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INTRODUCTION
As introduction to this report, first let me remind you what the transmission line model is
all about. This model will be used to analyze characteristics of different transmission
mediums such as the coaxial, parallel-plate, two-wire transmission lines to mention but a
few. The characteristics of the transmission line will then be interpreted and displayed
graphically using some sort of a visual display unit such as a computer monitor. That will
be achieved by the use of signals (voltage and current) that will be transported by the
physical transmission line and will pass through various stages until it will be interpreted
on the visual display unit.
The second topic will then show the block diagram of the whole system, and then an
explanation of each block will be discussed in detail. The report will also touch on the
progress I have made as far as the completion of the project is concerned. Pictures of
working modules will be presented in the process. Remaining tasks will also be
highlighted and when they will be accomplished.
1. BLOCK DIAGRAM AND SYSTEM SCHEMATIC
The following diagrams present the systems block and circuit diagrams.
Block diagram:
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Circuit diagram:
The various modules of the block diagram will be discussed in detail in the following
paragraphs.
1.1 Generator
Visual
display
unit
RS232
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This will generate and feed the signal to be transported by the transmission line. That
signal can be voltage or current which can be transmitted at a certain frequency from
the signal generator.
2.2. Transmission line
This is the actual transporter of the generated signal that will, in the end, have its
characteristics interpreted by the visual display unit. Here the type of transmission
line to be used will be specified together with the fundamental characteristics e.g
the characteristic impedance. Since the signal to be transported will be A.C and
since we want to feed it to a microcontroller, we first has to change it to D.C and
then regulate it to acceptable microcontroller levels (between 0V-5V)
2.3 Microcontroller
The analog signal from the transmission line will then be fed to the microcontroller.
The microcontroller will then use one of its peripherals to convert the analog data to
digital. It will also use another module to serially transmit the converted data to the
visual display unit.
2.4 Max 232 driver chip
Before data can be displayed visually it must first pass through the Max232. This is a
driver chip for the RS232 cable and what it does is it brings the microcontroller
voltage levels (0V-5V) up to the RS232 levels (usually +/-12V) then it inverts the
signal according to the type of data being sent.
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2.5. RS232 Cable
This cable acts as an interface between the microcontroller and the visual display unit
or monitor. For proper transmission some parameters such as the baud rate, character
length, parity bit and stop bit must be set properly. Also the RS232 voltage levels
must be known.
2.6. Visual display unit
This is where the characteristics of the transmission line will be represented
graphically. All the transmission line equations will be interpreted using software like
visual basic which will then plot the values of the parameter being interpreted.
3. PROGRESS
Having obtained enough information with regards to the project, the next step was to
implement all the things I have just explained in the second chapter above.
3.1 Signal generation
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The first thing I had to decide was to how I will go about when trying to input my
analog signal to the microcontroller for conversion. The microcontroller accepts
voltage levels from 0V to 5V D.C. So I used a potentiometer to vary the analog input
from the power supply between those acceptable levels. To access the input the
16F877 microcontroller was using pin AN0 (analog input).
But one thing to note is that the potentiometer was only used to observe if we are able
to achieve the analog-to-digital conversion. In the actual operation of the system
instead of using a potentiometer an A.C generator will be used produce the signal to
the transmission line, then that signal will be converted to D.C and upon doing that it
will be regulated to 5V D.C so that it can be within the microcontroller operating
voltages. Then that data will be input to pin AN0 of the microcontroller. Also the
desired transmission frequency will be obtained through the use of an oscillator or a
crystal.
3.2 Analog-to-digital conversion
Since I was now able to feed my analog input using the potentiometer through AN0,
now it was time to convert that data to a digital form. The following code for the
analog-to-digital conversion was written in MPLAB to accomplish the task.
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CODE 1: A/D conversion
__config _LVP_OFF & _XT_OSC & _WDT_OFF & _PWRTE_ON & _CP_OFF & _BODEN_OFF & _DEBUG_OFF
include "p16f877.inc"
list p=16f877
org 0x005
Start
bsf STATUS,RP0 ;bank 1
bcf STATUS,RP1
movlw B'00000000'
movwf TRISB ;PORTB [pin 0-7] is outputs
clrf ADCON1 ;clear a/d control reg, all inputs a/d
bcf STATUS,RP0 ;bank 0
movlw B'01000001' ;Fosc/8 [pin 7-6], A/D channel is AN0 [pin 5-3], a/d on
movwf ADCON0
Main
bsf ADCON0,GO ;Start A/D conversion
Wait
btfsc ADCON0,GO ;Wait for conversion to complete
goto Wait
movf ADRESH,W ;Write A/D result to PORTB
movwf PORTB ;light up LEDs
return
end
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What each line of the code does is explained on the far right of the line where
comments are located. The 0V to 5V analog signals will be represented by 0 to 255
digital words and each word is 8-bits long. The code above compiled successfully and
the PIC16F877 was later programmed with success too.
Upon programming, the circuit for testing if the code was functioning fine was wired.
The microcontroller was being clocked by a 4MHz crystal and the filtering capacitors
were 22uF. The input pin was AN0 and the output was taken from PORTB and was
being observed using LEDs connected between each PORTB pin and ground.
When the potentiometer was varied to produce analog voltages between 0V and 5V,
what was expected was that the LEDs will light up in succession from pin B0 till the
last pin B7. The meaning of LEDs lighting up is that a HI (1) bit is transmitted and if
it remains unlit a LO (0) is output.
At first this did not work out properly, but having worked on it thoroughly the
expected outcome was obtained. So one can safely say the analog-to-digital
conversion was achieved successfully. The following picture illustrates the results of
the analog-to-digital converter.
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Picture1: Digital output word (from LSB =>11100101)
3.3 Reception and transmission of digital data
For reception and transmission of digital data the PIC16F877 uses the USART (Universal
Synchronous Asynchronous Receiver Transmitter) module which is capable of receiving
and sending the available data serially.
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The following code is that of receiving and transmitting data serially, but the analog-to-
digital conversion code is also included since the USART reception register is supposed
to receive the digital data from PORTB.
CODE 2: Reception and transmission code
list p=16f877 ; list directive to define processor
#include "p16f877.inc" ; processor specific variable definitions
__CONFIG _CP_OFF & _WDT_OFF & _BODEN_OFF & _PWRTE_ON & _XT_OSC& _WRT_ENABLE_OFF & _LVP_OFF & _DEBUG_OFF & _CPD_OFF
;----------------------------------------------------------------------------
; Constants
SPBRG_VAL EQU .25 ; set baud rate 9600 for 4 MHz clock
;----------------------------------------------------------------------------
; Variables
CBLOCK 0x020
Flags ; byte to store indicator flags
ENDC
;----------------------------------------------------------------------------
; Macros to select the register bank
; Many bank changes can be optimized when only one STATUS bit changes
Bank0 MACRO ; macro to select data RAM bank 0
bcf STATUS,RP0
bcf STATUS,RP1
ENDM
Bank1 MACRO ;macro to select data RAM bank 1
bsf STATUS,RP0
bcf STATUS,RP1
ENDM
Bank2 MACRO ;macro to select data RAM bank 2
bcf STATUS,RP0
bsf STATUS,RP1
ENDM
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Bank3 MACRO ;macro to select data RAM bank 3
bsf STATUS,RP0
bsf STATUS,RP1
ENDM
; This code executes when a reset occurs.
ORG 0x0005 ;place code at this address
Reset Code: clrf PCLATH ;select program memory page 0
Start
bsf STATUS,RP0 ;bank 1
bcf STATUS,RP1
movlw B'00000000'
movwf TRISB ;portb [7-0] outputs
clrf ADCON1 ;left justified, all inputs a/d
bcf STATUS,RP0 ;bank 0
movlw B'01000001' ;Fosc/8 [7-6], A/D ch0 [5-3], a/d on [0]
movwf ADCON0
Main
bsf ADCON0,GO ;Start A/D conversion
Wait
btfsc ADCON0,GO ;Wait for conversion to completegoto Wait
movf ADRESH,W ;Write A/D result to PORTB
movwf PORTB ;LEDs
call SetupSerial ;set up serial port and buffers
MainLoop:
movf PORTB,W ;PORTB output to W reg
movwf RCREG ;W contents receiver reg
call TransmitSerial ;if so then go transmit the data
goto Main ;go do main loop again
;----------------------------------------------------------------------------
;Transmit data in RCREG when the transmit register is empty.
TransmitSerial: Bank0 ;select bank 0
btfss PIR1,TXIF ;check if transmitter busy
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goto $-1 ;wait until transmitter is not busy
movwf TXREG ;and transmit the data, RC6
return
;----------------------------------------------------------------------------
;Set up serial port.
SetupSerial: Bank1 ;select bank 1
movlw 0xc0 ;set tris bits for TX and RX
iorwf TRISC,F
movlw SPBRG_VAL ;set baud rate
movwf SPBRG
movlw 0x24 ;enable transmission and high baud rate
movwf TXSTA
Bank0 ;select bank 0
movlw 0x90 ;enable serial port and reception
movwf RCSTA
clrf Flags ;clear all flags
return
END
After the above code was compiled and programmed into the microcontroller, the
reception of the data by the USART module from PORTB and its transmission from themicrocontroller was tested, it seemed to be working properly. The picture below shows
the output from the PIC’s transmission pin (RC6) and from the multimeter we see that the
voltage is within the TTL voltage levels (0V-5V).
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Picture2: Output from the transmission pin (RC6) = 2.555V
3.4 Increasing voltage from TTL levels to RS232 levels
To increase from the microcontroller’s voltage levels to RS232 cable levels we need to
connect a max232 driver chip. The RS232 levels range between +/- 12V. The max232 is
wired as follows.
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Upon wiring the driver as in above, voltage measurements were taken from pin T2out
and the output depended on the data being transmitted. For words above 128 the voltage
is within the RS232 levels but it is inverted or negative whilst for those below 128 also
the levels are within the acceptable ones but now the voltage is positive. These can beseen from the pictures below.
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Picture3: RS232 levels when word above 128 is transmitted = -7.516V
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Picture4: RS232 levels when word below 128 is transmitted = 7.165V
From picture4 above it can be observed from the probe that both the LEDs representing a
LO and a HI levels are both lit. In principle those are expected to flicker to show that LO
and HI signals are transmitted, but because the voltage resolution of the analog signal is
19.6mV the human eye can not see that flickering. Otherwise the 1s and 0s are
transmitted.
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Again from picture4 it can be seen that even if you measure the voltage at the far end of
the RS232 cable (DB-9 connector on far right of picture) the voltage is the same as
measuring it from T2out. This means that the signal is transported up until it reaches the
DB-9 connector.
4. TASKS TO BE ACCOMPLISHED
The remaining task now is to implement the transmission line equations on visual basicsoftware so that wave patterns of transmission line parameters can be shown visually using
the same software.
To achieve what I have just mentioned above visual basic will continually check the
available data form the com port and that will be read and stored into some file. From that
file the data will then be interpreted by the transmission line parameter equations.
For example, say I want to draw the pattern of the reflection coefficient; I will sample the
voltage from the com port and store that in some file. We know that
Reflection coefficient = Vmax/Vmin
So from the stored voltage values I will pick the minimum and maximum then with that I
will be able to represent the reflection coefficient visually on a computer monitor.
5. CONCLUSION
One can conclude that most of the work has been done since the signal now has been
transmitted up until the DB-9 connector. The only remaining task is to implement the
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transmitted signal using visual basic so that the parameters are represented in form of wave
patterns. So I hope that before the last presentations occur that task will have been
accomplished.