RS-422 Interface...

[Hargen]

Hi!

I am now desinging an interface involving RS-422 line drivers and receivers
such as 26C31 and 26C32. I found the receiver from Harris has wider range
for the differential input voltage threshold (-400mV to 400mV) than that of
National. Does that mean that I have to make my differential input at least
higher than 400mV for logic "1" and lower than -400mV for logic "0"?
I also want to know about the input voltage hysteresis. What relation does
it have with the input voltage threshold?

Thanks in advance.
Hargen

 

[Stefan Heinzmann]

Hargen wrote:

I am now desinging an interface involving RS-422 line drivers and receivers
such as 26C31 and 26C32. I found the receiver from Harris has wider range
for the differential input voltage threshold (-400mV to 400mV) than that of
National. Does that mean that I have to make my differential input at least
higher than 400mV for logic "1" and lower than -400mV for logic "0"?
I also want to know about the input voltage hysteresis. What relation does
it have with the input voltage threshold?

Yes, you're only guaranteed to reliably recognize a logic 1 when the +
input is at least 400mV above the - input. Similar for the logic 0. In
practice less than that will suffice, but that's not guaranteed.

The hysteresis says that the receiver output switches from 0 to 1 at a
different input differential voltage than on a transition from 1 to 0.
This prevents output chattering when you have a slow input transition
and noisy signals. So in fact you have two separate thresholds, but both
lie within the range given in the datasheet.

--
Cheers
Stefan