I'm designing ESD protection for a circuit that will interface to a PXIe-8431/8, with it's loopbacks set to the RS-422 configuration. The NI serial hardware specification sheet indicates that the maximum common mode voltage in the RS422 configuration is +/- 7V and that the driver output is 2 to 6 V. In order to specify the breakdown voltage of the transient suppressors on the circuit I'm designing, I'd like to understand how the driver outputs relate to the PXI card's secondary ground, which I assume is the same as the shield ground for the interface cable.
1) I assume that the max common mode voltage spec of +/- 7V is simply the amount of common mode voltage that the RS422 signal will tolerate, as opposed to a built-in common mode voltage where the differential output signals might be biased above chassis return. I'm almost 100% sure of this, but am I right?
2) What is the driver output voltage with respect to the shield ground? Is a space 1 to 3 Volts above shield ground and a mark -1 to -3 Volts below shield ground? Or are the signals biased up, where a space is 6 Volts above shield and a mark is 3 Volts above shield. I think it's the former, but I have to be 100% sure.
Thanks!
