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Experiment to look at serial data communications

Discussion of experiment results

The terminal program sends out ASCII data in RS-232 asynchronous format. When you type out a character, the ASCII equivalent is transmitted. The characters are transmitted according to the rate at which you type them. They are not sent at fixed intervals. This type of transmission is known as asynchronous transmission. In RS-232 a ‘1’ or mark is represented by approx -12 V DC and a ‘0’or space is represented by approx +12 V DC.

The transmit pin TxD #3 normally sits at the mark voltage of -12 V DC. When data is sent, the first thing that happens is a so called ‘start bit’. This is a transition from the -12 V DC level to +12 V DC. The signal then stays at +12 V DC for a bit period. This sudden transition is used to notify the receiver that asynchronous data follows. After the start bit, then data bits are sent; according to the hex equivalent of the character to be sent. The LSB is sent first with the MSB at the end. After the MSB, a ‘stop’ bit is sent at the mark voltage of -12 V DC. This indicates to the receiver that the full character has now been sent.

Let us examine the transmission of ASCII ‘G’:

G = $47 (hex) = 1 0 0 0 1 1 1 (See Figure 8)
Bits/sec = 2400, Bit Period = 1/rate = 1/2400 = 416.7 µs

The ASCII value for ‘G’ can be found in Figure 8. Note there are different values for lower case and upper case letters. Figure 8 give 7 bit ASCII values. The transmitted waveform will consist of:
start bit(space) + mark(1=LSB) + mark(1) + mark(1) + space(0) + space(0) + space(0) + mark(1=MSB) + mark(stop)

The following waveform will be seen:

Theoretical output for ASCII character code 'G'

Figure 9: theoretical ASCII RS-232 eaveform for Letter ‘G’

Note that the theoretical waveform shown above in Figure 9 matches exactly the waveform captured on the ADC-200/20 shown in Fig 6. Note that for a bit rate of 2400 bps, the length of a bit = 417 µs.


1. Looking at Figure 8, ASCII ‘8’ = $38 = 0 1 1 1 0 0 0
The observed waveform would be: 
start bit(0) + space(0 = LSB) + space(0) + space(0) + mark(1) + mark(1) + mark(1) + space(0 = MSB) + stop(1)
This is exactly what is shown in Figure 6.
2. Since the data rate is twice as fast, the bit length would be half. Bit duration = 1/4800 bps = 208.3 µs.

Teachers’ notes

This experiment can easily be accomplished in a two-period lab. At Seneca College, our periods are 50 minutes each. The first period can be used to review the basics of RS-232 and give a demonstration of equipment set up.

An excellent reference that discusses the RS-232 and other serial data comm interfaces is: 
Telecommunications, Fourth Edition, Warren Hioki, Prentice Hall 2001. 
ISBN 0-13-020031-X. Section on RS-232 pp181-214.

Credits, comments and further info

This experiment was written by Jeremy Clark and Kyle James Mcneil at Seneca College, Canada.


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