diff --git a/README.md b/README.md
index 9b6f4ce..fcb17aa 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,25 @@ You need python, paho-mqtt-client and apache+php.
 
 To install the python dependencies, run 'pip install -r requirements.txt'.
 
+### Printer and cash drawer
+
+The POS serial port defaults to `/dev/ttyKASSA`. Install the udev rule on the
+machine connected to the Prolific PL2303 USB serial adapter:
+
+```sh
+sudo install -m 0644 udev/99-kassa.rules /etc/udev/rules.d/99-kassa.rules
+sudo udevadm control --reload-rules
+sudo udevadm trigger --action=add --subsystem-match=tty
+sudo udevadm settle
+ls -l /dev/ttyKASSA
+```
+
+Give the backend user membership of `dialout` and restart the backend. Set
+`pos.serial.port: /dev/ttyKASSA` in an existing `config.yaml`; local overrides
+remain supported. The adapter has no unique serial number, so this rule assumes
+only one `067b:2303` adapter is connected. With several identical adapters, narrow
+the rule to the appropriate USB port before installing it.
+
 ### data
 
 Data is stored in 'data'. In production it is also committed to git, but for
diff --git a/config.py b/config.py
index 8cb4909..d1ddf43 100644
--- a/config.py
+++ b/config.py
@@ -29,7 +29,7 @@
     },
     "pos": {
         "serial": {
-            "port": "/dev/ttyUSB0",
+            "port": "/dev/ttyKASSA",
             "baudrate": 19200,
         },
     },
diff --git a/config.yaml.example b/config.yaml.example
index 4be949e..3601400 100644
--- a/config.yaml.example
+++ b/config.yaml.example
@@ -23,7 +23,7 @@ stickers:
 
 pos:
   serial:
-    port: /dev/ttyUSB0
+    port: /dev/ttyKASSA
     baudrate: 19200
 
 logging:
diff --git a/tests/js/browser-smoke.test.js b/tests/js/browser-smoke.test.js
index bc24b04..81318da 100644
--- a/tests/js/browser-smoke.test.js
+++ b/tests/js/browser-smoke.test.js
@@ -20,19 +20,27 @@ function chromiumCommand() {
 }
 
 function runChromium(command, url) {
-  return spawnSync(command, [
-    "--headless",
-    "--disable-gpu",
-    "--disable-dev-shm-usage",
-    "--no-sandbox",
-    "--allow-file-access-from-files",
-    "--dump-dom",
-    "--virtual-time-budget=3000",
-    url,
-  ], {
-    encoding: "utf8",
-    timeout: 20000,
-  });
+  const profile = fs.mkdtempSync(path.join(os.tmpdir(), "kassa-chromium-"));
+  try {
+    return spawnSync(command, [
+      `--user-data-dir=${profile}`,
+      "--no-first-run",
+      "--no-default-browser-check",
+      "--headless",
+      "--disable-gpu",
+      "--disable-dev-shm-usage",
+      "--no-sandbox",
+      "--allow-file-access-from-files",
+      "--dump-dom",
+      "--virtual-time-budget=3000",
+      url,
+    ], {
+      encoding: "utf8",
+      timeout: 40000,
+    });
+  } finally {
+    fs.rmSync(profile, {recursive: true, force: true});
+  }
 }
 
 function skipIfChromiumSandboxed(t, result) {
@@ -103,6 +111,7 @@ test("kassa index boots in a real browser", (t) => {
   fs.unlinkSync(fixturePath);
   if(skipIfChromiumSandboxed(t, result)) return;
 
+  assert.ifError(result.error);
   assert.equal(result.status, 0, result.stderr);
   const match = result.stdout.match(/<pre id="smoke-result">([^<]+)<\/pre>/);
   assert(match, result.stdout);
@@ -148,6 +157,7 @@ test("kassa frontend posts keyboard and button input in a browser", (t) => {
   fs.unlinkSync(fixturePath);
   if(skipIfChromiumSandboxed(t, result)) return;
 
+  assert.ifError(result.error);
   assert.equal(result.status, 0, result.stderr);
   const match = result.stdout.match(/<pre id="smoke-result">([^<]+)<\/pre>/);
   assert(match, result.stdout);
diff --git a/tests/test_config.py b/tests/test_config.py
index fec84b7..64d47ab 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -9,6 +9,7 @@ def test_load_config_uses_defaults_when_missing(tmp_path):
     assert config["mqtt"]["host"] == "localhost"
     assert config["stickers"]["printer"]["port"] == 9100
     assert config["logging"]["level"] == "INFO"
+    assert config["pos"]["serial"]["port"] == "/dev/ttyKASSA"
 
 
 def test_load_config_merges_yaml_with_defaults(tmp_path):
diff --git a/udev/99-kassa.rules b/udev/99-kassa.rules
new file mode 100644
index 0000000..87f0aa3
--- /dev/null
+++ b/udev/99-kassa.rules
@@ -0,0 +1,3 @@
+# Printer and cash drawer USB serial adapter (Prolific PL2303).
+# This adapter has no unique serial number; use only one matching adapter.
+SUBSYSTEM=="tty", KERNEL=="ttyUSB*", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", SYMLINK+="ttyKASSA", GROUP="dialout", MODE="0660"