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Analog Switch for RS232 Level.

Hi everyone,

I need to work on an analog switch which will switch the RS232 (+/- 10VDC) signal from different RS232 serial communication.

I tried ADG333 with MAX232IDR, but it will draw too much current when RS232 is in Mark (-8.9VDC), then it will pull it to -0.89VDC.

Does everyone how to make analog switch for RS232 signals?

Thanks for help.

  • The ADG333 will be fine for this as long as you use supply voltages wider than your expected signal swing.

    So for example with VDD=15V and VSS = -15V and GND=0, you should nave no trouble switching most RS232 level signals. The RS232 standard allows for larger voltage swings, but most real life interfaces don't.

    That being said, I think you would be better off translating the RS232 levels to TTL and then do your switching (with logic) and converting it back into RS232. The advantage is that you have the robustness of the RS232 transceivers and an easy and clean way to switch between channels at low voltage.


  • Hi KRZ,

    Since I use a single power supply of +5VDC to MAX232, and +12VDC for ADG333(it was +5VDC incorrect on the schematics.

    when TTL is 0 (0VDC-Ground), RS232 Level is +8.57VDC, the ADG on COM is +8.57VDC, then NC/NO works as expected,

    However When TTL is 1 (+5VDC), if no connected to ADG333 on COM, RS232 Level will be -8.57VDC(negative). but once I connect it to ADG333 on COM, it drops to -0.67V, it did not stay -8.57VDC, and draw 0.02mA. What went wrong with TTL Logic Truth (1)?

    Please review this part of schematics. Thanks

  • The Vee connection (named Vss by ADI by the way) of the ADG333 limits how low your voltage can go on any of its pins.

    If you look at the absolute maximum ratings in the data sheet, you will see that this limit is Vss -2V or 20mA whichever occurs first. The device has protection features in it that start conducting if you exceed the supply voltage range. This is what you are observing. You will need to provide the switch with a negative supply.

    So instead of hooking pin 5 to ground connect it to pin 6 of the MAX232 which is its negative supply voltage.

    This will still limit your range for any incoming RS232 signals which might have larger voltage ranges.

    A sneaky way of solving this would be to just connect a 10nF cap to ground on pin 5 of the ADG333.

    This would charge it through the protection diodes to the lowest voltage seen on its inputs. You could add a zener to limit the maximum voltage. Still not really production ready, but Vss currents are specified at 5uA max so this will very likely work.


  • Hi KRZ,

    Thanks a lot for your help.

    I will have a try without -12V supply on Pin 5, and connect it to Pin 6 on MAX232.

    Or, can I used two MAX222s with /EN and /SHDN for signal switching?

    Thanks again for your inputs.

  • I think your best solution would be to simply use two of the MAX232 parts or equivalent.

    This assures you the ESD protection these devices will give you and makes switching a trivial exercise in digital logic. Having unprotected analog switches sitting out on an open connector is not a good idea...


  • Hi Klaus,

    How about using Optocouplers? I tried MOC208, and it seems working for RS232 communication. It works on both TXD and RXD as RS232 Level. I am just concerned that the Opto will be aged.


  • Probably, but the gate of the MOSFET is very exposed.

    Also, I think we're moving away from the purpose of this forum, since this design no longer has any ADI parts in it.


  • Hi Klaus,

    How can I do ESD protection on MOSFET such as 2N7000? I have been experiencing this failure on 2N7000.

    Please advise me.



  • Use series resistor from your connector (~10k) and a zener diode (~6.2V). Anode to ground, cathode to the gate of the 2N7000.