4.3.7 POWER SUPPLY
The power supply is detailed in page 8 of 9, Figure 8-2. It supplies regulated DC voltages of +5V, +12V, -5V, and -12V from both AC and DC inputs.
4.3.7.1 AC Power Supply
The AC power supply is a conventional passive regulator supply with a small PC mounted line transformer, T4 which has dual primaries for 115 or 230 VAC operation, and has dual 15 VAC secondaries which are wired in parallel. T4 and bridge CR36 along with capacitors C120 and C121 are configured as a positive full wave circuit supplying unregulated +BUS voltage (about 20VDC) to the +12V and two +5V regulators. One +5V regulator, U72, feeds the RTU-292 standard circuits. The other regulator, U71, feeds only the assemblies on the option tray. The positive regulators are standard three-terminal types, using a finned heatsink for cooling.
The negative voltage is supplied by switching regulator U73, which converts the +BUS voltage to -12V adjustable by R212.
4.3.7.2 DC Power Supply
The DC power supply allows the RTU-292 to be powered from either a +12V or +24V DC source as well as the standard AC supply line input. Slide switch S6 configures the supply for either +12V or +24/28V operation. This circuitry is reverse-polarity protected by schottky diodes in series with the voltage lines.
If sufficient AC and DC power are simultaneously applied, the unit will draw power from the AC line only. Its auto take-over feature will draw DC power only if the AC line sags.
The DC supply input is filtered by choke L2 and by capacitors C50 and C51. If the DC input is +12V, this filtered supply feeds the +BUS directly via CR34, and supplies +12V around U70 via CR33. It also supplies voltage directly to the inputs of the +5V regulators through CR39 and R213, and through CR40 and R214. If the DC input is +24/28V, the +BUS is fed through emitter follower Q24, which keeps the bus voltage from rising above about +24V when input voltages of 24V to 32V are used.
4.3.8 CPU
The CPU portion of the main board controls all functions of the RTU-292. This board uses the front panel switches and keypad, as well as the RS-232 serial communications port and signals received over the telephone as control inputs. The Main board lights front panel LEDs to display its current status, and controls the audio gates on the audio portion of the main board to route signals to their correct destinations. Refer to pages 5, 6 & 7 of Figure 8-2.
The RTU-292 is controlled by microprocessor U46. The program that runs U46 resides in EPROM U40. The unit’s current status is stored in EEPROM U41. Octal latch U39 creates address lines A0 through A7 from the data bus and the AS control line. Voltage supervisor U45 ensures that the microprocessor turns off and on in an orderly manner when power is turned off and on or during power glitches. RS-232 driver U37 interfaces the microprocessor with the serial communications port. The /IRQ (interrupt request) signal generated by portions of U47 and U48 informs the processor when the keypad is under use or a DTMF signal has been detected by the Audio Board.
Most of the rest of the circuitry comprises I/O devices. U43 and U44 create the chip selects required to control these devices. U38 sends commands to the optional Voice Prompt Board.
