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I was playing today with a very simple setup of pic16f876 and max232 connected to the COM port of a PC running a terminal emulator. I was just testing (the uc simply echoes anything it receives) when I decided to try using smaller capacitors for max232. For my amazement after I removed from the breadboard ALL FIVE capacitors (C1-4, Cbypass) it turned out that the communication was still working just fine! It works at 9600 and 28.8k (couldn't test with higher speeds because the 4MHz crystal I use is not suitable for generating the higher baudrates). But anyway, so far everything I tested works just fine. I even tried another analogous chip ST232CN and it also works without caps.

So my conclusion is that max232 already has inside all it needs for the generation of +/-8.5V. Until now I thought that the chip DEPENDS on the external caps to work. But it seems that they are not critical to max232 operation but ... I don't know, only provide extra stability? And yet with all my tests I never observed corrupt data transfer so I wonder what kind of interference do the caps actually protect from, what would the drawbacks be if I continue to use this setup with no caps at all?

PS: Since I didn't expect max232 to work without caps, at first I thought that I was mistaken and that I was observing some local echo at the terminal. But then I programmed the uc to echo not the same byte but byte+1. As expected, the incremented ASCII symbols were observed arriving at the terminal, so the local echo possibility was eliminated.

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    \$\begingroup\$ I think it has to do with the fact that the logic levels are sufficient. However, have you looked at the output with an oscilloscope? Those capacitors are needed for the voltage doublers. \$\endgroup\$ – Gustavo Litovsky Mar 17 '13 at 23:19
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    \$\begingroup\$ @Gustavo - I was thinking the same thing, I think you should add it as an answer. I assume the output will be 0 - 5V without the charge pump capacitors and apparently the PC is happy with this. Checking with a scope would confirm it, or with a multimeter at the Vs+ and Vs- pins. \$\endgroup\$ – Oli Glaser Mar 17 '13 at 23:31
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It's not likely that Max232 is pumping out +/-7.5V or more without the capacitors, more that your terminal accepts the logic level voltage coming out of your pic controller.

The RS232 spec allows for anything between +3 to +15 and -3 to -15. If the supply line to the Max232 is 5V then simply providing +/-5V for the max232 is acceptable to your terminal.

Some equipment (sometimes older) require +/-12V and the 7.5V that Max232 would boost to a 5V line would get you there. As Gustavo mentioned, if you hook it up to a scope you're unlikely to see a sustained +/-12V or more coming out of the max232.

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    \$\begingroup\$ I don't see how the MAX232 could generate a negative voltage without the negative charge-pump capacitors. It's not a split-rail part (+5V and GND). \$\endgroup\$ – Adam Lawrence Mar 18 '13 at 0:28
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    \$\begingroup\$ Thank you all! @Madman, Oli - you were right that negative voltage can not be generated without capacitors. V+ measures little less than 5V and V- is little more than 0V. Everyone was right that the true reason for the working comm is that the UART chip at the PC is "intelligent" enough to tolerate TTL levels. In fact, to be absolutely certain of this, I simply ttl-inverted the TX pin of the pic (because logic 1 is low/negative voltage in rs232), fed it directly to the PC, bypassing max232 completely, and everything works fine. So without caps max232 acts as something like ttl-inverter. \$\endgroup\$ – Cleric Mar 18 '13 at 20:44

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