Gtech Accutherm, Service Manual, Page: 23 / 132

Introduction
Service Manual for the Accutherm Ultra Printer 1-11
Sensor Interface
There are three, on-board infrared optical interruptive sensors, which are activated
by plastic flags attached to the mechanism. These devices have an open collector,
photo transistor output and provide a low level signal when blocked. The output
switches to a high level when the sensor is unblocked by a flag. The outputs of
these sensors are buffered with Schmitt triggers before being presented as inputs
to the MCF5253. The state of these sensors is as follows:
The controller board also has connections for two external optical reflective
sensors. One sensor is used to detect the Top of Form (TOF) mark and paper low
stripes on the back of the paper. The other sensor is used to detect paper jam
(JAM) conditions.
The LED current of the TOF/Paper Low sensor is adjustable by the processor in 8
steps from 8ma to 21ma and is auto calibrated using diagnostics resident in the
printer. The output of this sensor is presented in analog form to the MCF5253’s
AD converter for calibration and Schmitt trigger buffered on another input for fast
reading during normal operation.
The LED current of the JAM sensor is fixed to approx. 18ma. Its output is Schmitt
trigger buffered for fast reading by the MCF5253 processor.
Power Supply Circuitry
There are four different power supply voltages used in the Accutherm Ultra.
Table 1-1. Sensor State
Sensor Blocked Unblocked
OP1 Cutter home Cutter not home
OP2 Paper cover closed Paper cover open
OP3 Paper present Paper not present
Table 1-2. Power Supply Circuitry
Voltage Derived From Used For
24 VDC Modular Power Supply Print Head, Step Motors
3.3 VDC Derived from +24V using a buck
converter circuit Processor I/O,USB
communications, SDRAM, flash
memory, print head logic,
sensors