
write EAGAINs until it completes), and `usblp_release()` — i.e. our `close()` — **kills in-flight
URBs**. The printer drains bulk data at PRINT speed (tiny internal buffer on these clones), so
closing right after the last accepted write cancels the still-transferring tail — exactly where
the feed + `GS V` cut bytes live. Kernel-accepted ≠ printer-received.

Fix: the one-URB rule makes acceptance of write N a **completion certificate for write N−1**. So
`writeAllUsb` now writes the payload's FINAL BYTE alone: when that 1-byte write is accepted, every
byte before it is physically in the printer; a short drain pause (`USB_DRAIN_MS` 300 ms) covers
the lone final-byte packet, then close is safe. (usblp also implements `poll(POLLOUT)` as the true
completion signal, but Node cannot poll an arbitrary char-device fd without a native dep — the
hold-back + drain gets the same guarantee for all but the final byte, whose packet the printer
ACKs immediately after having just freed its buffer.)

> Driver-choice note for this clone: the ICS XP-K200L does NOT serve the Rongta `/prn_stat.htm`
> status page (checked on hardware at 10.0.10.11 — print socket 9100 open, status page absent),
> so on NETWORK the honest driver is **cashino** (reachability-only monitoring); under `rongta`
> the monitor would mark a perfectly working printer offline/degraded. Over USB the two drivers
> behave identically (reachability floor), so either works post-fix. See [[rongta-printer]].

Related: [[rongta-printer]], [[printer-status-monitoring]], [[printer-roles-failover]],
[[appliance-provisioning]], [[network-isolation]], [[technology-stack]].