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RS422 Help

stuart sutherland

Jul 6, 2015
12
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Jul 6, 2015
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Hi,
I am learning about serial comms and in particular RS422.

-2V to -6V = logic 1
+2V to 6V is logic 0.

What happens to data which is between -2 and 2V ?

Is this just ignored or is it random 1 and 0?
 

Nanren888

Nov 8, 2015
622
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Nov 8, 2015
Messages
622
The use of the word "data" suggests that you should first revise serial communication..
Search for tutorials on serial communication.
eg
https://learn.sparkfun.com/tutorials/serial-communication/all
.
Serial communication sends bits one after another.
After synch has been acheived, at certain times, usually evenly spaced, sometimes on a clock signal edges, the voltage on the line is sampled.
At that time, the values should fall within the voltage ranges specifiied above (for RS422) to be interpreted correctly. That is, the driving circuitry has a responsibility to drive it to a valid value at that sample time. That is, the spec says that a receiver should provide the correct response for any voltage level within the specified range.
.
I have not checked the RS422 standard for a long time, but I would guess that it is probably device-dependent what happen if the voltage, when sampled, is out of range.
Likely the device receiver would be implemented with a scmidt trigger providing a comparator-like performance but with some hysteresis.
If the standard does not specify the details of the receiver, then the device response would be device-dependent.
.
Others who have better knowledge of the standard may be able to give you a better answer.
 

AnalogKid

Jun 10, 2015
2,884
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Jun 10, 2015
Messages
2,884
The gap between -2 V and +2 V is a dead band created by the Schmitt trigger input stage of the receiver. If the signal line rests within this band, the receiver output remains in the state of the last valid input. This is a way to increase noise immunity. If the signal that was sent is below -2 V, it takes a noise pulse of at least 4 V to cause the receiver to see an incorrect input state.

ak
 
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