Electronic – Difference between usart and modem

digital-communicationsrs232uartwireless

This is a question of entrance exam for government telecommunications company:

A complex device used to send and receive serial data over telephone lines is

A)USART

B)Modem

C)SDLC

D)RS232

The given was USART, but according to me, it is modem that we use over telephone lines. What would be the justification to USART if it is the correct answer?

Best Answer

The modem actually sends parallel (2 or more) bits of data with trellis coding and 4, 64 to 128 level constellation of phase & amplitude modulation and NOT SERIAL data.

So USART is the correct answer.

  • USART uses logic to send serial data with an RS-232 a level shifter, then is modulated by one MODem and DEModulated by another modem in both directions.

  • the MODEM sends ANALOG signals converted to fit 56kbps into < 4kHz of audio bandwidth using up to 64 QPSK levels to modulate the data with channel equalization and single echo cancelling but contains synchronous QPSK up to 64 level ASK/PSK with no start/stop bits

So the USART sends the SERIAL DATA

Trick question due to "wording" because the MODEM is most complex with analog modulation but the USART communicates with logical serial data.

(opinions)

The USART is trivial by comparison which does 16x clock sync on start edge and can be easily monitored. (once upon a time) I designed a UART and I know it was simple for me at the time before USARTs even existed. circa mid 70's

EDIT ( is it too late?)

  • From the telephone line perspective the MODEM sends(& receives) the data.
  • From the User perspective, the USART sends the data (& receives) the data.
    • all three answers a,b,d are need to make it work
  • Since each answer has two interfaces, there is ambiguity ( its bad enough we mix up Send and Receive for the same wire.
  • but the keyword "over the phone lines" applies only to the MODEM , which is also most complex of all choices, so perhaps from the POTS or PSTN perspective could be the better answer, even though the signals cannot be decoded in many cases ( would not have may not same delay equalization and handshaking cannot be done) In many ways its like scrambling ,which is why it always sounds like white noise.

I concede both a) & b) can be correct, in this shamelessly ambiguous question.

Logical Explanation

All communication channels have two (2) sides in the OSI model TO and FROM both sides and also in both directions.(Rx,Tx)

  • The modem has two ports; a serial RS 232 port and an analog PSTN port
  • The USART has a parallel port and a serial TTL port.
  • All serial ports send and receive serial data.
  • Therefore both the USART and MODEM share this complex function
    • when sending data over the phone line as they each have a serial port, albeit different types as TTL logic serial data is inverted with respect to the RS-232 bipolar data, in spite of having the same logic threshold.
    • but there was no stipulation or implied statement saying which type of serial port.
    • there was also no specific perspective who is responsible for sending the data. The Modem is just a medium used on the phone lines.
      • therefore a) & b) can be correct since there are acceptable ambiguous perspectives.
Related Topic