The issue quickly explained
I would like to switch between two fully opto-isolated Tx differential signals (RS422). How can I achieve that if the grounds are different for the signals and the analog switch ?
The issue explained
This question comes after another one but a reminder is always good. To be specific : I have 8 opto-isolated RS422 Ports coming out from a PC card. This is a full opto-isolation : every port I have is opto-isolated from each other and opto-isolated from the PC ground. Consequently I have 9 separated ground : 1 for the PC et 8 for every port.
I would like to multiplex the TX (transmit) differential signals from two of those ports, let’s say P1 and P2 but I don’t know what to use since I have to keep the ground isolation intact and separated.
The picture above depicts the basis of what I am trying to achieve. The signals Tx+ and Tx- at the top-left come from the PORT1 of my PC card, this PORT1 is isolated from the PC Ground and from the other PORTx. At the bottom-left are the Tx signals from the PORT2 of the PC Card. It is also isolated from the PC Ground and the other PORTx. Thus, PORT1 and PORT2 are isolated from each other.
I would like to have only one signal at the output of my custom card, signal I could choose between PORT1 and PORT2 via a command signal on a MUX.
The Analog Switch ?
First of all I thought of using a simple analog switch 2 x SPDT (or 2 x (2:1) ) such as the TS3A27518E from Texas Instrument or the ADG5234 from Analog Devices (or equivalent). But the more I thought about those components the more I could feel something went wrong with this solution. After all, both PORT1 and PORT2 have their proper ground (respectively GND B and GND C if we refer to ma drawing above). The analog switch will also have its proper ground and probably it will be PC ground (GND A). How can the switch work properly if its ground is different from the signals I try to switch (signals Tx I connect to its inputs) ? It can’t, am I right ?
The electromechanical relay ?
I thought about a second the solution with an electromechanical relay. But it is too expensive, too heavy, it takes too much space and it is not power I want to multiplex but signals. I don't think it is a good solution but maybe I am wrong.
What else ?
I didn’t manage to find how different isolated signals can be multiplexed. Is it because I am not looking in the right direction when I did my research ? Is it because actually a simple analog switch will do the job (despite the FET tech, BJT or whatever needs a reference) or is it because it can’t be done (but I doubt) ?
Thanks (again) !