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.

Opto-isolation schematic

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.

Mutliplexing two isolated signal schematic

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) !

  • \$\begingroup\$ How do you plan on synchronizing each of the 8 paths? bit sync? byte sync? or no sync? This is critical to method of Mux/deMux \$\endgroup\$ Commented Mar 19, 2018 at 23:06
  • \$\begingroup\$ Its not obvious why you would want to do this. i.e why would you be turning two ports back into one? I can imagine using a mux to turn P1 into P1a and P1b , but a reason to do what you want doesn't come to mind... \$\endgroup\$
    – Henry Crun
    Commented Mar 19, 2018 at 23:23
  • \$\begingroup\$ use a reed relay ... one for each channel ... here is a picture that shows what the contacts look like (the middle one is 4PST) .... normally the contact assembly would be inside the wire coil ... it can also be operated by an external magnet as a proximity sensor (not what you are looking for though) ............. here is a supplier page eu.mouser.com/Electromechanical/Relays/Reed-Relays/_/… \$\endgroup\$
    – jsotola
    Commented Mar 19, 2018 at 23:31
  • \$\begingroup\$ In the order : I don't plan to sync anything. The MUX only purpose is to give the possibility to use either TX+/- from P1 or TX+/- from P2. Actually I have two cards, it is for redundancy but it is not relevant here, the issue is the same with one or two card. Reed relay... ok I will check that. \$\endgroup\$
    – vionyst
    Commented Mar 20, 2018 at 7:45

2 Answers 2


Relays are a very good solution, as all ports are isolated from each other, and the withstand voltage of the switches is high. They are DPDT so only one is required. Look at NAIS TQ2 series. They are not expensive relative to multiple specialist optos. They can be held at low power, or use single coil latching if you need minimum energy.

Opto-fet isolators/switches are an easy solution to a lot of problems like this, if a relays are not. But your job needs a total of 4 switch contacts / fets, per channel pair. It will cost more than relays, and probably use more power, and have lower withstand voltage

  • 1
    \$\begingroup\$ Reed relays do the trick. Also very small. \$\endgroup\$
    – Janka
    Commented Mar 20, 2018 at 1:39
  • \$\begingroup\$ @janka Am I missing something about Reed Relays? Apart from a small number of heinously expensive ones, they seem to be an SPST device i.e. 4 would be required vs a single ordinary dpdt relay \$\endgroup\$
    – Henry Crun
    Commented Mar 20, 2018 at 2:41
  • \$\begingroup\$ I understood the OP has to multiplex 8:1, not 2:1, so a DPDT wouldn't help much. There are also 4-contact reed relays, and even 4PDT reed relays. \$\endgroup\$
    – Janka
    Commented Mar 20, 2018 at 4:11
  • \$\begingroup\$ I have always just assumed that reeds are a sort of legacy telecoms thing with a $30+ price for anything but spst. Am I missing a good source of them? \$\endgroup\$
    – Henry Crun
    Commented Mar 20, 2018 at 4:35
  • \$\begingroup\$ From what I read so far reed relays are more expensive than (signals) relays as proposed by @Henry Crun. Your solution seems to fit my needs, I'll check a little bit more. \$\endgroup\$
    – vionyst
    Commented Mar 20, 2018 at 7:58

Any relay, etc. will have bonded grounds at some point. The best solution I have in mind is to isolate the outputs again into logic gates then isolate the gates' output into the mux. I cannot think of any other way that might keep them all completely isolated. Surface mounted isolators are cheap, very small, and fairly easy to use.


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