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Print Flow

The PC printer driver intentionally returns quickly from /print. Actual printer I/O happens in the background worker for the target printer.

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End-to-End Flow

  1. Plato Console renders a receipt, ticket, or report to a PNG blob.
  2. The Console WIN_DRIVER adapter converts the image to black/white raster rows.
  3. Raster rows are split into chunks of 128 rows.
  4. Each chunk is encoded as an ESC/POS raster command and base64 encoded.
  5. Console sends POST /print with ip_address, port, and chunks.
  6. The driver wraps chunks with small delays, appends feed/cut, and optionally adds beep commands.
  7. The job is added to that printer’s BullMQ queue.
  8. The worker opens a TCP socket to the printer.
  9. The worker sends ESC/POS initialize, then each chunk.
  10. The worker closes the printer connection after the job.

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Console Chunking

The Console adapter uses:
SettingValue
Driver typeWIN_DRIVER
Raster chunk height128 rows
Status timeout3000ms
Print request timeout5000ms
Cash drawer timeout3000ms
The browser sends already-encoded raster chunks. The driver does not render HTML.

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Driver Chunk Wrapping

receipt-encoder.ts turns incoming chunks into job chunks:
  • Each incoming chunk is decoded from base64.
  • Each print chunk gets delayAfterMs: 25.
  • A feed and partial cut command is appended.
  • If beep is true, 18 beep commands are appended.
  • Each beep chunk has delayAfterMs: 150.

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ESC/POS Commands

CommandBytesPurpose
Initialize1B 40Reset printer before each job.
Raster image1D 76 30 00 ...Sent by Console chunks.
Feed0A 0A 0A 0AFeed before cut.
Partial cutEncoder-generatedPartial paper cut.
Beep1B 42 01 01Beep command repeated by the driver.
Cash drawer1B 70 00 19 FAKick drawer pulse.

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Socket Lifecycle

The current worker opens and closes the printer connection for each job.
ConstantValuePurpose
OPEN_ATTEMPT_TIMEOUT_MS2000Max time to open TCP socket.
CONNECTION_COOLDOWN_MS250Minimum delay after close before another open.
CLOSE_TIMEOUT_MS250Max wait for graceful close before destroy.
WRITE_TIMEOUT_MS6000Max time for one socket write.
There is also an idle-release timer of 3000ms, but the current process path closes the connection immediately after each successful job.

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Failure Path

If opening or writing fails:
  1. The printer is marked offline.
  2. The socket is released/destroyed.
  3. The BullMQ job fails.
  4. BullMQ retries after queue.retryDelayMs.
  5. When the printer is reached again, delayed retry jobs are promoted unless they are intentionally scheduled with a future runAt.

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Status Probing

POST /status can probe the printer only when queue work is not active. This avoids opening a probe socket while the same printer is busy printing.