1 - Asynchronous serial communications

Question 1

What are the key steps in transferring data from user to user?

What are the methods used to transfer the data between these steps

Answer 1

User -> Transmitter -> Transceiver -> Cable -> Transceiver -> Receiver -> User

Transmitter -> Transceiver : TTL
Transceiver -> Cable : EIA-232
Cable -> Transceiver : EIA-232
Transceiver -> Receiver : TTL

The transceivers use FIFO logic and have shift registers.
The transceivers convert logical level signals to bauds.

Question 2

Explain how the signal levels works in EIA-232

Answer 2

Signal is received

EIA-232
0 baud - negative voltage (-12V)
1 baud - positive voltage (+12V)

TTL
0 - baud<threshold
1 - baud>threshold
- inverts the signal and changes the voltage

Question 3

How do the sender and receiver know the rate of transmission?

Answer 3

They each have a shift register.
Both registers have clocks that are set to the SAME RATE.
The clock determines the duration of each baud

Question 4

What are the components of an asynchronous slot (in order) ?

Answer 4

Mark - gap between slots (TTL High level)
1 start baud (TTL low level)
8 data bits
2 stop bauds (TTL high level)

Question 5

In what scenario when can baud rate be equal to bit rate?

Answer 5

If each baud contains a bit

Question 6

what is the difference between baud and bit

Answer 6

A baud is the PHYSICAL TRANSITION per second on a cable. It represents the number of signal changes per second, this includes any errors or overheads.

A bit is the smallest unit of digital information. Represented as a binary value.

Question 7

What is the transmission order of ASCII Character H = 0x48?

Answer 7

H in binary = 0100 1000
Transmission order = 0001 0010 (reverse of binary representation)

Question 8

calculate the slot duration of a standard asynchronous slot with a 250kbaud clock

Answer 8

baud rate = 250 000 baud

duration of one baud = 1/ baud rate = 1/250 000 = 4*10e-6

slot size = 8 data bits + 2 stop bauds + 1 start baud = 11

slot duration = slot size * duration of one baud = 11 * 410e-6 = 4410e-6

Question 9

Benefits and drawbacks of asynchronous transmission

Answer 9

Benefits
- One common standard (widely used)
- Simple UART implementation as there is no clock recovery.

Drawbacks
- Low efficiency as 3/11 of the capacity is used for slot framing.
- Poor error detection, so bytes/slots may be lost.
- The rate is limited by the clock stability and cable quality.

Question 10

Benefits and drawbacks of asynchronous transmission

Answer 10

Benefits
- One common standard (widely used)
- Simple UART implementation as there is no clock recovery.

Drawbacks
- Low efficiency as 3/11 of the capacity is used for slot framing.
- Poor error detection, so bytes/slots may be lost.
- The rate is limited by the clock stability and cable quality.

Question 11

Fundamental aspects of binary communications

Answer 11

• The receiver needs sufficient voltage to differentiate between a 0 and 1 baud
• If a 0 is detected when 1 was sent, or vice versa, it is an error.

Question 12

How can a received signal be reduced?

Answer 12

Cable attenuation

Question 13

how does the EIA-232 receiver work?

Answer 13

Features
- Input impedance of 12k Ohms
- Shield not connected at receiver

Process
TTL Logic 0 - difference is between +3V and +15V.
TTL Logic 1 - difference is between -3V and -15V.

Question 14

Explain how a signal is altered during transmission and received.

Answer 14

• Signal can be perfect when sent
• cable causes attenuation. This increases with increase in distance
• Interference from other signals add to the received signal and Noise distort the waveform
• A large 24V transmit voltage swing ensures there is a signal at the end of a LONG cable

Received - Using receiver hysteresis

• The receiver uses voltage threshold hysteresis to avoid oscillation in the output when the signal varies.
• A signal must cross UPPER threshold to count as 0
  A signal must cross LOWER threshold to count as 1

Question 15

What is the difference between EIA-232 and EIA-423 Transmission?

Answer 15

• EIA-423 is an update to EIA-232 for office use
• EIA-423 has a voltage level of -6V to +6V whereas EIA-232 is from -15V to +15V.
• EIA-423 can be used over longer distances. Its Open ended cable length is increased to 1200m
• EIA-423 does not require termination, whereas EIA-232 does.
• EIA-423 is not suited to industry use as it is sensitive to noise and interference.