Fig. 5 shows the operation of the receiver unit schematically
in a block diagram. The central electronic component
of the receiver unit is a PIC microcontroller 18F4520 model
[25]. The PIC was programmed in Assembler language to
accurately perform several controlling functions. The PIC
gets local and remote unit current measurement data and
internally performs calculations of absolute and percentage
differences between the measured current values.
Finally, it sends the four values (local and remote unit measures,
absolute and relative differences) to the receiving
unit’s liquid crystal display.
The electronic circuits described in this text were developed
as part of the interface. The mention of these circuits
is necessary due that the software/firmware is embedded
in the microcontroller’s program memory that makes up
the hardware.
Fig. 6 shows a photograph of the prototype board 2.4
MCTX (Current meter transmitting at 2.4 kbps), which
has been developed based on a ‘‘universal board’’. It should
be noted that the center plate is inserted in a PIC.
The Clamp-ammeter is the electric current transducer
for power cables under testing considering a voltage as a
reference signal. The MCTX 2.4 board will obtain this voltage
signal, and it will process the information. The board
converts the electric current intensity measured in digital
signals, which will be transmitted to the Human Machine
Interface (HMI) via a plastic optical fiber at a rate of
2.4 kbps. Fig. 7 shows a photograph of the HMI board.
A mobile unit containing a modified 2.4 MCTX board
will transmit digital signals to the HMI by radio frequency,
also at a rate of 2.4 kbps, using a 433 MHz carrier with a
modulation frequency-shift keying (FSK). The HMI board
will receive the data from the two MCTX 2.4 measures
cards (the card connected by an optical fiber and another
connected by an RF link). The HMI shows both current
readings on the 16 2 cm LCD display. Beyond the current
intensity, the HMI also shows the difference between the
currents, in addition to the percentage corresponding to
the current difference delivered by both meters (relative
error). It should be noted in Fig. 7, the presence of another
PIC and a LCD display. The source program HMI and MCTX
2.4 board were written in the C language. These programs
when compiled into object code in which the instructions
are represented in machine language and finally transferred
to the microcontrolator program memory.