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I am working on a communication switch that is digitally controlled and I am using four MUX506IPWR from TI to choose the output channel. The switch has on the input a DSUB9 for a CAN channel and a DSUB9 for a RS232 channel.

So I want to switch RX/TX on the RS232 and CAN_LO/CAN_HI on the CAN channel and each of these lines are connected into a MUX506(so a total of four MUX506).

On the output side I have DSUB9 connectors for the communication where the communication lines merges so that RX goes with CAN_HI and TX goes with CAN_LO. This circuit behaves very odd when all the MUX506 are soldered to the PCB. If I choose one output by sending the right adress and enable the chip all of the outputs will show a voltage level that is about 50% av the signal voltage(I use +-12V for RS232 and 3V3 for the CAN). I suspect that there might be something wrong whenever there is a higher voltage that wants to go from the output on the RS232 to my output connector and then back into the output on CAN chip.

The circuit seems to work fine whenever I remove the IC's for either CAN or RS232. I have tried connecting the MUX506's VDD for the CAN channel to 12V(single ended supply is from 10V to 36V so its wrong in the schematic) but it still behaves the same way. Can anyone point me in the right direction for solving what I have done wrong here?

Datasheet for the MUX506

From my schematic showing the inputs and outputs of the circuit

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  • \$\begingroup\$ Are you sure you want to spend money making a CAN to RS232? RS232 is somehow incompatible, low speed, different protocol. \$\endgroup\$ Feb 16, 2020 at 11:21
  • \$\begingroup\$ Are you trying to swap the pin-out of RS232 standard DB9 to CAN standard DB9 and vice versa? I don't see how that's possible, given that RS-232 has rx on pin 3 but CAN has signal ground there. \$\endgroup\$
    – Lundin
    Feb 17, 2020 at 9:35
  • \$\begingroup\$ @Lundin This is designed for a testrig where some devices communicates through a CAN interface and some devices communicates through a Rs232 interface. I am building a automated switch that can easily switch between the interfaces for the nine outputs. Only one output at a time will be enabled. \$\endgroup\$
    – snake
    Feb 17, 2020 at 15:36

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The MUX chip must have larger supply voltages than the signal voltages. If you route RS232 with the RS232 MUX which is about +/- 12V signal to any of the nine output connectors, there it will get connected to CAN MUX. Therefore the CAN MUX must also have +/- 12V supplies so it does not clamp the signal. You also can't connect CAN and RS232 simultaneously to same connector with the MUXes as that will connect the CAN bus to RS232 lines.

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  • \$\begingroup\$ Ok,thanks. This is as I suspected. But if I have both the CAN MUX and the RS232 MUX connected to a +/-12V supply and then just make sure to not enable the CAN MUX when the RS232 MUX is enabled an vice versa will that work? Shouldn't the disabled chip cut off the signal path so no voltages/current can go through it? The MUX'es are never be be enabled at the same time. \$\endgroup\$
    – snake
    Feb 16, 2020 at 11:47
  • \$\begingroup\$ It should work if you enable only one path at a time, RS232 or CAN. \$\endgroup\$
    – Justme
    Feb 16, 2020 at 12:31

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