Problem with db9 serial port to max232 to xbee connection

Hello Guys,
I have a project about communication between two xbee modules but I have some trouble. I used FT232RL for one xbee module to programme it and I didn't have any problem. I could reach to module. To programme other one, I used db9 and max232 but I couldn't reach to modem. After I pressed Test/Query button I had that message "unable to communicate with modem". I used that circuit "http://www.egr.msu.edu/classes/ece480/capstone/spring10/group02/files/Jason_Grimes_AN_ECE480_SS10.pdf" (page5).

I added one more capacitor from pin16 to ground and all of the values of the capacitors are 1 µF. Max 232 works with 5V and my xbee modules work with 3.3v. Do I have to add any resistor between input of xbee and output of db9 or vice versa? I asked this question because I connect pin2 of xbee to pin11 of max 232 and pin3 of xbee to pin12 of max232 directly. I know that xbee modules work with 3.3v but I don't know how much volt is there on pin 11 and 12 of max232. Btw, I am using X-CTU. I tried everything and I need help. I am not sure if I could explain it clearly because I am really confused. I am sorry for that. If you need another information that makes you more clear, feel free to ask. Thank you very much.

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Btw, I am giving data to xbee from PC through max232 and giving power from external power supply to feed my xbee module and max232. Might it be my problem?

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I used that xbee module : "http://www.google.com.tr/imgres?um=1&hl=tr&biw=1454&bih=704&tbm=isch&tbnid=N6KtT8nauRzrKM:&imgrefurl=http://www.warungrobotika.com/&docid=8ar0odfsapC5dM&imgurl=http://4.bp.blogspot.com/-Ul7iemdbeiA/ToUsitBRb-I/AAAAAAAAAdc/-kUJdKylO1k/s1600/XBP09-DPSIT-156.png&w=337&h=283&ei=GPk0UPetJtC0hAf644HADA&zoom=1&iact=hc&vpx=715&vpy=388&dur=2951&hovh=207&hovw=246&tx=116&ty=158&sig=115542301877489106039&page=1&tbnh=162&tbnw=193&start=0&ndsp=21&ved=1t:429,r:17,s:0,i:120"
I don't know if it changes stg

The volt levels that distinguish a high from a low are different in 5V devices, as compared to 3.3V devices.

When sending data from one to the other, you must make it compatible.

To step up is often possible with a simple transistor. To step down it seems like a simple resistor divider ought to work, but I've seen schematics that are more complicated for some reason.

Customary advice says you should not apply higher volt levels at any pin on an IC, than its supply voltage. Nor lower than its ground pin.

Hi BradtheRad,
Thank you for your reply but I couldn't fix the problem. I still have a problem to connect my Xbee module. I need to take power from DB9 not to use any external power supply. I need to feed max232 and my Xbee module through DB9. Could you help me about to take power from DB9? Thank you for your interest.

The method below shifts 3V pulses to 5V.



This assumes you have 5V supply available in the bundle.

Notice the incoming signal can be cleaned up even when its peaks do not go all the way to zero or 3V.

If you use one stage it inverts the incoming signal. Use two stages to restore polarity.

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In case you want to reduce 5V pulses to 3.3V, here are schematics showing crude methods.



It's important to limit the input at your 3.3V device, because it should not wander outside a device's supply rails. A zener diode does the job. However it may be tricky to obtain a threshold of 3.3 V. There are ways to 'bend' a zener value.

If your signal does not go all the way to ground, then it may not be interpreted as a low. A diode inline reduces your signal by about .6V.

These simple methods may or may not work with your setup, at your communication speeds.