Datamax ST-3210 Operator's Manual page 53

Standard Media Sensor option –
Using a soft-bristled brush or compressed air, sweep or direct an air
stream into the Media Sensor to remove any debris. (In cases of
extreme build-up, a cotton swab dampened isopropyl alcohol can be
used to wipe off the sensor; however, if this method is used, allow
the sensor to dry before proceeding.)
Cotton Swab
Media Sensor
Printhead Assembly
Lower the printhead assembly and slide the printhead latch into the
'locked' position.
Plug in and turn 'On' the printer. Load ticket stock; see Section 3.2.
ST Models: Close the access cover. This completes the procedure.
S-Class 45