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RS-232 Shield Issues


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#1 chain

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Posted 16 April 2014 - 07:29 AM

I've combed through the documentation, the product wiki, and TI's datasheet on the MAX-232 chip to try and identify the source of this issue. I've got an Arduino Uno R3 connected to the RS-232 Shield (exact model and distributor https://www.sparkfun.../products/11958 )that I need to get communicating to a touch screen controller. After numerous attempts at bug fixes I eventually used the RS-232 tutorial (http://arduino.cc/en...noSoftwareRS232) to set up a serial console and confirm that the Arduino is using the serial port properly, and the serial console wouldn't operate. I'm now sitting with an oscilloscope and tracing voltage levels, and am finding that the output on the Tx line of the RS-232 port is identical to the output of the pin that is feeding it. That is to say that the logic doesn't seem to make the switch from TTL logic to EIA-232-F logic as described in the TI datasheet. 
I'm not sure if this indicates something wrong with the shield that I received, or if I'm misinterpreting the intended purpose of this shield, but if someone could point me where I might be making my mistake, or what I should check for, it would be much appreciated.

Some background: I'm a UMass Lowell senior Computer Engineering Student designing an LED Touch Panel, using a 3M Microtouch capacitive touch screen and firmware controller controlled by an Arduino Uno. The 3M controller sends touch screen data over RS-232, and I'm writing the firmware to be able to control the chip and interpret these signals. 



#2 chain

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Posted 16 April 2014 - 07:58 AM

RS232 shield is a level shifter. You can see its schematic:

RS232 schematics
 
This shield shift uart TTL serial(D0,D1) level of  Arduino UNO to RS232 level.
 
You can compare DB9 of your touch screen controller and DB9 of RS232 shield to see if the connection is right.


#3 chain

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Posted 23 April 2014 - 12:57 AM

So to be clear, I should have TTL serial logic alternating from ~0V to ~5V for logic 0 and 1 respectively at the arduino's Tx pin, which feeds into T1in of the MAX232 chip, and then T1out should instead alternate from ~+5V to ~-5V for logic 0 and logic 1 respectively. That's what you mean by shifting TTL serial logic to RS232 serial logic.

Does that mean, then, that if my T1out line is identical to my T1in line, except with a small voltage drop (.5-.75V, presumably from going through the chip), that my shield/chip is defective? My output lines on the DB9 of the shield aren't shifted, just have a voltage drop from propogating through the shield's circuitry.



#4 jim

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Posted 13 May 2014 - 04:07 AM

Can you please use a RS232 to USB cable to proof test?






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