ISRO-2013-37

Question

The voltage ranges for a logic high and a logic low in RS-$232$ C standard is

• A. Low is $0.0 V$ to $1.8 V$,  High is $2.0 V$ to $5.0 V$
• B. Low is $-15.0 V$ to $-3.0 V$,  High is $3.0 V$ to $15.0 V$
• C. Low is $3.0 V$ to $15.0 V$,  High is $-3.0 V$ to $-15.0 V$
• D. Low is $2.0 V$ to $5.0 V$,  High is $0.0 V$ to $1.8 V$

Answer 1

ans is (c)

The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines. Valid signals are either in the range of +3 to +15 volts or the range −3 to −15 volts

https://en.wikipedia.org/wiki/RS-232

Comments

It should be B

because -15 to -3 is less than +3 to +15 ?

Answer 2

http://www.lammertbies.nl/comm/info/RS-232_specs.html

Option B

Comments

I think answer is Option C

Check in Wikipedia

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This link seems to support the option c.  Under the heading 'Voltages',

"A high bit, or mark state is identified by a negative voltage and a low bit or space state uses a positive value. This might be a bit confusing, because in normal circumstances, high logical values are defined by high voltages also."