EFY December 2002
PC-BASED OSCILLOSCOPE
M.M. VIJAI ANAND tive side itself, so both the half cycles are whether the cycle is positive or negative.
read as positive when it is given as input to It is the most critical part of the circuit
T
his circuit conditions different sig- the ADC. During positive half cycle, diode and if it operates improperly, the symme-
nals of frequency below 1 kHz and D3 is on and diode D4 is off, and op-amps try of the analogue signal displayed in the
displays their waveforms on the PC’s A1 and A2 act as inverters. Thus the output PC monitor gets affected. At the zero-cross-
screen. The hardware is used to condition is a replica of the input. During the nega- ing instant when the input signal transits
the input waveform and convert it to the tive half cycle, diode D3 is off and diode D4 to negative side, the zero-crossing detec-
digital format for interfacing to the PC. The is on. With R2=R3=R4=R5=R6=R=330 tor informs the PC by taking pin 15 of 25-
software for acquiring the data into the PC ohms, the voltage (V) at output pin 1 of op- pin ‘D’ connector of the parallel port high.
and displaying the same on its screen is amp A1 is related to the input voltage (Vi) The input at pin 15 of ‘D’ connector goes
written in Turbo C. as follows: low when the input signal transits to posi-
The input waveform (limited to 5V Vi/R +V/(2R)+V/R=0 tive side. The zero-crossing detector com-
peak-to-peak) is first applied to a full-wave V= -(2/3)Vi municates with the PC through bit D3 of
rectifier comprising op-amps A1 and A2 The final output voltage (Vo) at pin 7 the status port 379Hex.
of quad op-amp LM324 (IC4) and a zero- of op-amp A2 is given by the following The zero-crossing detector has been
crossing detector built around LM3914 dot/ relationship: realised using LM3914 IC. You may adjust
bar display driver (IC8) simultaneously. Vo=(1+R/2R)(-2Vi/3)= -Vi VR1 such that the last LED (LED10) goes
The full-wave rectifier rectifies the in- As Vi is negative, the output voltage is off when the input signal transits negative
put signal such that the negative half cycle positive. side of the input waveform. The LM3914
of the input signal is available in the posi- The zero-crossing detector detects itself rectifies the input signal and allows
PARTS LIST input voltage to 8-bit digital output. The control port address is 0x037a. The port
Semiconductors: data bus is tristate buffered. With eight addresses for parallel ports are summarised
IC1 - 74244 bits, the resolution is 5V/255 = 19.6 mV. below:
IC2 - 7805
IC3 - ADC 0804 The inbuilt clock generator Printer Data port Status port Control port
IC4, IC5 - LM324 circuit produces a frequency of
IC7 - CD4016
IC8 - LM3914 about 640 kHz with R1=10 kilo- LPT1 0x0378 0x0379 0x037a
T1 - Transistor SL100 ohms and C4=150 pF, which are LPT2 0x0278 0x0279 0x027a
D1-D4 - 1N4001 switching diode
L1-L10 - Red LED the externally connected timing LPT3 0x03bc 0x03bd 0x03be
Resistors (all ¼-watt, ±5% carbon, components. The conversion
unless stated otherwise): time obtained is approximately 100 µs. The (EFY Lab note. For details of the par-
R1 - 10kilo-ohm
R2-R6 - 330-ohm functions of other pins are given below: allel port pins, refer ‘PC-based Dial Clock
R7 - 1Mega-ohm Pin 1 (CS): This is active-low chip- with Timer’ project published in June 2002
R8-R17 - 470-ohm
R18 - 1kilo-ohm select pin. issue of EFY.)
R19 - 10kilo-ohm/1watt Pin 2 (RD): This active-low pin en- The software, written in C program-
VR1, VR2 - 10kilo-ohm Preset
ables the digital output buffers. When high, ming language, is user-friendly and easy-
Capacitors:
C1 - 1000µF, 25V electrolytic
the 8-bit bus will be in Hi-Z state. to-understand. It gets data from the devel-
C2 - 0.1µF, ceramic Pin 3 (WR): This active-low pin is used oped hardware circuit and displays it in
C3 - 1500PF, ceramic
C4 - 150PF, ceramic to start the conversion. the graphical screen with some changes.
Miscellaneous: Pin 9 (Vref/2): This is optional input The C program includes two user-de-
25 PIN D-Connector Female pin. It is used only when the input signal fined functions with the main function:
2 PIN SIP CONNECTOR range is small. When pin 9 is at 2V, the graphics( ) and settings( ). The settings( )
PCB
range is 0-4V, i.e. twice the voltage at pin 9. function is used to adjust the voltage and
only positive half of the cycle. Pin 6 (V+), Pin 7(V-): The actual in- time scale. The graphics( ) function is used
The output from the full-wave rectifier put is the difference in voltages applied to to display the waveform on the screen. The
is applied to the input of a sample-and-hold these pins. The analogue input can range sample control signal is used to close the
circuit comprising op-amps A3 and A4 of from 0 to 5V. switch in the sample-and-hold circuit, so
the LM324 (IC5), capacitor C3, transistor In this circuit, pins 1 and 2 are always the capacitor charges towards the analogue
T1 (SL100), and analogue switch IC6 made low, so the IC and the buses are input voltage. After the sampling is over,
(CD4016). This circuit samples the input always enabled. Pin 9 is made open, as the switch is opened using the same signal.
signal, i.e. it divides the waveform into a we use analogue input with 0-5V range. Then the start-of-conversion control signal
number of voltages or points and inputs Pin 7 is grounded. is given to start the conversion. The sam-
each voltage level (with a delay) to the Pin 5 (INTR): This active-low pin indi- pling time is approximately 20 µs and the
ADC for conversion into the digital format. cates the end of conversion. It is connected conversion time is approximately 100 µs.
Op-amps A3 and A4, along with a switch to pin 17 (bit D3 of I/O port 37A) of ‘D’ After the conversion is over, the 8-bit
from IC CD4016 and a 1500pF capacitor connector. (Note that this bit is inverted.) binary data for the specific voltage sample
with sampling time of 20 µs, are used as The start-of-conversion command via is available in the data bus of the ADC.
voltage followers/buffers. pin 16 of ‘D’ connector is applied to pin 3 Since the PC accepts only 4-bit data through
When the base of transistor T1 is made of the ADC0804. Since we cannot read 8- the status port (379H), the 8-bit data must
low via strobe pin 1 (bit Do of I/O port bit digital data output from ADC through be split into two 4-bit data, which are
37A) of 25-pin D connector of the parallel the 4-bit status port at a time, we divide it accepted one after another. This is done by
port, the transistor stops conducting and in two 4-bit parts and read. Hence the IC 74244, which is controlled by D0 and D7
the voltage at its collector goes high. The ADC data output is multiplexed through bits of the data port. Then the two 4-bit
high voltage at the collector of transistor two 4-bit sections of octal buffers of IC1 data are packed to get the final 8-bit data.
T1 closes the switch inside CD4016. As a (74244) with the help of output-enable sig- The default BGI directory path is set as
consequence, the analogue input signal is nals from pins 2 and 9 of ‘D’ connector to ‘c:\tc\bgi’. The sampling time is decided
applied to the capacitor, which charges pins 1 and 19 (OE1 and OE2, respectively) by the ‘for’ loop that uses the samp value.
towards the signal voltage. of IC1. The digital data output from IC1 The maximum delay produced should be
When the switch is subsequently is interfaced to the PC via pins 13 (S4), 12 greater than 20 µs, which is the maximum
opened by applying a logic-high voltage (S5), 10 (S6), and 11 (S7) of status input acquisition time of the capacitor. When the
from pin 1 of ‘D’ connector to the base of port 379H of ‘D’ connector. sample value is increased, the number of
transistor T1, the capacitor retains the volt- The circuit uses 9V and 5V regulated points on the input signal decreases and
age with a loss of about 20 mV/sec and DC supply voltages as shown in the cir- therefore the accuracy decreases. The time
this voltage is given to input pin 6 of the cuit diagram. scale may be calibrated with 50Hz sine
ADC0804 (IC3) via buffer A4 for conver- A PC printer port is an inexpensive wave as reference.
sion to the digital format. When the num- platform for implementing low-frequency Note. Mount a 25-pin D-type female
ber of sampling points in the input signal data acquisition projects. Each printer port connector on the PCB. Use 25-pin ‘D’ male-
waveform is increased, the reconstructed consists of data, status, and control port to-male connector for connecting PCB to
waveform becomes more accurate. addresses. These addresses are in sequen- computer’s female 25-pin LPT port ‘D’ con-
The ADC0804 is compatible with mi- tial order; for example, if the data port nector. It is a demo PC based oscilloscope
croprocessors. It is a 20-pin IC that works address is 0x0378, the corresponding sta- only and may not be suitable for real time
with 5V supply. It converts the analogue tus port address is 0x0379 and the application.
PROGRAM IN ‘C’ FOR PC OSCILLOSCOPE
/* PROGRAM FOR PC OSCILLOSCOPE */ may=getmaxy(); outtextxy(10,170,”TYPE ‘C’ TO CHANGE AND ‘D’ TO
/*by M.M.VIJAI ANAND B.E (E.E.E) C.I.T */ may=may-20; DEFAULT”);
#include<dos.h> outtextxy(0,may,”OSCILLOSCOPE”); c=getch();
#include<time.h> settextstyle(0,0,1); if(c==’c’)
#include<stdio.h> setcolor(BLUE); {
#include <graphics.h> outtextxy(max-200,may+2,”press ‘a’ for next sample”); outtextxy(10,200,”TYPE 1 for 1 unit = 2 volt”);
#include<string.h> outtextxy(10,240,”TYPE 2 for 1 unit = 4 volt”);
#include<stdlib.h> setcolor(BROWN); outtextxy(10,300,”TYPE 3 for user defined”);
#define data 0x0378 outtextxy(max-200,may+10,”press any key to exit”); switch(getch())
#define stat 0x0379 setcolor(GREEN); {
#define cont 0x037a settextstyle(0,0,0); case ‘1’ :
for(a=0;a<=may;a+=get) { scale=2;
void graphics(int[],int[]); //FUNCTION TO DISPLAY {line(0,a,800,a); break;
GRAPH AND WAVEFORM } }
for(a=0;a<=max;a+=get) case ‘2’ :
void settings(); //FUNCTION TO CHANGE { {scale = 4;
THE SETTINGS(TIME AND VOLT- line(a,0,a,may); break;
AGE) } }
setcolor(BROWN); case ‘3’ :
long int samp=7000; //PLEASE CHECK THESE VAL- setlinestyle(0,3,0); {
UES WHEN CONVERSION IS line(max/2,0,max/2,may); outtextxy(10,340,”TYPE VALUES FROM 1 TO 9 (mini-
// NOT PROPER(+-3000) line(0,may/2,max,may/2); mize) or m to (magnify)”);
setcolor(RED); d=getch();
float scale=1; for(a=0,c=0;a<=max;a+=50,c++) if(d==’m’)
float times=1; { {
char again=’a’; putpixel(a,may/2,BLUE); outtextxy(10,360,”TYPE a (1 unit = 0.5 volt) or b (1
int number=800; itoa((a-c*30)*times/2,str,10); unit = 0.25 volt)”);
outtextxy(a+3,may/2+3,str); switch(getch())
void main() } {
{ for(b=(may/2)-45,c=1;b>=0;b-=45,c++) case ‘a’:
int i,j,k,a[1700],b[1700],c[1700],e[1700]; //This value { {
1700 is given when we want to compress the waveform itoa((c*scale),str,10); scale=0.5;
putpixel((max/2),b,BLUE); break;
//done when we compress the time scale outtextxy((max/2)+3,b+3,str); }
long int b1; } case ‘b’:
clrscr(); for(b=(may/2)+45,c=1;b<=800;b+=45,c++) {
settings(); { scale=0.25;
while(again==’a’) itoa((c*scale),str,10); break;
{ strcat(st,str); }
for(i=0;i<number;i++) putpixel((max/2),b,BLUE); }
{ outtextxy((max/2)+2,b+2,st); }
outportb(cont,0x05^0x0b); strcpy(st,”-”); else
outportb(cont,0x04^0x0b); } { e[0]=’0';
e[i]=(inportb(stat)^0x80)&0x08; setcolor(MAGENTA); e[1]= ‘0’;
for(b1=0;b1<=samp;b1++) //sampling time e[2]=d;
is approximately 50 µsec outtextxy(max-80,may/2+30,”time(msec)”); scale=atoi(e);
{} settextstyle(0,1,0); break;
outtextxy((max/2)-10,0,”volt(s)”); }
outportb(cont,0x05^0x0b); }
outportb(cont,0x01^0x0b); setlinestyle(0,0,0); }
outportb(cont,0x05^0x0b); setcolor(RED); }
while((inportb(cont)&0x08)==0x00) //converstion time moveto(0,may/2); setcolor(BROWN);
is approximately 100 µsec for(b=0,c=0;b<=number;c+=1, b++) outtextxy(10,380,”TYPE C TO CHANGE TIME SET-
{ { TINGS”);
} if(e1[b]!=0x08) m=getch();
{ if( m==’c’)
outportb(data,0xf0); lineto(c*times,((may/2)-a1[b])); {
a[i]=(inportb(stat)^0x80)&0xf0; } cleardevice();
outportb(data,0x01); else outtextxy(10,20,”X AXIS 1 unit= 10msec CHANGE TO
b[i]=(inportb(stat)^0x80)&0xf0; { x(10msec)”);
outportb(data,0xff); lineto(c*times,((may/2)+a1[b])); outtextxy(10,40,”TYPE ‘a’ IF x IS (2 to 9) ,’b’ IF x IS (10
} } to 99) AND ‘c’ IF x IS (.5 TO .9)”);
for(i=0;i<number;i++) } switch(getch())
{ again = getch(); {
a[i]=a[i]>>4; closegraph(); case ‘a’:
c[i]=a[i]+b[i]; restorecrtmode(); outtextxy(10,60,”x value is ....”);
c[i]=c[i]*0.0196*45/scale; n[0]=getch();
} } times=atoi(n);
graphics(c,e); itoa(times,n,10);
} void settings() outtextxy(10,70,n);
{ break;
} int gd=DETECT,gm,error,max,may,b; case ‘b’:
char c,d,e[2],m,*n; outtextxy(10,60,”x value is ....”);
void graphics(int a1[],int e1[]) times=1; n[0]=getch();
{ initgraph(&gd,&gm,”c:\\tc\\bgi”); //default bgi n[1]=getch();
int gd=DETECT,gm,max,may,a,b,c,im,error,get=5; directory path times=atoi(n);
error=graphresult(); itoa(times,n,10);
char str[10],*st=”-”,d; if(error != grOk) outtextxy(10,70,n);
{ break;
clrscr(); printf(“Graphics error %s /n”,grapherrormsg(error));
initgraph(&gd,&gm,”c:\\tc\\bgi”); //use printf(“PRESS ANY KEY TO EXIT”); case ‘c’:
default bgi path getch(); outtextxy(10,60,”x value is...”);
error=graphresult(); exit(1); getch();
if(error != grOk) } n[0]=getch();
{ max=getmaxx(); times=atoi(n)*0.1;
printf(“Graphics error %s /n”,grapherrormsg(error)); / setbkcolor(LIGHTBLUE); outtextxy(10,70,”scale decremented”);
/reports error when settextstyle(1,0,0); break;
setcolor(BROWN);
//graphics is not set outtextxy(max/2-60,20,”SETTINGS”); }
printf(“PRESS ANY KEY TO EXIT”); line(0,60,800,60); number=800;
getch(); setcolor(MAGENTA); if(times<1)
exit(1); settextstyle(1,0,1); {number=number/times;
} outtextxy((max/4)-70,80,”Voltage Scale”); }
setbkcolor(LIGHTCYAN); settextstyle(0,0,0); getch();
setcolor(MAGENTA); setcolor(BROWN); }
outtextxy(10,120,”DEFAULT :”); closegraph();
settextstyle(0,0,2); outtextxy(10,120,” 1 unit = 1 volt”); restorecrtmode();
max=getmaxx(); setcolor(RED); }
