RS422

I'm having a hard time finding the logic voltage levels for RS422.

Anyone know if a high voltage in RS422 is a one or zero?

 

I don't know the exact details, but it is a differential method, not
single-ended one like RS-232. You can't look at it quite the same
way.

Jon

 

Differential output -- A is non-inverting, B is inverting. The nominal
signal levels are +5V and 0V.

Cheers
Chris

 

What exactly does that mean to say the nominal signal levels are +5V
and 0V?

Isn't one signal always the negative of the other? So, if one is at
3v, the other is at -3v?

 

No, when one is +5 the other should ideally be zero.
When it is at zero, the other should ideally be at +5.

They should, ideally cross each other at 2.5 volts, though
this happens so fast, it is not really important. When one
signal is more positive than the other, that is one logic
state. When it is more negative than the other, that is the
zero state. The exact voltage of either is unimportant,
since all that matters is one relative to the other.

 

The output will go from 0v to 5v.
To determine your value of high and low, you would have to compare your A
and B outputs
The relationship of the levels on the A and B output determines your logic
level
If A is negative of B you have a mark(off), or binary low.
If A is positive of B you have a space(on), or binary high

 

No they are the 'logical inverse' of each other like any digital signal. Not
'inverted' !

Graham

 

And if your logic states stay stable for a long period of time a voltmeter
works great. If on the other hand your logic is a train of pulses, a 2
channel oscope may be better.

 

Thanks for the help.

So, if A > B, it's a one, and if A < B, it's a zero?

Also, I remember reading that the noise cancellation properties of
differential signaling were due to the currents being opposite. Hence,
I thought one was the exact negative of the other. If the voltage is
always positive though, then the current is always going in the same
direction. So, I guess currents being opposite isn't that important
for noise cancellation?

 

Depends on which line is A and which is B. ;-)

With conductors taking a similar path through electric and magnetic
fields, similar noise voltage tend to get added to both signals.
Taking the difference of the two resultant voltages subtracts these
similat noise voltages with an approxmately zero effective error.
Regardless of the amount of common voltage added to each side, the
difference between them remains essentially + or - 5 volts, so the
decoded state is unaffected.

This method has a big advantage over single ended systems (like RS 232)
that subtract the signal from the local ground voltage to determine
whether the signal is positive or negative.