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MAX3221: Extreme cable length driver

Jesse Meyer
Prodigy 20 points
Part Number: MAX3221
Other Parts Discussed in Thread: OPA211, OPA132,

Tool/software:

Hello everyone,
I have an interesting project where I need to create a line driver that can signal across thousands of feet of cable. Cable will be between 75 and 110 ohm and between 1.2 and 1.7 uF.

Our target is between 40 and 200 kbaud.

For reference we are currently do 50kbaud by feeding our CMOS level outputs into a OPA211 that then buffers to a push pull BJT to signal 30 VP2P.  Simple but effective:
Current Design

I am wondering if switching to OPA132 or possibly MAX3221 (we are open to sending RS232) would yield better results or should we take steps to improve the circuit design from another IC.

Min and max baud rates will be 50k and 200k so I am calculating a minimum slew of 4.7V/us and to reach full speed 18.8V/us.

Also, to lower ISI we only pulse on transition instead of entire clock cycle. For example:

I'd like to be able apply pre-emphasis to the pulses but don't see any solutions that build that into an IC beyond the MAX3221.

If anyone has experience with extreme cable length signaling, open to try new ideas.

  • 0
    Guru 296360 points

    Would it be possible to use differential signalling? RS-485 can do 100 kbps at 4000 ft.

  • 0
    TI__Expert 8127 points

    Hi Jesse,

    Thanks for posting on E2E!

    If you are already considering swapping to RS-232, I would recommend RS-485 instead. Because of it's differential signaling implementation, it can provide much better signal integrity allowing for longer distances as Clemens mentioned. The RS-232 standard recommends a limit of 50ft.

    If you are curious about RS-485 and want to learn more, I would recommend this design guide:

    https://www.ti.com/lit/an/slla272d/slla272d.pdf

    For actual devices, I would recommend either SN65HVD05 or THVD2410 (at 5V VCC). These have higher drive strengths best for long distances. Any RS-485 device can support up to 4000ft and <=100Kbps if setup correctly according to standard. Past 100Kbps AC losses take effect. Beyond 4000ft, a repeater should be used.

    Whether you end up using RS-232 or RS-485, feel free to post schematics here on E2E, and we can help review those. 

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Prodigy 20 points

    Thanks for the suggestion Ethan but we are stuck with RS232 because the cable is single ended mono conductor cable. 

  • 0 in reply to Jesse Meyer
    TI__Expert 8127 points

    In that case, RS-232 would be worth attempting even though its designed for short ranged peripherals. You would be going outside the RS-232 spec since 2500pF is the "guaranteed" limit. But from some quick research, I am seeing others push the distance up to 2000-4000ft with the right cabling. If you have control of the cabling, lowering the capacitance rating will improve your chances.