2018KCL SERIES SIGNAL MONITOR
OPERATIONS MANUAL
Eberle Design Inc.
Page 29
6.7 REAL TIME CLOCK
The DS1305 Real Time Clock (U29) provides the timekeeping function. It transfers data to
the main MPU using the serial SPI interface. During normal operation the RTC is
synchronized to the 60 Hz AC Line for long term accuracy. The lifetime lithium battery
(BATT1) provides power to the RTC when AC Line voltage is not present. Should the
battery fail, no data will be lost other than the time and date should the AC Line voltage
drop out.
6.8 DISPLAY
The front panel LED display is multiplexed by the row driver U3 and the column driver U1.
The scan rate is approximately 60 Hz. Display data is shifted into U3 while individual
columns are selected by U1 using the SPI interface.
6.9 RMS-ENGINE
The RMS-Engine is a Microchip PIC18F242 (U37) single-chip high performance RISC
microcomputer executing proprietary firmware that calculates true RMS voltages for each
input channel. The resulting data is transferred to the main MPU via the SPI interface. The
analog to digital converter (U39) provides the voltage measurement data for the RMS-
Engine approximately every 6 microseconds. Analog multiplexers (74HC4051N) select the
appropriate channel for sampling. Op amp circuits U40 provide a buffer circuit.
6.10 POWER SUPPLY
The switching power supply is based on a MIC38C43 (U20) fixed frequency current-mode
PWM controller. The AC Line is full wave rectified and charges capacitor C78 to
approximately 170 Vdc. The PWM controller switches current from C78 through the
transformer T1 using transistor Q4 at approximately 200 KHz. The transformer couples this
energy to the secondaries and provides the following output voltages: VDD=+5 Vdc,
VCC=+8 Vdc, VGG=-8 Vdc, VEE=-5 Vdc, VCCL=+16 Vdc, and VDDS=+5 Vdc. Opto-
coupler U26 provides feedback to the controller for regulation.
6.11 EIA-232 COMMUNICATIONS PORT
The EIA-232 communications port is isolated from the main circuitry of the monitor through
opto-couplers U7 and U12. The MAX202 interface circuit U2 provides a level translation
from EIA-232 voltage levels.
6.12 ETHERNET COMMUNICATIONS PORT
The Ethernet communications port (J2) is isolated from the main circuitry of the monitor
through opto-couplers U7 and U12.