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I'm having an issue programming the Arduino Uno with Sparkfun's MAX3232 breakout board attached. Sometimes the Arduino IDE won't see the Uno plugged in. I'm thinking of connecting a switch to the GND or the 3V - 5.5V pin so I don't have to disconnect the breakout board when programming. Are there any better solutions?

Sparkfun link: https://www.sparkfun.com/products/11189

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  • Switching off power to the MAX2323 might work. I assume you understand the MAX2323 is connected to exactly the same pins as the host PC, via USB, and that is the problem source. Yes? Would you post a link to the MAX2323 you are using? Is there a schematic diagram? I would like to see the schematic diagram so that I could see if the MAX2323 might still present a load to the Tx Rx lines when it is powered off. For serial communication, the Tx and Rx lines may be pulled up by resistors, and you would like them to be defeated or disconnected too. We should probably look at the MAX2323 data too
    – gbulmer
    Commented Aug 12, 2014 at 22:54
  • Hi gbulmer. I meant the MAX3232 breakout, but I think you probably figured that out. I've included a link to sparkfun's breakout board to the original question as well.
    – user2218339
    Commented Aug 12, 2014 at 23:31
  • Thank you for the schematic. The breakout has no resistors on Tx or Rx to cause any problems. It looks like switching off power would work, and it is an okay solution. To be more sure, we need to look at the datasheet for MAX3232. I would expect it to be designed well enough that it won't be harmed by having lines driven while it is off, or that it could pull-up a signal line while off, but it is worth a check. I apologise, but I am off till tomorrow. Good night from the UK.
    – gbulmer
    Commented Aug 12, 2014 at 23:33
  • Good night, thanks for the help and tips!
    – user2218339
    Commented Aug 12, 2014 at 23:35
  • PS - My alternatives were: use an enable/disable on the RS232 converter (there isn't one, so that is dead), disconnect MAX2323 Tx (as that interferes with the Host PC's signal), or disconnect both (if necessary).
    – gbulmer
    Commented Aug 12, 2014 at 23:39

1 Answer 1

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Switching the power off to the MAX3232 appears to be okay.
There are no components connected to the Rx and Tx lines on the breakout board to complicate it.

Unfortunately the break out does not give access to pin 1, EN which is "Enable Control", or pin 18 SHDN "Shutdown Control". AFAICT they could have been used to turn off the receiver and transmitters of the MAX3232. That would have been ideal. Maybe contact Sparkfun and suggest the improvement to put them on pads for folks to use, and offer to beta test it :-)

The datasheet for the MAX3232 says that it is able to withstand a continuous short on the Tx line, so it sounds pretty tolerant to minor abuse. Also, the diagrams dont show any pull-ups on the MOS/TTL Tx line. So I would expect it t be okay if you shut the power off, but continued to wiggle the Tx Rx pins.

An alternative would be to disconnect the Tx pin from the Arduino, so that the MAX3232 wouldn't interfere with the host PC's USB comms. It sounds like the thing on the RS232 connection will ignore data it receive while the Arduino is being programmed. If that is not true, then it would need to be disconnected too.

At the moment, switching off power looks like a reasonable approach. Disconnecting Tx and Rx also looks reasonable, and slightly better than powering the MAX3232 off.

Do you know how you want to do that?

NB: The MAX232 datasheet shows that part should not be driven by a high when the chip is unpowered. However the MAX3232 datasheet does not carry that warning. My preference would be to use the MAX3232 signals 'EN' and 'SHDN'. However those are not available on the breakout. I'd recommend having the part powered, and disconnecting Tx and Rx from the Arduino.

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  • Hey, you're still awake! Thanks for interpreting the datasheet for me, I'm not very good at that. I'm thinking of just a simple switch at either the GND or power. I can also have switches at the RX and TX lines like you suggested earlier. I think I'll do more testing with the things you suggested.
    – user2218339
    Commented Aug 13, 2014 at 0:23
  • @ChrisStratton - Is MAX232 a typo? The question is about MAX3232. If it is a typo, that is an extremely important point, and I will correct my answer. I looked for that information, but may have missed it.
    – gbulmer
    Commented Aug 13, 2014 at 18:48
  • Sorry, I did indeed look up the wrong part, and the specs on the MAX3232 appear to be different without the MAX232's input-above-Vcc restriction. It would be interesting to use an ammeter to see what if any current the chip draws through the TIN pin when unpowered.
    – Chris Stratton
    Commented Aug 13, 2014 at 18:51
  • @ChrisStratton - I amended my answer to show a WARNING pending clarification. I will remove that warning.
    – gbulmer
    Commented Aug 13, 2014 at 18:52
  • @ChrisStratton - I have amended my answer. I have removed that warning, but kept a note to reduce the chances of the same concern being re-raised.
    – gbulmer
    Commented Aug 13, 2014 at 18:58

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