View Full Version : MAX232 (RS232) based Serial to Propeller interface - how safe ?

01-21-2009, 02:22 PM
Although I have built the Parallax Serial to Propeller Interface which works great in place of my USB propplug, I came across a MAX232 Serial to Propeller circuit (WWW.tdus.de (http://www.tdus.de)) incoporated a Prop D-40 board.

Question: Are the signal levels safe for the Propeller. I seem to remember that for 3.3V circuits, they recommend to use the MAX3232 ic which is the 3.3 volt version of the chip.

I can only find MAX232 chips here locally unless I order·the MAX3232·from the US.

01-21-2009, 02:31 PM
Well - you *could* use max232s with voltage dividers and/or with other techniques others will no doubt be able to suggest.

The max3232 though is the neater solution. I got some from http://www.futurlec.com I'm in Australia and Futurlec ship out of Thailand so their shipping time is about 8 working days whereever you are.

$1.90 each, postage for small quantities is a flat $4. I buy about 2 orders a week because their shipping is so low you can afford to get a few test chips then come back and get more if they work fine.

Post Edited (Dr_Acula (James Moxham)) : 1/21/2009 7:37:26 AM GMT

01-21-2009, 03:16 PM
Well, you can also order MAX232 from the US at a price lower than $0.90 per piece. The TI ones are much cheaper than the Dallas ones.

Regarding the voltage levels have a look at the hardware stickies in this forum: 'HOW TO SAFELY INTERFACE A 5V SIGNAL TO THE PROPELLER?' I would not connect it directly to RS232. Stay with the MAX232 or build a converter yourself from two transistors and a few resistors.


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01-21-2009, 05:14 PM
Have a look at these two options .. from 'St Microelectronics' and 'Intersil' - Both are Max3232 drop in replacements .. and are 3v - 5.5v devices.

I have included pdf's for both.

The ST Microelectronics option is available from Radionics - radionics.rs-online.com/web/ (http://radionics.rs-online.com/web/)
Specifically - radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4152111 (http://radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4152111)

The Intersil version is available from Farnell - www.farnell.com/ (http://www.farnell.com/)
Specifically - ie.farnell.com/intersil/icl3232cpz/rs232-transceiver-3232-dip16/dp/1201546 (http://ie.farnell.com/intersil/icl3232cpz/rs232-transceiver-3232-dip16/dp/1201546)

Incidentally I have used both without issue.

Hope this helps..


Edit: As a reference I have added a·Pdf of the the RS-232 PropStick schematic as used by parallax.



'Necessity is the mother of invention'

Those who can, do.Those who can’t, teach.

Post Edited (QuattroRS4) : 1/21/2009 10:29:07 AM GMT

01-21-2009, 10:22 PM
Thank you for all the replies. I needed these info to clear up some things in my mind.
The links and files are very helpful

01-22-2009, 05:45 AM
...and here is another link: www.parallax.com/tabid/254/Default.aspx (http://www.parallax.com/tabid/254/Default.aspx). The second image headlined 'Serial to Propeller' shows a level converter Propeller to RS232. I knew that I've seen a schematic for a discrete converter recently... http://forums.parallax.com/images/smilies/smilewinkgrin.gif

However, I would put diodes in parallel to the resistor between basis and emitter of the transistors taking RS121 as inputs. Haven't looked up the datasheets but usually they don't like negative VBE.

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