MAX3232 on RadioShack 276-159B, RS232-to-TTL Conversion

[TheDonald]

I'm waiting on a couple of pre-built RS232-to-TTL converter boards to come on a slow boat from China. In the interim time, I plan on building a few of my own.

Most of the DIY plans I've found are very similar or the same (with a few variations) and are built on regular prototyping boards. It's been a while since I've had many EE courses, so due to those variations and the layout that I'd like to build on a Radio Shack 276-159B board, I have a couple of questions.

Here is a sample circuit I found on SoDoItYourself:

http://i40.tinypic.com/s33a11.jpg

And a different one from X-Box Scene:

http://i41.tinypic.com/30hlclf.jpg

These are mostly the same except that the one for the X-Box LiteOn drive does not seem to be interested in the Rx side and the SDIY circuit includes a connection between the +3.3v and GND with a 10uF cap on it.

I think I've translated the circuit correctly to the IC board from the shack below (rendered in lovely MS Paint). A) would someone like to error check it to make sure I didn't foul it up and B) what is the deal with the extra connection with the cap (shown in yellow). Is this part of the circuit optional?


http://i41.tinypic.com/20tklc7.jpg


ETA: Also, are polarized caps necessary?

ETA2: According to Table 2 in the Maxim data sheet, should I be using 0.1uF caps instead of 1uF? http://datasheets.maximintegrated.com/e ... AX3241.pdf

[danb35]

Polarized caps aren't necessary. They're shown in the schematics because, in times past, you'd almost certainly need a polarized cap to get 1 uF. Today, you can get cheap nonpolarized ceramic caps in that range, which makes the process easier.

I'd generally follow the recommendations in the data sheet, unless I knew of a specific reason not to. A MAX232 calls for 1 uF caps in the data sheet, and maybe that's what your other guides are thinking about.

[hellbender]

Hi,
the 10uF is there for stability. The 4x 1uF are used to pump the voltage of VCC to the needed voltages for RS-232. Problem is that this is a bit current demanding and that will make the RS-232 line used to power the whole thing fluctuate. This will result in unpredictable behavoir. So to prevent this the 10uF gives a little extra "reserve" to compensate for the moments the chip requires more power, keeping the powerline stable. Usually there is also a small 10nF/100nF transient suppressing capacitor between the VCC and GND to cancel out any high frequency transients on the powerline.

Both capacitors (10uF and 10nF) should be placed as close to the powerpins as possible. I normally solder them in a straight line between the VCC and GND pins of the chips.