Payment Flow

A payment is a full request-response exchange: connect, send command P or X, wait for terminal ACK, stream progress, parse the final result, then optionally drain receipt lines.

sequenceDiagram participant JS as React Native JS participant Cpp as C++ Session participant TCP as Native TCP participant POS as POS terminal JS->>Cpp: pay({ amountCents }) Cpp->>TCP: ensure connected Cpp->>POS: STX P payload ETX LRC POS-->>Cpp: ACK POS-->>Cpp: SOH progress EOT POS-->>Cpp: STX result ETX LRC Cpp-->>POS: ACK Cpp-->>JS: PaymentResult
  1. Call status() during pairing or startup.
  2. Display terminal state to the operator.
  3. Call pay() only from an explicit checkout action.
  4. Show progress events as transient payment status.
  5. Persist resultCode, stan, onlineId, and authCode with the sale.
  6. If the connection drops during a financial command, reconnect and call sendLastResult().
Financial commands are not idempotent

pay, payExtended, reverse, preAuth, incrementalAuth, and preAuthClosure can affect money movement. Automatic replay can duplicate effects.

Minimal payment handler

async function collectPayment(amountCents: number) {
  try {
    const result = await client.pay({ amountCents });
    return result;
  } catch (error) {
    await client.connect();
    return client.sendLastResult();
  }
}

The catch block recovers the last terminal result. It does not reissue pay().

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