# MAX232 circuit not working

I am designing a circuit using a MAX232. I have connected Tx and Rx of the MCU to the 11th & 12th pin and connected the 14th & 13th pin to the RS232 cable. I do not have a UART port in my laptop, so I have used a UART to USB Cable. However, I am not getting data. Vcc is 3.3V. I have used 1uf capacitors. Below is the circuit:

Where am I going wrong?

• Where are you no getting data? Have you measured on Pin 14 if you can see the data coming from MCU (with oscilloscope) ? – Botnic Jun 30 '16 at 7:42
• I am using PIC MCU which works on 3.3v and I am using MAX232 for UART which I think is the wrong IC because the voltage level it requires is 5v. For 3.3v, it is MAX3232. Sorry for everything, I have used the wrong IC. I will try the circuit with MAX3232 – S Andrew Jun 30 '16 at 9:06
• Even if you change the IC the fact is that the capacitors on the VCC and GND lines are still going to prevent the IC from working. Also, you need to remember the TX coming OUT the MAX232 needs to be connected to the RX going IN to the PC (Pin 2 on your 9 Way D Connector) – Doodle Jun 30 '16 at 9:29
• Out of interest, what is your overall goal? If, for example, it's to connect a PC to a 3.3V microcontroller's UART, you could use one of these FTDI cables, and avoid the need for a MAX232 altogether. – Steve Melnikoff Jun 30 '16 at 15:30
• Additional note: you should include all of the pins of a device in its schematic symbol or use symbols that make it evident that the (usually large) component has multiple sub-symbols. Showing the power and ground pins for this device would improve the ability to understand the circuit. – user2943160 Jun 30 '16 at 17:44

MAX232 does not work with 3.3V supply voltage. You should either use MAX3232 (which is 3.3V equivalent part of MAX232) or you should supply 5V to your IC.

Besides that there is only one thing wrong with your circuit: You should connect (+) pin of C4 to GND not VCC.

For those who misinterpret the circuit: Pins 2 and 6 are NOT the supply pins of the IC. They are respectively, positive and negative charge pump outputs for storage capacitors.

• I'll just note, that while C4 should be connected to GND, C3 could be connected to VCC instead of GND. The TI datasheet shows it connected to GND, but mentions that C3 can be connected to VCC or GND, but the Maxim datasheet seems to prefer connecting to VCC. – ilkkachu Jun 30 '16 at 16:11
• I have replaced the MAX232 and now using MAX3232. Does 1uF Capacitors ok with the circuit or should I use 0.1uf cermaic capacitors – S Andrew Jul 1 '16 at 8:19
• Although it should work with both 1uF and 0.1 uF caps, I would consult the datasheet and do what it says. TI suggests 0.1 uF caps. – Durmus Jul 1 '16 at 9:31

If your board matches your schematic, then your chip has no power. C3 and C4 are blocking Vcc and GND.

• I don't think we can tell that from the schematic, since the Vcc and GND pins are not pictured. V+ and V- of MAX232 store voltages from the voltage doubler and inverter, they are not the same as Vcc and GND (which would be pins 16 and 15). See e.g. this image picked from the Maxim datasheet. The image on the question looks like it's from Eagle, and the component there might have the Vcc and GND pins as a separate drawing unit. Which of course might be disconnected if the poster didn't notice them. – ilkkachu Jun 30 '16 at 13:16
• /agree ilkkachu - this is yet another wrong answer here (probably upvoted by people who never used any RS232 voltage level converter with built-in RS232 voltage level generators); V+/V- ain't Vcc/GND pins; check for yourself, Mark: ti.com/lit/ds/symlink/max232.pdf (page 10, Typical application) – user20088 Jun 30 '16 at 15:24