Marquette Hellige GmbH
MicroSmart V 2.xx
Page 25
Servicing Instructions 227 470 35 B - 97.12
Thermal array control
As the data output to the printhead is relatively time-consuming, special hardware has been provided
which relieves the processor of this task.
To drive the printhead, the CPU data for a printline are written block by block and at high speed
into a FIFO. A start signal generated by the CPU informs the printhead control TPH_CONTROL
(seated in a CPLD) that output to the thermal printhead can begin. Several "state machines" within
the TPH_CONTROL read 80 bytes from the FIFO and transmit the serialized data to the printhead.
At the end of the transfer the CPLD generates the latch signal for the array and the trigger signal
for the heat duration generation.
The speed-related heat duration is software-selected. The heat duration value is gained via the pulse-
pause ratio of a TPU channel functioning as PWM channel. After the PWM signal has been routed
via a low pass, a DC voltage proportional to the PWM ratio which is used for setting the heat
duration. With each trigger pulse for the heat duration generation, a capacitor charged via a constant
current source is discharged and a heat duration cycle is started. The linear voltage increase at the
capacitor is compared in a comparator with the analog value supplied by the PWM channel. If the
analog value is exceeded, the heat duration pulse is terminated. In addition, the heat duration is
adjusted as a factor of the printhead substrate temperature. The temperature-dependent voltage
obtained via the array thermistor is added to the PWM voltage supplied by the TPU channel in a
summing amplifier.
The supply voltage of the thermal array can be switched off via the power switch if:
- array voltage < 19.2V
- reset active
- motor not running
- array overheated
Temperature monitoring
An array excess temperature monitoring device is fitted to protect the thermal array. Using a
comparator, the voltage of the thermistor is compared with a reference value. If the array
temperature of 60°C is exceeded, the comparator signals this to the processor.
