diff --git a/deploy/nixos/hardware/upboard.nix b/deploy/nixos/hardware/upboard.nix
index 26e2842..f96050a 100644
--- a/deploy/nixos/hardware/upboard.nix
+++ b/deploy/nixos/hardware/upboard.nix
@@ -49,7 +49,13 @@
 
     kernelParams = [
       "i915.enable_psr=0"
-      "console=ttyS4,115200n8"
+      # NOTE: do NOT route the kernel console through ttyS4 on Sintra.
+      # ttyS4 is the SoC's MMIO 16550A (the only real UART besides the
+      # legacy ttyS0 at I/O 0x3f8) and is wired to the Fujitsu F56
+      # dispenser's RS-232 header on Sintra. Holding it as console
+      # prevents userspace from opening it at 9600 baud and HAL fails
+      # with "Input/output error setting custom baud rate of 9600".
+      # If you want serial debug, point console at ttyS0 instead.
       "console=tty0"
       "quiet"
       "splash"
@@ -118,8 +124,14 @@
     KERNELS=="1-7.3:1.0", SYMLINK+="ttyJ5"
     KERNEL=="ttyUSB1", SYMLINK+="ttyJ5"
 
-    # Dispenser (ttyJ7)
+    # Dispenser (ttyJ7).
+    # ttyS1 / ttyS5 cover earlier UP Board variants where the dispenser
+    # lands on those kernel-enumerated serial nodes; ttyS4 covers the
+    # Sintra (UP Board Atom/Apollo Lake) where the dispenser is wired
+    # to the SoC's MMIO UART. Whichever device actually exists at
+    # runtime gets the ttyJ7 symlink.
     KERNEL=="ttyS1", SYMLINK+="ttyJ7"
+    KERNEL=="ttyS4", SYMLINK+="ttyJ7"
     KERNEL=="ttyS5", SYMLINK+="ttyJ7"
 
     # Legacy ttyAMA0 alias