TTL 3,3V to RS232 Level converter – part 4

I just finished building a first version of the level convertor on a breadboard. You can see the schematics and a few pictures above (click the thumbnails to enlarge).

I used:

  • 2 x 100nF non polarized capacitors
  • 3 x 470nF non polarized capacitors
  • 1x DE-9 Female connector
  • 1x 4-pins PCB connector
  • 1 x MAX3232CPE+
  • 1 x bread board

To connect to the PC, you can use a standard crossed serial cable.  For La Fonera, the standard settings are:

  • baudrate: 9600bps
  • data bits: 8
  • stop bits: 1
  • parity: none
  • flow control: none

This level convertor can be used for speeds up to 115200bps, and works with TTL inputs from 2,7V up to 5,5V.

Next up I’ll make the same schematic in a more permanent and less cluttered form.

    12 Replies to “TTL 3,3V to RS232 Level converter – part 4”

    1. I am getting into the fonera. I have it hacked but I would like a serial interface installed. I went to maxim and got a MAX3222CPN+ sample. Will your circuit work for it as well. I have very little electrical engineering skills but i would love to get this working. Thanks.

    2. The MAX3222CPN+ is the low power version of the MAX3232CPE+. It is typically used in battery operated devices. Because of this it has a few more pins more pins. I believe your MAX3222CPN+ has a 20-pin package, right? Those pins can be used to turn off the device and put it in low power mode.

      It is not drop in compatible with the drawings here. You can see how to connect the chip on the datasheet page 12.

      There are extra pins:
      enable (EN): connect to GND to power up the device
      shutdown control (SHDN): connect to Vcc to deactivate the automatic power shutdown
      pins 11 and 14 are not connected (NC)

    3. there are 9 pins on each side. I will check the datasheet. Thanks for the info.

    4. My mistake, I’m sorry.
      The datasheet doesn’t have a pin layout for the 18 pins DIP package. But if you check the typical operating circuit on page 12, you can see that:
      pins 2-9 on the MAX3222CPN+ match the pins 1-8 on the MAX3232CPE+
      pins 10-17 on the MAX3222CON+ match the pins 9 to 16 on the MAX3232CPE+

      This makes it easy to drop it in the schematic I drew in part 5. You just need to connect pin 1 to GND and pin 18 to Vcc. The other pins are connected the same way as the MAX3232PCE+.

      You can choose the same values for capacitors.

      Have fun!

    5. Thanks, I will try it. BTW, the caps in the page 12 diagram are polarized. Should that make a difference in the design?

    6. You can use unpolarized or polarized caps, whatever you have laying around. But if you use polarized caps, you should respect the polarisation as on the drawing in the datasheet.

      I used unpolarized caps in my schematic, so I didn’t need to worry about exploding components. The drawing I made is actually the same as the drawing in the datasheet, I only added the names of the wires to the Fonera router and how to connect it to the computer.

    7. Thanks for this circuit plan and description, it helped me what components to buy and how to solder it together. It works very well. My first attempt with a max323 failed because that one is for 5V only it seems.. good to have one for 3.3V now 🙂

    8. I’m glad it was of any help to you.

      In part 5 I have posted a circuit board layout you can solder on an experimentation board yourself without overlapping paths, this makes it even easier 🙂

    9. Do the values of the capacitors matter? I haven’t been able to find .1 uF and .47uF capacitors.

    10. No they don’t. They can be higher if you like, but there are limits. Some of the caps need to have to same value. It’s in the datasheet somewhere, but I don’t know all the details from heart 🙂

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