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Author Topic: (Newbie) MAX233 is not outputting negative voltages  (Read 3390 times)

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Offline klaxxonTopic starter

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(Newbie) MAX233 is not outputting negative voltages
« on: July 26, 2007, 07:45:56 AM »
Hello all, just wanted to see if anyone else has experienced this or has any suggestions...  I ordered some free samples of the MAX233 chip to build myself a TTL-to-RS232 serial converter. (The MAX233 is the same as a MAX232 but with internal capacitors).

After constructing the cable and triple-checking all my connections, when I try to send data over the line, the computer is receiving inverted bits.  I got out the voltmeter and discovered I could not measure ANY negative voltages coming out of the MAX233, not even on the V- pin.

Notes:
- I am providing the chip with regulated +5V
- I am connecting the TTL end to an AVR mega48 (actually a Baby Orangutan controller)
- I have checked and re-checked all my connections, and as far as I can tell, everything is hooked up as specified in the MAX233 datasheet
- I'm using a 1.0 uF electrolytic capacitor between Vcc and GND (as specified in the datasheet)
- Pins 6 and 9 (both GND) are tied together
- I'm using T1 in/out, and R1 in/out, T2 and R2 are left floating (should they be tied high or low?)
- I have tried the connection with two separate MAX233 chips, they both provide the same results

The only thing I can imagine is that I ordered the wrong variation of the part...?  The chip is labelled MAX233CPP, and I'm afraid I know nothing about IC suffixes (what does the CPP mean?).

Any advice or assistance would be appreciated.
In the meantime, I went ahead and ordered a properly-manufactured TTL-to-RS232 conversion cable from RobotShop.ca, but I would like to solve this problem, if only as a learning experience.

Datasheet: http://datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdf

Thanks,
- Adam

Update:  Attached is a quick-and-dirty diagram of my connections.  Can anyone see anything obviously wrong with this?  Even if my connections are incorrect, I still can't figure out why the chip doesn't appear to be providing any negative voltages.
« Last Edit: July 26, 2007, 11:59:55 AM by klaxxon »

Offline brijesh

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Re: (Newbie) MAX233 is not outputting negative voltages
« Reply #1 on: July 26, 2007, 01:55:22 PM »
MAX233 works with single +5V supply voltage by internally generating negative voltages that are required to generate the negative output signal.

In your case, in the figure you have shown you have put only one capacitor between supply and ground.
The circuit requires 4 more capacitors as shown in Figure 8 of the data sheet you have attached.
The other capacitors are needed for internal negative voltage generation.

Take note of the polarity of the capacitors.

Brijesh

Offline klaxxonTopic starter

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Re: (Newbie) MAX233 is not outputting negative voltages
« Reply #2 on: July 26, 2007, 02:28:36 PM »
From the datasheet:
"The MAX225, MAX233, MAX235, and MAX245/MAX246/MAX247 use no external components and are recommended for applications where printed circuit board space is critical."
...and...
"MAX233: No. of Ext. Caps: 0" (from the chart on page 1)

>The circuit requires 4 more capacitors as shown in Figure 8 of the data sheet you have attached.
Figure 8 refers to the 223, not the 233 that I am using.

Figure 11 indicates that the capacitors are integrated internally on the MAX233.

Am I mistaken?
« Last Edit: July 26, 2007, 02:38:17 PM by klaxxon »

Offline brijesh

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Re: (Newbie) MAX233 is not outputting negative voltages
« Reply #3 on: July 27, 2007, 06:58:47 AM »
opps..true Fig 8 is for 223, I misread in a hurry.

In Figure 11, note that capacitor pins and negative voltage pins are connected.
Have you done the same?

brijesh

Offline klaxxonTopic starter

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Re: (Newbie) MAX233 is not outputting negative voltages
« Reply #4 on: July 27, 2007, 08:42:59 AM »
Wow, I can't believe I missed that....  I've been staring at the datasheet for so long, I never actually noticed the connecting lines in amongst the jumble of pin numbers and labels.

I'll give it a try and followup with my results.

Thanks for your help!

- Adam

 


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