diff --git a/docs/equipment/inline-optical-qc.md b/docs/equipment/inline-optical-qc.md
index 2fecec9..104186e 100644
--- a/docs/equipment/inline-optical-qc.md
+++ b/docs/equipment/inline-optical-qc.md
@@ -258,6 +258,20 @@ October 2026). The conveyor uses the same kit, so one spare covers both.
   drive's touch-probe input, which latches the servo position at the same
   mark every turn. The scan, the laser traces and the servo then share
   one angle reference.
+- **STO version, looked at in October 2026.** StepperOnline sells the
+  A6-EC drive with Safe Torque Off only from 400 W: A6-EC400H2B1-M17-STO
+  with a brake (USD 157.91,
+  [StepperOnline](https://www.omc-stepperonline.com/a6-series-400w-sto-safe-torque-off-ethercat-ac-servo-motor-kit-3000rpm-1-27nm-w-brake-17-bit-absolute-encoder-ip67-a6-ec400h2b1-m17-sto))
+  or the same without a brake, A6-EC400H2A1-M17-STO (USD 129.60). The STO
+  is two 24 V inputs, STO1 and STO2, on CN5, but its manual names no
+  safety rating: no SIL or PL figure and no certificate
+  ([A6-EC-STO manual](https://files.omc-stepperonline.com/A6-EC-STO_manual.pdf)).
+  On that document alone it cannot replace the contactor in the safety
+  circuit. The brake holds nothing on a horizontal spindle or conveyor.
+  Recommendation: stay with the 100 W kit. If StepperOnline confirms a
+  rated STO in writing, switch both axes to the version without a brake
+  and drop the contactor; its 0.60 kg·cm² rotor then needs only a 1:2 to
+  1:3 belt.
 
 **Sensor list.** Everything that measures the record, and everything the
 station controller needs to run the station. Lights, drives and cylinders are in the