How to find common mode voltage in RS-422 Communication Line

[kalaianand]

Hi..

Can anyone tell me, how to estimate the common mode voltage in a RS-422 Interface Line?

For a standard RS-422 Interface, The differential voltage level is ± 6V.

The alowable common mode voltage is ±7V. What does it mean?

If There is no disturbance or transient occur in the transmission, what will be the common mode voltage?

 

[erikl]

The alowable common mode voltage range is ±7V. What does it mean?

That the differential voltage levels can be +7/-7 or +7/-5 or +5/-7 or +6/-6V .

what will be the common mode voltage?

I don't think that a common mode voltage expression makes much sense for a differential voltage signal. The std. logic levels are given by +6/-6 resp. -6/+6V.

Of course you could define its common mode voltage as the center of both differential voltages, i.e. 0V for a ±6V (or ±7V) system, or e.g. +1V for a +7/-5V system.

 

[crutschow]

A strict interpretation means the the common-mode (average voltage) can be ±7V with a ±6V differential voltage added to that, giving a maximum voltage ranging from +13V to -13V. But it may mean the the maximum average CM voltage can only be ±1V with maximum peaks of +7V to -7V.