NI serial cards work with GND & +5V interface?

[ScottWenzel]

My company has a product where we want to use the GND & +5V interface pins of an FPGA to implement a bidirectional serial interface to the external world to be used during preship testing only.  We want to use as little additional circuitry in the product itself to implement this serial interface.

The digital designer tells me that although non-standard the GND and +5V logic levels usualy works with RS-232 hardware receivers and transmitters.  He does not know if this is true for RS-422.

Does any one know if this GND & +5V interface does work with NI RS-232 or RS-422 cards that have isolation?

[RossC]

Scott,

RS-232 signaling is between +5V and -5V minumum (driver side), so GND to +5V is not appropriate. See this link for more RS-232 info. RS-422/485 is a differential standard, with signaling between +2V to -2V minimum (driver side). GND to +5V will not work for this bus either, unfortunately. See this link for more info on RS-422/485. You could certainly implement a serial signaling system with +5V and GND, but it would not fall into the RS-232 or RS-422/485 spec.

Sincerely,
RossC

[ScottWenzel]

Thanks RossC.  

But, I guess I was not too concerned about wheather the interface technicaly meets the spec. definitions we will only be using the interface for our own internal product testing.  I am more interested in making something work reliably without having to design, build and document extraneous level shifitng circuitry if I do not have to.

Any one have personal experience or knowledge of the NI cards?

[RossC]

Scott,

I see what you are getting at now.  Well, my first instinct would be to go for RS-232 signaling by AC coupling the signal, then amplifying it with a gain of two (or more, as desired).  This could be accomplished with an in-line capacitor and an op-amp circuit, just to keep things simple.  The circuit drops the signal to +/- 2.5V, then boosts it up to +/- 5V (or whatever level you set the gain to).  I've never actually tried this technique though, so I'll let others chime in with their solutions.  Have a good day!

Sincerely,

RossC