Control Magnetic Levitation System Control - SLIDING MODE CONTROL

SLIDING MODE CONTROL OF A MAGNETIC
LEVITATION SYSTEM
Introduction
Magnetic levitation systems have practical importance in many engineering systems such
as in high-speed maglev passenger trains, frictionless bearings, levitation of wind tunnel
models, vibration isolation of sensitive machinery, levitation of molten metal in induction
furnaces, and levitation of metal slabs during manufacturing. The maglev systems
can be classified as attractive systems or repulsive systems based on the source of levitation
forces. These kind of systems are usually open-loop unstable and are described by
highly nonlinear differential equations which present additional difficulties in controlling
these systems. Therefore, it is an important task to construct high-performance feedback
controllers for regulating the position of the levitated object.
more



H∞CONTROL AND SLIDING MODE CONTROL OF MAGNETIC LEVITATION SYSTEM
ABSTRACT
In this paper, H∞disturbance attenuation control and sliding mode
disturbance estimation and compensation control of a magnetic levitation
system are studied. A magnetic levitation apparatus is established, and its
model is measured. Then the system model is feedback linearized. A H∞
controller is then designed. For comparison, a sliding mode controller and a
PID controller also were designed. Some experiments were performed to
compare the performance of the H∞controller, the sliding mode controller and
the PID controller.
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High performance variable structure control of
a magnetic levitation system
Abstract- In this paper the position-tracking problem of a
voltage-controlled magnetic levitation system is considered. It is
well known that the control problem is quite complicated and
challenging duo to inherent nonlinearities associated with the
electromechanical dynamics. A sliding mode control is employed
for controlling the system. The proposed controller exhibits
satisfactory robustness in response to parameter uncertainties.
Simulation results reveal the effectiveness of the proposed robust
controller.
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ADVANCED SLIDING MODE STABILIZATION OF A LEVITATION SYSTEM
Abstract
Levitation bearings are intrinsically unstable, nonlinear and
highly uncertain systems. In this paper, we focus our attention
on sliding mode controllers which allow robust design and
more particularly on second order sliding mode control which
appears very relevant with respect to the process structure.
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Schematic Heart rate (beats) Meter with Microcontroller AT89c51

This is revised version of heart beat monitor located in this blog ob post.
http://microcontroller.circuitlab.org/2010/07/heart-beat-monitor-with-microcontroller.html
There were some question asked related to this project. So i decided to redesign the project and make some necessary changes in the algorithm to measure the heart pulses per minute.

The heart rate meter is used to measure the heart beats per minute from finger placing between the sensor. The sensor is made of simple photo resistor and LED. The pulses from the circuit are them amplified and converted into TTL logic pulses using comparator Operational Amplifier.
The analog section of the project is same and taken as such from the last post on this project. Student can take the circuit diagram from that post, if it is not clear. However the LCD connection to Microcontroller are changed in this post. As describe earlier this post is written in the response of student questions so the hardware is slightly changed. If you are familiar of electronics and lcd PIN connection, then you will notice there are not major changes. for LCD details like PIN connection and interfacing with microcontroller, you can just read some related post in this blog. Sufficient material is uploaded for the interfacing of LCD with microcontroller.
In this project, we have used one line 16 character LCD, but any other similar LCD can be connected.
Circuit diagram of the heart rate monitor is shown below.

The code is written in keil C51 compiler. The c code listing for heart rate (beat) monitor is given below.

#include // plz ad the reg51 . h file
#include // plz ad the string . h file

#define lcdport P2 // chnage it for ur hardware
sbit rw = P3^7; // LCD connection may be different
sbit rs=P3^6; // LCD interface with microcontroller
sbit en=P3^5; // Enable pin of LCD
unsigned char sec,sec100;
unsigned int bt,tick,r,bpm;
void lcdinit();
void lcdcmd(unsigned char);
void lcddata(unsigned char);
void send_string(unsigned char *s);
void msdelay(unsigned int);

void extrint (void) interrupt 0 // external Interrupt to detect the heart pulse
{
bt=tick; // number of ticks are picked
tick=0; // reset for next counting
}
void timer0 (void) interrupt 1 using 1 // Timer 0 for one second time
{
TH0 = 0xdc; //The value is taken for Ssc/100 at crystal 11.0592MHz
sec100++; // It is incremented every Ssc/100 at crystal 11.0592MHz
tick++; // This variable counts the time period of incoming pulse in Sec/100
if(tick>=3500){tick=0;} // tick are limited to less trhan 255 for valid calculation
if(sec100>=100) // 1 sec = sec100 * 100
{
sec++;
sec100=0;
}
}

void main()
{
P0=0xff;
P1=0xff;
P2=0xff;
P3=0xff;
rw=0;
EA = 1;
TMOD = 0x21;
IT0 = 1;
EX0 = 1;
ET0 = 1;
TR0 = 1;

msdelay(1000);
lcdinit();
msdelay(1000);
send_string("Heart beat ");
msdelay(1500);

msdelay(500);

//delay(15000);
bpm=0;bt=0;

while(1)
{

if(sec>=1)
{
sec=0;
/*
The sampling time is fixed 1 sec.
A variable "tick" is incremented with one tick per 100mSc in the timer 0 interrupt routine.
Each on occurring of external interrupt the value in the "tick" is picked up
and it is set to zero for recounting.
The process continues till next external interrupt.
Formula for calculating beats per minutes is

as tick is the time period in Sec/100. so extract the frequency of pulses at external interrupt
Frequency = (1/tick)* 100 i.e pulses /sec
Then
bpm = frequency * 60 for one minutes i.e pulses per minute
in short we can do it as
bpm = 6000/ bt

*/
lcdcmd(0x02);
if(bt>=7){
bpm = 6000/bt; // for valid output bt is limited so that it should be greater than 6
msdelay(500);
send_string("Pulse. ");
lcddata((bpm/100)+0x30);
r=bpm%100;
lcddata((r/10)+0x30);
lcddata((r%10)+0x30);
send_string(" bpm ");
}
else {
send_string("out of range");} // otherwise bpm will be shown zero, if limit does not fit for your project you can change it.
}
}
}
void lcdinit()
{
msdelay(100);
lcdcmd(0x01);
msdelay(500);
lcdcmd(0x38);
msdelay(500);
lcdcmd(0x38);
msdelay(500);
lcdcmd(0x38);
msdelay(500);
lcdcmd(0x06);
msdelay(500);
lcdcmd(0x0c);
msdelay(500);
lcdcmd(0x03);
msdelay(500);
msdelay(500);
}
void lcdcmd(unsigned char value)
{
rs=0;
lcdport=value;
msdelay(100);
en=1;
msdelay(100);
en=0;
msdelay(100);
rs=1;
}
void lcddata(unsigned char value)
{
rs=1;
lcdport=value;
msdelay(10);
en=1;
msdelay(100);
en=0;
rs=0;
}
void msdelay(unsigned int i)
{
//unsigned int i;
while(i --);
}
void send_string(unsigned char *s)
{
unsigned char l,i;
l = strlen(s); // get the length of string
for(i=1;i<=l;i++)
{
lcddata(*s); // write every char one by one
s++;
}
}

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Control Magnetic Levitation System Control

Design of a Robust Controller for a Magnetic Levitation System
Abstract
A Magnetic Levitation System (Maglev) is considered as a good test-bed for the design and analysis of control systems since it is a nonlinear unstable plant with practical uses in high-speed transportation and magnetic bearings. The objective of this project is to design a robust controller and implement it on a test-bed to help students learn the robust control design. In this project a robust controller for a maglev system is designed, using H-infinity optimization [3]. Complete mathematical models of the electrical, mechanical and magnetic systems are also developed. The design and simulations are performed under a Matlab/Simulink platform. Wincon control software of Quanser Inc. [7] is used to establish the link between the Matlab/Simulink models and the actual magnetic levitation system.
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Design and Implementation of a Controller for a
Magnetic Levitation System
Abstract
This paper reports on the design of a controller for keeping a steel ball suspended in the air. In
the ideal situation, the magnetic force produced by current from an electromagnet will counteract the weight of the steel ball. Nevertheless, the fixed electromagnetic force is very sensitive, and there is noise that creates acceleration forces on the steel ball, causing the ball to move into the unbalanced region. The main function of this controller is to maintain the balance between the magnetic force and the ball’s weight. According to the analytical method, the mathematical models of this magnetic levitation system were established with the goal of designing the control system. System linearization and phaselead compensation were employed to design the controller of this unstable nonlinear system. The algorithm
proposed in this paper provides a robust closed-loop magnetic levitation system which can stabilize the system over a large range of variations of the suspended mass. The design methods of this system are presented in this paper. And lastly, the hardware is implemented for a scientific demonstration.
more



PREDICTIVE CONTROL OF A MAGNETIC LEVITATION SYSTEM WITH EXPLICIT TREATMENT OF OPERATIONAL CONSTRAINTS
Abstract.
This paper concerns the application of a predictive control methodology to the stabilization and referencefollowing operation of a magnetic levitation process. From a control engineering point of view, the problem is challenging owing to the nonlinear and unstable nature of the plant, the required positioning accuracy and the operational restrictions on the manipulated and controlled variables during transients.


The formulation employed in this work is based on a linear prediction model obtained by linearizing the plant dynamics around the center of the working range of the position sensor.
Offset-free tracking is achieved by augmenting the cost function with a term associated to the integral of the tracking error. Operational constraints on the input (current in the electromagnet coil) and output (width of the air gap between the electromagnet core and the suspended object) of the process are enforced in the optimization process. The optimal control sequence is implemented in a receding-horizon strategy, in which the optimization is repeated at every sampling instant, by taking into account the new sensor readings. The design and validation of the predictive control loop are carried out
by using physical parameters from a real magnetic levitation process. The results obtained by simulation show that the explicit treatment of operational constraints, especially those related to the input variation rate, is fundamental to an appropriate control of the system.
more




MAGNETIC LEVITATION SYSTEM
IN CONTROL ENGINEERING EDUCATION
Abstract.
This paper deals with the magnetic levitation control system of a metallic
sphere, which is an interesting and visually impressive equipment for demonstrating
many intricate problems. In order to stimulate future research, after short description
of the system operation in analogue and digital mode, several open problems in areas
of electrical and control engineering are offered. Also, the paper presents some initial
outcomes in creating a laboratory environment for remote monitoring of the magnetic
levitation equipment.
more







Modeling and Control of a Magnetic Levitation System
ABSTRACT
Magnetic levitation technology has been receiving increasing attention
because it helps eliminate frictional losses due to mechanical contact. Some
engineering applications include high-speed maglev trains, magnetic bearings and
high-precision platforms. The objectives of this project are to model and control a
laboratory-scale magnetic levitation system. The control algorithm is
implemented using assembly language on Intel 8051 microprocessor to levitate
and stabilize a spherical steel ball at a desired vertical position.
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Inverse Model Based Adaptive Control of Magnetic Levitation System
ABSTRACT
This paper presents, an adaptive finite impulse response
(FIR) filter based controller used for the tracking
of a ferric ball under the influence of magnetic
force. The adaptive filer is designed online as approximate
inverse system. To stabilize the open-loop unstable
and highly nonlinear magnetic levitation system,
PID controller is designed using polynomial approach.
To improve the stability, an adaptive FIR filter
is added along side the PID controller while the
use of the proposed controller has improved tracking.
Since adaptive FIR filters are inherently stable so the
controller remains stable. Experimental results are included
to highlight the excellent position tracking performance.


AFIR addition to improve the stability

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Schematic Better Kitchen Timer, PIC16F877

The project aim is to build a better kitchen timer. It has four independent count-up and count-down timers, display of current value of user-selected timer, start/stop and digit entry controls, and alarm buzzer.

The project uses PIC16F877 as main processor. The output part consist of single 7-segment LED display and piezo speaker, while the input part consist of 10-position “BCD” rotary switch, two pushbuttons for set and start/stop, two 8-position DIP switch for timer selection. The project software written in PIC assembly and compile it using gpasm. To write the PIC, it uses XWisp and WLoader.

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Schematic Software Freuency meter and pulse width measurement

Software Freuency meter and pulse width measurement.
#include //please includes at89x52 . h
#include //please include intrins . h for _nop_() instructions
#include //please include stdio . h
/********* Function Prototype Declarations********************/
void init_serial_port(void);
void lcd_start_messeges(void);
void init_timer(void);
void init_interrupts(void);
void serial_start_messeges(void);
void waitms(unsigned int );
void waitUS (unsigned char );
void init_lcd(void);
void clearlcd(void);
void putcharlcd(unsigned char);
void putstringlcd(unsigned char *);
void print_str_lcd(unsigned char *);
void write_lcd(unsigned char ) ;
void positioncursor(unsigned char);
/******** END of Declarations*********************************/
sbit rs_lcd = P3^6; // Register Select LCD, 1= Data, 0 = Instruction
sbit en_lcd = P3^7; // Enable LCD H->L enable
sbit rw = P2^2;
sbit osc = P0^1;
sbit heart_beat = P0^0;
sfr16 TIMER2=0xCC; sfr16 RCAP2=0xCA;sfr16 DP=0x82;
/******* END of Definitions ******************************/
/************ Text Messeges ********************************/
unsigned char code msg0[]= "microcontroller.circuitlab.org ";
unsigned char code msg1[]= " At89c52 $";
unsigned char code msg2[]= " Frequency meter $";
unsigned char code msg3[]= " Microcontroller PROJECT $";
unsigned char code msg4[]= " Engineering students$";
unsigned char code msg5[]= "Frequncy Measurement$";
unsigned char code msg6[]= "Cum Digital Clock $";
unsigned char code msg7[]= " Heart beats $";
unsigned char code msg8[]= " are counted$ ";
unsigned char code msg9[]= " which are then $ ";
unsigned char code msg10[]=" displayed on LCD$";
unsigned char code msg11[]=" and as well as$";
unsigned char code msg12[]=" sent to PC$";
unsigned char code msg13[]=" Through Serial port$";
unsigned char code msg14[]=" RS-232$";
unsigned char code msg15[]="Freq=$";
unsigned char code msg16[]="P.W=$";
unsigned char buff [10]; //define 10 byte buffer
unsigned char n; //sprintf return variable
/* end of text messeges block */
// global variables declarations
unsigned char beat1,beat2; //counter for heart beat
unsigned int frequency,pw;
bit count_flag=0, disp_time=1, serial=0, time_incorrect=1;
struct timestruct
{
unsigned int sec,minute,hour;
}time;
unsigned char t2count=20;
/******** Start of Main*****************************/
void main (void)
{
rw = 0 ;//LCD is always Written and never read;
waitms(20); //wait for lcd to get ready
init_serial_port();
serial_start_messeges();
init_lcd();
lcd_start_messeges();
init_timer();
init_interrupts();
RI=1;
while (1)
{
if(disp_time)
{
bit temp_flag; temp_flag=ES; ES=0;
n=sprintf(buff,"%02d:%02d:%02d",time.hour,time.minute,time.sec);
//save time to buffer;
disp_time=0;// clear flag
positioncursor(0x00);
putstringlcd(msg13);// Clear the First LCD Line
positioncursor(0x05);
print_str_lcd(buff);
putchar(0x0D);
puts(buff);
/* time updated*/
if(count_flag)
{
positioncursor(0x40);
putstringlcd(msg13); // Clear LCD Second Line
count_flag=0;
n = sprintf (buff, "%03d",frequency); //conversion to buff
positioncursor(0x40); //Second line
putstringlcd(msg15);
print_str_lcd(buff); //print frequency at second line
putstringlcd("KHz$");
positioncursor(0x4A); //2nd line
putstringlcd(msg16);
puts(msg15);
puts(buff);
n = sprintf (buff, "%05d",pw);
print_str_lcd(buff);
putstringlcd("uSec$");
puts(msg16);
puts(buff);
// printing complete,
/************************************************/
EX0=1; //enable interrupt for further measurements
}
ES=temp_flag;
}
if(beat1++==250){
if(beat2++==100){
beat1=beat2=0;
heart_beat = ~heart_beat;
}}}}
void timer2(void) interrupt 5
{
//every 50 msec
t2count--;
TF2=0;
if(t2count==0){
t2count=20; // 20x50=1000ms=1 Sec
disp_time=1;// update the display on return from interrupt
time.sec=time.sec+1; // increment second
if(time.sec>=60){
time.sec=0; //if sec=60, increment minutes
time.minute=time.minute+1;
if(time.minute>=60){
time.minute=0; time.hour=time.hour+1;
if(time.hour>=13){
time.hour=0;
}}}}}
void exter_intr_0(void) interrupt 0
{
unsigned char temp=TMOD;
TR1=TF1=TF0=0; //clear timer flags
EX0=0; //Disable further interrupts for one sec
serial=0; //timer1 is not available for baud rate generation now
DP=-1000; //1msec delay
TMOD=0x15; //timer 0,1 both 16 bit mode, timer0 as counter
TH1=DPH; TL1=DPL; //load timer 1 to create 1 sec delay
TH0=TL0=0;
TR1=TR0=1; //start timers
while(!TF1); // wait for 1msec
TR0=TR1=TF1=0; // Stop timers
DPL=TL0; DPH=TH0; frequency=DP;
/************ Frequency measured, now measure pulse width*****/
TMOD=0x09; //timer 0,1 both 16 bit mode, timer0 as counter
TH0=TL0=TF0=0;
while(!T0); //wait for pulse to go high
TR0=1; //start timer0
while(T0); // wait for pulse to go low
TR0=0; //stop timer
DPL=TL0; DPH=TH0; pw=DP; //save the timer count to pulse width
/*******************End of measurement*****************************/
TMOD=temp;
TH1=TL1=-26; // for 1200 bps, but it is doubled by setting SMOD bit
TR1=1; // spare timer to generate baud rate
count_flag=1; // display frequency and pulse width on return from interrupt.
}
void print_str_lcd(unsigned char *d)
{
while (n>0)
{
write_lcd(*d);
d++;
n--;
}
}
void clearlcd(void)
{
rs_lcd =0;
write_lcd(0x01);
rs_lcd =1;
}
void positioncursor(unsigned char c )
{
rs_lcd = 0;
write_lcd(0x80 | c);// 1xxx xxxx set address of cursor
waitms(50);
rs_lcd =1;
}
void putstringlcd(unsigned char *d)
{
while(!(*d == '$'))
{
write_lcd(*d);
d++;
}
}
void write_lcd(unsigned char a)
{
P1 = a;
waitUS(250);
en_lcd = 0;
waitUS(250);
waitUS(250);
waitUS(250);
waitUS(250);
en_lcd = 1;
}
void waitUS(unsigned char a)
{
while(--a != 0); /* wait = a * 2 + 5 usec @ 12 MHz*/
}
void waitms(unsigned int a)
{
while (--a !=0)
{
waitUS(247); //500us delay
waitUS(247);
waitUS(247); //500us delay
waitUS(247);
}
}
void lcd_start_messeges(void)
{
positioncursor(0x00); //first line
putstringlcd(msg1);
positioncursor(0x40); //2nd line
putstringlcd(msg2);
waitms(800);
clearlcd();
positioncursor(0x00); //first line
putstringlcd(msg3);
positioncursor(0x40); //2nd line
putstringlcd(msg4);
waitms(800);
clearlcd();
positioncursor(0x00); //first line
putstringlcd(msg5);
positioncursor(0x40); //2nd line
putstringlcd(msg6);
waitms(800);
clearlcd();
positioncursor(0x00); //first line
putstringlcd(msg7);
waitms(800);
clearlcd();
positioncursor(0x00); //first line
putstringlcd(msg8);
positioncursor(0x40); //2nd line
putstringlcd(msg9);
waitms(800);
clearlcd();
positioncursor(0x00); //first line
putstringlcd(msg10);
positioncursor(0x40); //2nd line
putstringlcd(msg11);
waitms(800);
clearlcd();
positioncursor(0x00); //first line
putstringlcd(msg12);
positioncursor(0x40); //2nd line
putstringlcd(msg13);
waitms(800);
}
void init_serial_port(void)
{
SCON = 0x50; /* SCON: mode 1, 8-bit UART, enable rcvr */
PCON |= 0x80; /* set SMOD = 1 for double buad rate */
TMOD |= 0x20; /* timer 1 autoreload mode */
TH1 = -26; /* TH1: -26reload value for 1200 baud @ 12MHz */
TR1 = 1; /* TR1: timer 1 run */
TI = 1; /* TI: set TI to send first char of UART */
}
void init_timer(void)
{
TH2=0x0FF;
TL2=0x00;
T2CON=0x00; // Timer2 auto reload mode
RCAP2=-50000; // for debugging only , actual value =-50000;
TR2=1; // start timer2 to start generating 50ms delays
}
void init_interrupts(void)
{
IE=0xA1; // enable globle, timer2 interrupt, external0 interrupt
PT2=1; //set priority for timer2
ES=1; //Enable serial interrupt
TCON |= 0x05; // low edge triggered for external 0 and external 1 int
}
void init_lcd(void)
{
//LCD module
// D0-D7 -> P1
// RS -> P2.0
// EN -> P2.1
rs_lcd = 0 ; //for cmd
waitms(500);
write_lcd(0x38); //Function Set 0011 1000
waitms(100);
write_lcd(0x38); //Function Set 0011 1000
waitms(100);
write_lcd(0x38); //Function Set 0011 1000
waitms(100);
write_lcd(0x0C); //display off/ON No Cursor No Blinking at cursor
waitms(100);
write_lcd(0x01); //clear Display
waitms(100);
write_lcd(0x06); //Entry Mode Set
rs_lcd = 1 ; // for data
}
void serial_start_messeges(void)
{
bit temp_flag; temp_flag=ES; ES=0;
puts(msg0);
puts (msg2);
puts (msg3);
puts (msg4);
puts (msg5);
puts (msg6);
puts (msg7);
puts (msg8);
puts (msg9);
puts (msg10);
puts (msg11);
puts (msg12);
puts (msg13);
ES=temp_flag;
}
void serial_recieve(void) interrupt 4
{
if(RI){RI= 0;
if(SBUF == 'i'| SBUF == 'I'){puts("Please Enter the Current Time\n");
scanf("%d %d %d",&time.hour,&time.minute,&time.sec);
while(time.hour>12)
time.hour-=12;
ES=0; //once time has been entered the serial interrupt should be disabled
}}}

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Schematic Circuit diagram of Frequency Counter and Pulse Width Measurement System, Cum Digital Clock

As we discussed in previous pages the functionality of each individual part of the project of Frequency Counter and Pulse Width Measurement System, Cum Digital Clock. Now it is time to discuss the actual circuit diagram of the project based on microcontroller AT89c52.

So here is the Circuit diagram of Frequency Counter and Pulse Width Measurement System, Cum Digital Clock. Introduction of the project is given here.

First of all in this post we are going to list the main components of the pulse width measuring project based on the microcontroller 8052.

List of Componenets of the Microcontroller Project is as follows:

1. The heart of the project is microcontroller 8052.
The microcontroller 8052 is discussed here. The 8052 microcontroller is used for local processing and controling of LCD. The measured resuts are shown on the local liquid crystal display. The microcontroller 8052 is also used to get the incoming pulses and process these, extract useful information from the pulse trains, like frequency, pulse width, duty time, etc. and display these on the LCD. as well as the results are then sent to PC through RS-232.
2. Liquid Crystal Display LCD.
This the main display we used in this project. The project measures the paramters from field or from the incoming pulse trains and then after calculation these paramters are then shown on LCD. The incormation about the LCD is here.
3. Max-232.
As discussed earlier that the measured parameters are displayed on local LCD, as well as these parameters are sent to PC. So there was some special circuitry involved in this phas which is based on MAX-232 IC. The RS-232 can also be used here. The time is also sent to microcontroller from PC. So this is two serial communication though MAX-232. The information about the RS-232 is presented here.

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Schematic MAX 232 Interfacing with Microcontroller 8052

MAX232 is used to interface the microcontroller to standard RS-232 port of personal computer. It is a signal level converter necessary for conversion between TTL and RS-232 standards.

The MAX232 requires 5 external 10uF capacitors. These are used by the internal charge pump to create +10 volts and -10 volts. The MAX232 includes 2 receivers and 2 transmitters so two serial ports can be used with a single chip. We will only use one transmitter for this project. The only connection that must be made to the 8052 is one jumper from pin 3 of the 8052 to pin 11 of the MAX232.
The circuit diagram shown below illustrates the connection of RS232 with microcontroller and serial port DB9 connector.Data is transmitted and received on pins 2 and 3 respectively. Data Set Ready (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD) indicates that a good carrier is being received from the remote modem.

Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (Clear To Send - from the Data set) are used to control. In most Asynchronous situations, RTS and CTS are constantly on throughout the communication session. However where the DTE is connected to a multipoint line, RTS is used to turn carrier on the modem on and off. On a multipoint line, it's imperative that only one station is transmitting at a time (because they share the return phone pair). When a station wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds for carrier to stabilize, and then raises CTS. The DTE transmits when it sees CTS up. When the station has finished its transmission, it drops RTS and the modem drops CTS and carrier together.

Clock signals (pins 15, 17, & 24) are only used for synchronous communications. The modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate.

Note: Transmit and receive leads (2 or 3) can be reversed depending on the use of the equipment - DCE Data Communications Equipment or a DTE Data Terminal Equipment.

CTS Clear To Send
DCD Data Carrier Detected (Tone from a modem)
DCE Data Communications Equipment eg. modem
DSR Data Set Ready
DSRS Data Signal Rate Selector (Not commonly used)
DTE Data Terminal Equipment eg. computer, printer
DTR Data Terminal Ready
FG Frame Ground (screen or chassis)
NC No Connection
RCk Receiver (external) Clock input
RI Ring Indicator (ringing tone detected)
RTS Request To Send
RxD Received Data
SG Signal Ground
SCTS Secondary Clear To Send
SDCD Secondary Data Carrier Detected (Tone from a modem)
SRTS Secondary Request To Send
SRxD Secondary Received Data
STxD Secondary Transmitted Data
TxD Transmitted Data
The RS232 connector was originally developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel.

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