8/11/2019 Expt 1 the Serial Data Interface

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Experiment no. 1The Serial Data Interface

Joseph Kerby J. Nava

Department of Electronics Engineering

Faculty of Engineering, University of Santo Tomas

Sampaloc, Manila, Philippines

 josephkerbynava@yahoo.com.ph

 Abstract— The experiment tackles about the introduction of

serial communication ports to the students and what the interfacesand how these interfaces are used for serial data transfer betweencomputers. This includes the study of the physical, electrical,mechanical and functional characteristics of the cable used that isthe EIA-232-A also known as RS-232. 

 Index Terms—Serial Interface, Data Terminal Equipment

(DTE), Data Communications Equipment (DCE), Breakout box,Registered Standard (RS), Gender changers and converters.

I. I NTRODUCTION 

Serial interface serves as a communication interface between two data transmitting digital systems  [1]. The data is

transmitted as a series of voltage pulses in a particular wire. The

serial cable used in the experiment is EIA-232-A or formerly

known as RS-232. EIA-232-A has two types depending on the

number of pins involved: the DB25 and the DB9 that has 25 pins

and 9 pins respectively. Logic 1 is a negative voltage termed as

“mark” and logic 0 is a positive voltage termed as “zero”. Forcontrol signals, the positive voltage is the active state and the

negative voltage is the inactive state [2]. A breakout box is adevice that splits a multiple-cable line into several compound

connectors. It is designed to streamline the wiring system of

electronic devices and to simplify troubleshooting problems [3].

II. I NTERPRETATION OF DATA AND RESULTS 

 A. Cable Wire Mapping

The continuity tester of the digital mutimeter is used to

determine the pin configuration of a DB25 to DB9 serial datacable. The pinhole in the DB9 side of the connector are checkedas to what is the corresponding pinhole of the said DB9 end in

the DB25 side of the connector. The same applies with the DB25male to DB9 female connector, but this time with the use of

gender changer or a gender converter.

TABLE I. CABLE WIRE MAP 

 DCE (DB25

 Female) Pin no.

 DCE (DB9

 Female) Pin no.

 DCE (DB25

 Male) Pin no.

 DCE (DB9

 Female) Pin no.

1 - 8 1

2 - 3 2

3 2 2 34 7 20 4

5 8 7 5

6 6 6 6

7 5 4 7

8 1 5 8

9 - 22 9

10 - - -

11 - - -

12 - - -

13 - - -

14 - - -

15 - - -

16 - - -

17 - - -

18 - - -

19 - - -20 4 - -

21 - - -

22 9 - -

23 - - -

24 - - -

25 - - -

 B. Voltage Measurement Reading

With the use of a breakout box to help ease in the voltage

measurement, voltage readings are written in the table below

This was made by connecting the ground node of the voltmeter

to pin 7 of the breakout box then measuring the voltages in each

of the pins on the DTE side. Same procedure was done with the

DCE side of the breakout box.

TABLE II. VOLTAGE MEASUREMENTS 

 Pins DTE

Side

Voltage (V) Pins DCE

Side

Voltage (V)

1 -0.045 1 -0.001

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2 -9.5 2 -9.42

3 0.068 3 -0.07

4 -9.5 4 -9.42

5 0.064 5 -0.07

6 0.067 6 -0.07

7 0 7 0

8 -0.068 8 -0.07

9 -0.024 9 -0.01

10 -0.0010 10 -0.01

11 -0.0014 11 -0.01

12 -0.009 12 -0.01

13 -0.009 13 -0.01

14 -0.012 14 -0.01

15 -0.011 15 -0.02

16 -0.011 16 -0.02

17 -0.013 17 -0.02

18 -0.014 18 -0.01

19 -0.016 19 -0.02

20 -9.5 20 -9.9

21 -0.008 21 -0.02

22 -0.001 22 -0.07

23 -0.007 23 -0.01

24 -0.001 24 -0.01

25 -0.001 25 -0.02

C. 

 Breakout Box LED Status Checking

LED status was obtained in this part of the experiment. It

was done by observing which pin/s in the DTE and DCE part of

the breakout box are lit.

TABLE III. BREAKOUT BOX STATUS 

DTE DCE

 Pins Green LED

 Red LED

 Pins Green LED

 Red LED

1 1

2 / 2 /

3 3

4 / 4 /

5 5

6 6

7 78 8

9 9

10 10

11 11

12 12

13 13

14 14

15 15

16 16

17 17

18 18

19 19

20 / 20 /

21 21

22 22

23 23

24 24

25 25

III. 

CONCLUSION 

The experiment enables the students to familiarize with the

electrical, mechanical, functional and procedural characteristics

of the serial data interface. It was known that the 9 pins,

specifically pins 1 to 8, pin 20 and pin 22, of the DB25 cable is

already enough to make a serial or series transmission of data.

Gender changers and converters are used to compensate for the

 ports and convert a DB25 to DB9 and vice versa. DB25 cable is

typically used for modem handshaking.

Lighted green LED on the breakout box means the pin is

active and red LED means it is on standby.

For simplification, the table below shows the pin cable

layout of EIA-232-A (DTE side) DB25 and DB9 and their

respective uses in the data transmission [4].

 DB9 Pin DB25 Pin SIGNAL DESCRIPTION

1 8 DCD Data Carrier Detect

2 3 RXD Receive Data

3 2 TXD Transmit Data

4 20 DTR Data Terminal Ready

5 7 GND Ground

6 6 DSR Data Set Ready

7 4 RTS Request To Send

8 5 CTS Clear To Send

9 22 RI Ring Indicator

R EFERENCES 

[1]  M. Lemmon, “What is a serial interface?”, 2009http://www3.nd.edu/~lemmon/courses/ee224/web-manual/web-

manual/lab9/node4.html  

[2]  ARC Electronics, http://www.arcelect.com/rs232.htm  

[3] 

Technopedia,http://www.techopedia.com/definition/728/breakout-box-bob.  

[4]  Electrodroid Android App.