3.3v Propellers being able to drive external serial devices?

Marksu

Hi, I’m trying to drive a Nubotics Wheelcommander (serially) from a Parallax Propeller Demo board without much luck.·
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I’ve calibrated the Wheelcommander using the Arconame USB/Serial device, and the Wheelcommander works fine from a BasicStamp demo board I have. ·
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So what I have tried so far is to –

• Set the Wheelcommander communication speed to 9600 baud
• Tested the Wheelcommander settings from a BasicStamp at 9600 baud. ·Works great….
• Created a SPIN (Propeller program) to drive a 9600 baud serial connection to a LCD panel (Tx only)
• The SPIN program wrote a ASCII ‘K’ with a linefeed (also tried CR) to the LDC panel, worked fine.
• Connected the Tx pin from the Wheelcommander to the pin I used with the LCD panel..
• Nothing…

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I was reading a thread below on debugging using the Propeller, and it made me think that maybe the 3.3v from the Propeller might not be able to drive the serial port of the Wheelcommander? ·
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Is there a circuit I could create the ‘amplify’ the signal?
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Any help would be greatly appreciated!
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Thanks!
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Mark
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Background info - ·
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http://forums.parallax.com/showthread.php?p=575016 ·(Forum discussion on driving serial devices from a 3.3v devices, and specifically ‘unverted signals’.. .)


After using Jon Williams' debug object for awhile, I found that I needed something simpler yet: just a self-contained object that twiddled an output pin directly -- no inputs -- and provided an inverted signal that would drive my PC's RxD pin without additional circuitry. Here's the result:
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My PC works fine with the Propeller's 0 and 3.3V signal levels (in lieu of -10 and +10), but this may not work for everyone.

mcstar

You could use any of the maxim max232 devices with appropriate capacitors and resistors. It uses the capacitors as a charge pump to get the voltage up to around 12volts. I have a plastic tube full of them since they come in so handy.

http://www.hemmy.net/2006/04/26/disney-animation-reuse/
http://www.alldatasheet.com/datasheet-pdf/pdf/27224/TI/MAX232.html

Marksu

Very cool, thank you!

I’m trying to understand the typical operating circuit for the MAX232. ·Would it be as simple as putting capacitors between the MAX232’s pins 1 & 3, 4 & 5 and then connecting my Prop’s serial tx to MAX232’s pin 8, and my external serial device’s receive pin to the MAX232’s pin 7? ·(Not sure why I would need to 8.5V comming out of the MAX232)
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Any suggestions on the size capacitors I would need to drive a typical serial device?
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Finally, do I need to ‘step down’ the voltage coming out my external serial device before I connect it directly to the Prop chip?
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(You can probably tell I’m a software guy and my pile of over 'powered' electronics is growing and I dont want to add my Prop board to it!…I've never·'smoked' a·software program by applying too much voltage/amperage...)
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Thanks again for your help!
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Mark