Laboratory 2- experimentation Asynchronous data communications via null modem




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Laboratory 2- Experimentation
Asynchronous data communications

via null modem

We use the lab set-up shown in Fig. 3. Two cables connect the RS232 ports of the 2 PC’s to a lab board. The TD pin of PC1 cable is connected to point V1 of the board. The RD pin of PC2 cable is connected to point V4 of the board. The SG pins of the 2 cables are connected to a common ground of the board. A picture of the board is shown in Pic.1, where points V1 and V4 are labelled.



Fig. 3: Lab set up

Pic.1: Picture of lab board


The PC’s have a communication software called Hyperterminal (to find it: StartPrograms AccessoriesCommunications). This software is normally used to connect a machine to another machine across the PSTN via modems. Here, we use it to send data (e.g., an ASCII text file) from one PC1 to PC2 via a null modem (as if it’s through the PSTN).

In this first lab, you may need to practice with Hyperterminal by going through its various windows to set-up the parameters needed for communications.


First, choose a connection name, say “lab1”, as shown below.

Next, select the serial port (COM1, COM2, TCP/IP) as shown in the following screen. For serial communications, choose COM1 (or COM2).



Then, select the communication parameters: data bit rate (e.g., 2400 bps), data_bits per character (e.g., =7), parity bit (even, odd, none), Stop_bit=1 (can be >1), flow control = hardware, as shown on the following screen.



  1. Lab Experiment




    1. Objective: observe and decode the EIA232 signal, wire a null modem, and transmit

ASCII data between 2 PC’s via the null modem.
2.2 Apparatus: Lab board, 2 RS232 cables, 2 PC’s, oscilloscope
2.3 Procedure



  1. Measuring & decoding the data signal in the serial port




  1. Connect the TD pin of the serial port of PC1, from point V1 of the board, to the oscilloscope (and connect the SG pin to the common ground of the scope). Using the Hyperterminal communication software, set the bit rate to 300 bps, data_bits=7, parity=even. Then type character “m” repeatedly on the screen. The resulting signal is transmitted on the TD pin.




  1. Capture and observe the signal for “m” on the scope. Note that the signal runs from right to left, starting with the Start_bit pulse, the LSB bit pulse, etc., as was shown in Fig. 1-b. Zoom on one character (set of 10 pulses). On your answer sheet do the following:

  • Sketch the 10-pulse signal for “m” that you see on the scope

  • What is the polarity of the Start_bit and the Stop_bit?

  • Measure the amplitudes (in V) of the + and – pulses of the signal

  • Measure the width (in second) of the + and – pulses. How is this width related to the bit rate?

  • Decode the signal to find the ASCII code for letter “m”. Verify that this is the correct ASCII code by consulting an ASCII table.

  • Verify that the even parity bit observed is correct.

4. Repeat the steps above with bit rate=1200 bps parity=odd.




  1. Serial data transmission via null modem

1. Using a jumper cable, connect the TD pin of PC1 cable (point V1 on board) to the RD

pin of PC2 cable (point V4 on board). Make sure the SG pins of both cables are

connected to the ground. This creates a null modem.


2. On each PC, using Hyperterminal, write a text file consisting of characters

“mmmmmmm…”. Save it as file as file1.txt on PC1, and as file2.txt on PC2.




  1. Set Hyperterminal parameters as bit_rate=300 bps, data_bits=7, parity=even on both

PC’s. Use the “Transmit text file” of Hyperterminal to transmit file1.txt from PC1 to

PC2. Retry until successful transmission, i.e. when “mmmmmmm…” appears on the

screen of PC2. Fill in answer sheet.


  1. Repeat Step 3 above by changing bit_rate=1200 bps on PC2. If no successful transmission, explain (on answer sheet)

5. Rewire the connections on the board to transfer file2.txt from PC2 to PC1.





  1. Lab answer sheet


Preparation

  1. Decimal code for letter “m” is: ……………




  1. 7-bit binary ASCII code for letter “m” is: ……………….




  1. a) ASCII code with even parity: ………….

b) ASCII code with odd parity:


  1. Signal sketch for letter “m” is:


Experiment

Step A-2:

  1. Sketch the 10-pulse signal for “m” that you see on the scope




  1. What is the polarity (+-)of the Start_bit and the Stop_bit?

Start_bit:

Stop_bit:




  1. Measure the amplitudes (in V) of the + and – pulses of the signal

+ pulse:

- pulse:



  1. Measure the width (in second) of the + and – pulses. How is this width related to the bit rate?

Pulse width:

Pulse width is related to bit rate as:




  1. Decode the signal to find the ASCII code for letter “m”.

Decoded ASCII code for “m”:

Decimal value of code:

Verified by ASCII table?:



  1. What is the polarity of the ( even ) parity bit?

Polarity:

Why is it even parity?:


Step A3:


  1. Measure the width (in second) of the + and – pulses. Does it verify the bit rate?

Pulse width:

Width verifies bit rate?:




  1. What is the polarity of the (odd ) parity bit?

Polarity:

Why is it odd parity?:



Step B3:

Is the data transfer successful?:

If yes, capture screen of PC2 and insert below.
Step B4:

If data transfer unsuccessful, why?


Step B-5:

Is the data transfer successful?:



If yes, capture screen of PC1 and insert below.
4. References
[1]. http://www.asciitable.com


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