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RS422 Multiplexer - 2 inputs 1 output

Steven Ng
Prodigy 10 points
Other Parts Discussed in Thread: TMUX1574, TMUX6234, TMUX7234, SN74CBTLV3257, AM26LV32, AM26LV31

I am currently working on a satellite that uses RS422 communication protocol for S-Band and X-Band communication and as part of this we have both nominal and redundant connections for redundancy.

We aim to connect the satellite On-Board Computer (OBC) to an Electrical Ground Support Equipment (EGSE) to interpret the data and convert it into more human readable data for our ground team. Since only one of the nominal or redundant links can be used at any given time, we require a switcher to be made to switch between the nominal and redundant links outside of physically unplugging one harness and connecting the second one up.

I'm quite a newbie when it comes to using RS422 protocol and hardware to get it all working - is there a solution that multiplexes the two inputs, and then just sends one output? The transmitter and receiver RS422 drivers will be handled by the EGSE/OBC respectively, so I don't think there will be any added circuitry to get the RS422 signal working?

For S-Band, there will be an added complexity in that the multiplexer circuit is required to be bidirectional since it will be sending telecommands upstream (from EGSE to OBC) and then also receiving telemetry downstream (from OBC to EGSE.)

Currently the connections used are Sub-HD 44 Way connectors, and I believe up to 5 pairs of signal lines (5 pairs on S-Band, and only 2 pairs on X-Band) on the connector will be used for the RS422 communication protocol. If there's a way to completely switch from from one of the inputs (Sub-HD 44 Way) to the output (Sub-HD 44 Way) without having to individually multiplex each of the lines in the harness, then that would be ideal. My question is, is there a simple off-the-shelf solution with a bunch of multiplexers and maybe an FPGA to control this switching. Or, will the circuit have to be custom designed with a bunch of multiplexers and the FPGA on a PCB to perform this?

So, effectively:

S-Band: 2 inputs (Sub-HD 44 Way) to 1 output (Sub-HD 44 Way) but bidirectional so has to be used in 1 input to 2 outputs as well. Operator will just have to select whether the nominal or redundant link will be used.

X-Band: 1 input (Sub-HD 44 Way) to 2 outputs (Sub-HD 44 Way). Operator will have to select whether the nominal or redundant link will be used.

Hopefully this makes sense!

  • 0
    Guru 316240 points

    As far as I can see, you can simply use passive switches. What device to use depends on the required bandwidth and on the voltages that actually happen on the RS-422 bus (receivers allow a large common-mode range to allow for ground shift, but your actual range might be lower).

    For RS-422 voltages, there are no switches with more than four channels. You will probably need 4× TMUX1574 if you can stay below 5 V, or 4× TMUX6234/TMUX7234 for higher voltages.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Hi Clemens,

    Thanks for the reply.

    I presume that since the transmitter and receiver drivers are on the OBC and EGSE respectively, that this is not required in this circuitry, and passive switches can be used for my use case?

    With regards to the levels of my circuit, please find the following information from our spec sheet:

    For the X-Band communication protocol, it seems that I was incorrect about the protocol, and it is in fact LVDS and not RS-422. Please find the corresponding datasheet below:


    My question is, can I use the same circuitry for LVDS too? Are passive switches sufficient?

    Thanks in advance!

    Steven.

  • 0 in reply to Steven Ng
    Guru 316240 points

    Yes, you can use passive switches; they work pretty much like mechanical switches (for voltages between their supply rails).

    For LVDS, you can use low-voltage switches, e.g., SN74CBTLV3257/TMUX1574 (but a higher-voltage switch would not hurt).

    Your RS-422 specification allows a much wider common-mode voltage range than for LVDS. This is probably because of the capabilities of commonly-used receivers, and not because of actual characteristics of your connections. In any case, the switches' power supplies define the range of voltages that can be passed through.

    (Please note that the DS26LV3x transceivers are being discontinued; use the AM26LV31(E)/AM26LV32(E) instead.)

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Hi Clemens,

    Thank you very much for the information.

    So for RS422, since my transmitter is +/-5.5V, and the receiver is +/-10V, you recommend the TMUX6234/TMUX7234 line of passive switches? Can I use the same switches for both transmitter and receiver?

    And for LVDS, you recommend SN74CBTLV3257/TMUX1574 low voltage switches - which of the datasheet specifications are you looking at when you spec the ICs? (apologies, when I said I was a complete newb to this, I really am!)

    And for LVDS, does impedance matching matter more in the circuit for this type of signalling? As in, when putting the ICs onto a PCB, do I have to factor in trace length to match the impedance across the different lines?

    For the number of passive switches in the circuit - do you recommend switching every single pin, or just the signal lines? So, for the Sub-HD 44 way connector, do you recommend 11x of the passive switches to total 44x connections, or shall I just focus on the actual RS422/LVDS lines for switching? The majority of the other pins in the harness are either not connected, or have an internally pulled high resistor to indicate which harness this is.

    And one final question - once again, a complete newb, but we are planning on building a circuit that can potentially communicate via a LAN interface to a central computer to control the switching between the nominal and redundant communication links - is it better to build a custom standalone PCB and design it with a couple other components to do this, or is a FPGA an "easier" way of approaching this?

    Thanks,

    Steven.

  • 0 in reply to Steven Ng
    Guru 316240 points

    I do not understand why you want separate switch for transmitter and receiver; analog switches are passive and bidirectional, and you would have one between the transmitter and the receiver.

    You can choose any switch that supports the required data rate and that covers the required voltage range (the switch's supplies determine the voltage range).

    Please note that the differential output voltage is the difference between the two lines, so it does not tell you the absolute voltages. (A transmitter always outputs voltages somewhere between its supplies.)

    The characteristic impedance of the switch is likely to be different of the characteristic impedance of the cable/traces, but for very short distances, this does not matter.

    You need to have switches only for those signal lines that you want to read.

    The switches need a single control signal to select one of the two sources/destinations. I do not think that you would need an FPGA do generate this control signal. (But to manage a LAN interface, you would need a microcontroller.)

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 24440 points

    Hello Steven,

    I agree with everything Clemens said above.

    1.Just for more background the TMUX6234/TMUX7234 will work for rs422 because they can support the voltages that will be passed through them. 

    For example if the supplies for the TMUX6234 are +/-10V, they it will pass any signal between -10 and 10 volts.

    2.To determine what mux is best in your lvds application you can take a look at this faq.

    The devices that Clemens suggested (SN74CBTLV3257/TMUX1574) can support lvds and if you are operating with lvds signals of 327.5MHZ you should use TMUX1574.

    Please let us know if you need any further assistance

    Regards,

    Kameron