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SECTION 4
THEORY OF OPERATION
4.1
INTRODUCTION
The F270X is an exchange side Tele-Data Modem with ST single or multimode fiber optic connectors.
These Modems are designed to work with F271X subscriber side Modems in pairs and interface with audio
and RS-485 or RS-422 data grade signals. These units are housed in VERSITRON FOM II chassis: HF-1,
HF-2, HF-2SS and HF-20. Chassis descriptions are found in paragraph 1.2.3.
4.2 POWER SUPPLY AND ALARM CIRCUITRY
4.2.1
Power Supply Circuit
When 12VDC is applied to the circuit, the unit should draw between 100mA and 130mA depending on
the Modem state. The unit has a switching power supply circuit converting 12V to 5VDC and a charge pump
circuit converting 5V to
−
5VDC. The switching power supply has a switching frequency of 44.4Khz
determined by capacitor C28 (1800pF) and an under voltage lockout feature. This feature is produced by CR5
(1N755) and CR6 (1N4148). The under voltage lockout is accomplished as follows:
When the input voltage becomes lower than the zener breakdown voltage, the output transistor is turned
off. This occurs because diode CR6 at that time becomes forward bias, allowing resistor R42 to sink a greater
current from the non-inverting input than is sunk by the parallel combination of R43 and R44 at the inverting
terminal.
4.2.2
Alarm Circuit
Alarm LED goes off when one of the following occurs:
1.
+5V power supply circuit is not working
2.
–5V charge pump circuit is not working
3.
+12V power supply is not working
4.
A problem exists with data transmission and a mux clock is not present at U5 (HC123)
These conditions activate the base of Q3, which activates the base of Q2 and starts current flowing
through DS6, the alarm LED. When Q2 is activated this also changes the voltage level of connector P1 pin 7
that activates the system alarm circuit on the power supply.
