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MAX3243E: Schematic review

Billy
Prodigy 30 points
Part Number: MAX3243E
Other Parts Discussed in Thread: MAX3243

Tool/software:

Hello,

    I have never used the MAX3243 before.  Could someone review the schematic below and provide any feedback?  I know most of the IC pins are omitted and this is intentional as I wanted to simplify the schematic for this question.

    I am mainly concerned with the RIN, DIN, ROUT, and DOUT pins and ensuring they are connected to the appropriate pins on the RS232 and TTL ends.  Unused DIN pins will be connected to GND.

    The questions I have are:

  1. Are the MAX3243 pins connected to the correct pins on the MCU and RS232 connector based on the labels?
  2. Can I leave ROUT2B floating?
  3. Does FORCEON/FORCEOFF require pullup resistors?

    Thank you for your assistance.

  • 0
    TI__Guru 58860 points

    Hi Billy,

    I currently have a high volume of e2e threads I'm working through, I will try my best to get this reviewed by tomorrow. 

    -Bobby

  • +1
    Guru 279850 points

    1. D=Driver and R=Receiver are from the point of view of the MCU. (And table 5-1 tells you which pins have logic or RS-232 voltage levels.) So these connections are correct.

    2. Yes, unused outputs can be left open. (The output is driven, so it will not be floating.) (Inputs must not be allowed to float, but the bus inputs have internal resistors to GND.)

    3. No; they are connected to the transceiver's power supply, so it is not possible for them to be floating when the transceiver tries to read them.

  • +1
    TI__Guru 58860 points

    Hi Billy,

    Looks like Clemens answered your 3 questions. 

    Here is my checklist on the schematic review

    U? MAX3243ERHB 
    Checked Status (Good, bad, okay, ?) Additional comments
    Schematic matches datasheet pinout Okay Some pins are missing but everything present I checked aligned with the datasheet
    Schematic nets match expected pins Okay Same comment as above, looks like the nets were designed around the MCU perspective
    Capacitors are sized correctly Good
    Floating inputs ? Some pins are missing from this schematic, the main thing I would verify is if the Dx pins are floating, those pins should be biased to Vcc or GND (we generally recommend through a resistor like 10k but you can short to Vcc/GND if you really wanted).
    Local decoupling capacitors Good
    Pull up resistors on the TTL output pins Okay I don't see any populated but sometimes when the RS232 device is unpowered and the MCU or whatever is connected to the RX pin sees it floating or GND, it can cause issues with the MCU thinking theres a problem or garbage data is received from the MCU (or break condition is seen). We sometimes see customers use pull up resistors on the Rx pins to ensure a known state. 

    Please let me know if you have any follow up questions.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 30 points

    Hello Bobby,

        Thank you for your quick response!  I have created an updated schematic that includes the serial connectors, pull-down resistors, and a faux MCU symbol.  The MCU, for simplicity, does not include the majority of it's pins such as the crystal pins.

      I am a little confused on the checklist saying to add pull-up resistors on the TTL side.  It is my understanding that TTL operates as HIGH = 1, LOW = 0, whereas RS232 is HIGH = 0, LOW = 1.  For this reason I used pull-down resistors.  Would this be correct or have I made a mistake?

    Thank you again!

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Hello Clemens,

        Thank you for your quick response and verifying my thought process!

  • 0 in reply to Billy
    Guru 279850 points

    The idle state of a (TTL) UART is high (corresponding to a negative voltage on the RS-232 bus), so in general, you would want a pull-up resistor.

    Anyway, you need a pull-up/-down only when the transceiver is powered off. This is not the case in your circuit; the ROUT pins always drive a value, so the resistors have no effect.