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Original question:

MAX3232: About unused pins

MAX3232E: Impact of unused pins not being treated as recommended

Shinichi Inoue
Guru 20536 points
Part Number: MAX3232E

Dear Specialists,

My customer is connected unused pins with a different way than recommended.

I would be grateful if you could advise.

---

According to the related thread, DIN2 must be tied to VCC or GND, with or without a resistor.

Circuit (channel1) is used and (channel2) is not used. 

In this case,

DIN(channel2) is pulled up to Vcc,
RIN(channel2), ROUT(channel2) are floating,
DOUT(channel2) is connected to RIN(channel1) via a resistor *Circuit (channel1) is used and (channel2) is not used. 

DOUT(channel2) is not recommended treatment.

Some past products had DOUT that was different from the recommended one.
We are checking to see if there is any impact.

This connection may be more stable for unused terminals, or there may be problems.
Please let me know if there are any possible advantages or disadvantages.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • +1
    Guru 281410 points

    No DIN is floating, so as far as the MAX3232E itself is concerned, this is OK.

    DIN2 is pulled up, so DOUT2 will always output a negative voltage. How this affects the RIN1 bus signal depends on the size of the resistor.

  • +1
    TI__Mastermind 46797 points

    Hi Shinichi,

    Yeah so that is highly inadvisable - an unused DOUT needs to be left floating - they should not connect it to an active receiver - as DOUT will be outputting a voltage, even if unused. 

    The risk is two fold:

    1. As previously mentioned unused DOUT can influence what is measured at RIN - which could negatively impact communication- by how much really depends on the resistor value - but it is not going to provide any benefit by doing this. 

    2. The DOUT communicating to the active RIN pin will also have to contend with the resistor to DOUT - depending on the resistor value between unused DOUT and the used RIN you could actually overload the transceiver that is communicating with this device. RS-232 drivers are only meant to drive 1 RS-232 receiver (which has a minimum impedance of 3k) 

    Ultimately this is very risky and failures are much more likely to occur without providing any benefit. Just keep unused DOUT floating - otherwise you add a lot of unneeded risk to the design. 

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson 

  • 0 in reply to Parker Dodson
    Guru 20536 points

    Hi Clemens and Parker,

    Thank you for your reply.

    I understand it is not recommended DOUT(channel2) is connected to RIN(channel1) via a resistor.

    I'll share with the customer.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 46797 points

    Hi Shinichi,

    No Problem - I am glad it was clear! 

    If you have any other questions please don't hesitate to reach back out!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Guru 20536 points

    Hi Parker,

    There has been no response from the customer at this time.
    So let's close the thread.

    If they have an additional question, I consult new thread.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi