Vaisala WS425FG - Manual (Page 63)

Chapter 5 _________________________________________________________ Operating Principle
VAISALA_______________________________________________________________________ 61
Power Supply Operating Principle
The 12 V linear regulator provides power to the sensor and some
internal circuits. The 5 V linear regulator powers the fiber optic modem
and the internal logic. The 24 VAC heater power is switched on or off
to the sensor enclosure heater and to the transducer and tube heater. The
switches are solid state relays that are controlled by two heater control
lines originating in the sensor.
The state of diagnostic logic is determined by the DTR and RTS lines
of the maintenance ports marked TEST. With the maintenance port
disconnected, the fiber optic modem is switched to the sensor (normal
system operating mode). When a personal computer is connected to the
maintenance port, the following states exist:
RTS DTR Description of States
LO LO Normal host system operation (TEST, connector
unplugged).
HI LO PC talks to the sensor, host system is disconnected
LO HI PC talks to the host system, sensor is disconnected
HI HI Host system is connected to the sensor with the PC
listening to both the transmit and receive lines