Radio Shack TRS-80 PT-210 Service Manual page 32

Thermal Head
The thermal head is used for printing with dots when the thermal head is heated by data pulses which are
processed by the pCPU and then driven by the transistors.
Control Logic
I
Head
Figure 23.
VTT 4-5.0-6.1V is used as the power supply for driving the thermal head.