I'm trying to connect TI's MAX232 to my Arduino so that I can communicate with my PC. I have connected it exactly as the picture shows:

MAX232 circuit schematic

Tx/Rx are not connected because I just wanted to see if I can get proper V+/V- on pin 2 and 6. At Vcc=5V I'm reading 1.9V on both - pin 2 and pin 6. I connected my 1uF capacitors so that the longer leg represents the white part of the capacitor symbol, and the shorter leg is in place of the black part. Should I connect some other signals so that I can read +10/-10V on pin 2 and 6?

See an image of my circuit below. The brown cable is Arduinos GND and the grey cable is Vcc.

My MAX232 circuit

  • \$\begingroup\$ C4 doesn't look correct. Should connect between pin 2 and 16 (Vcc) not ground \$\endgroup\$ – JIm Dearden Mar 18 '14 at 11:11
  • \$\begingroup\$ I reconneted C4 so that the -side of the capacitor is connected do Vcc, the +side to Pin2. Now Pin2 reads 2.9V and Pin6 still 1.9V. Those values seem very strange, especially because the Pin6 should get me -V and not +. \$\endgroup\$ – Bart Mar 18 '14 at 11:16
  • \$\begingroup\$ @JImDearden According to the datasheet, C4 negative lead can be connected to either VCC or GND. See the circuit in my answer. \$\endgroup\$ – Ricardo Mar 18 '14 at 11:18
  • \$\begingroup\$ @JImDearden Capacitor C4 can be connected either to Vcc or ground, it doesn't make a difference and the circuit works correctly both ways. \$\endgroup\$ – alexan_e Mar 18 '14 at 11:19
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    \$\begingroup\$ When following Ricardo's advice to try and replace the IC, I realized that one of my 7-segment LED display drivers got mixed up in a pile of MAX232... I replaced it with MAX232 on my breadboard and now I'm reading 8.8V on p2 and -8.6V on p6, so everything works fine. Sorry for your time, I didn't anticipate an error of this kind. Thank you for your help! \$\endgroup\$ – Bart Mar 18 '14 at 11:47

I'm assuming you're using TI's MAX232. Other than missing the bypass capacitor, your schematic seems fine. See below the Typical Operating Circuit from the datasheet for a comparison.

MAX232 Typical Operating Circuit

Also, regarding Jim's comments, according to the datasheet, your C4 capacitor negative (short) lead (which corresponds to C3 in the circuit below) can be connected to either VCC or GND (see footnote).

So, I'm guessing that either -

  1. your IC has failed
  2. your connections are different from the schematics you presented

A good quality picture of your board or breadboard would help us determine if the problem is in your circuit or not.

Also, if you have access to an oscilloscope, you should see a nice square wave on the kHz range at the capacitors leads (not sure which ones, but C1 and C2 certainly). That would mean the IC's charge pumps are working fine. I'll post a shot when I get home so you can see what it looks like. From memory, I would say that they are about 1V in amplitude.

In any case, I would add the bypass capacitor as well, just to make sure the MAX232 power supply is stable. Adding the correct bypass capacitor made a difference for me once.

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  • \$\begingroup\$ Thanks for the tips, I uploaded the picture (link in comments below the first post). I will connect another MAX232 and see what happens. \$\endgroup\$ – Bart Mar 18 '14 at 11:37
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    \$\begingroup\$ Maybe you would like to wait before connecting a new MAX232 there just to make sure your circuit isn't burning them. \$\endgroup\$ – Ricardo Mar 18 '14 at 11:44

I had the same issue. The Arduino cannot supply enough power. Try powering your Arduino with its wall adapter and not with the USB. For me it works but no debug is possible.

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