diff --git a/apps/machine/src/config/device.ts b/apps/machine/src/config/device.ts
index 9a22ab8..ae4ac00 100644
--- a/apps/machine/src/config/device.ts
+++ b/apps/machine/src/config/device.ts
@@ -139,7 +139,12 @@ export const MACHINE_PRESETS: Record<MachineModel, Omit<DeviceConfig, 'fiatCode'
     model: 'batm3',
     validator: {
       type: 'ebds',
-      device: '/dev/ttyACM0',
+      // MEI bill acceptor (EBDS) on a USB-serial bridge, exposed via the
+      // stable udev symlink /dev/ttyMEI (batm3.nix, serial A9YW78OC). The old
+      // /dev/ttyACM0 default assumed a CDC-ACM BNR; this hardware enumerates as
+      // ttyUSB* instead, so ACM0 never existed and cash-in was silently
+      // disabled ("[HAL] No validator"). Per-box override: VITE_LAMASSU_VALIDATOR_DEVICE.
+      device: '/dev/ttyMEI',
     },
     dispenser: {
       type: 'f56',