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Electronics => Electronics => Topic started by: airman00 on April 24, 2008, 09:52:21 PM

Title: RS232 Interface Voltages
Post by: airman00 on April 24, 2008, 09:52:21 PM

I have this DTMF decoder  http://www.dschmidt.com/dtmf.html

It says it has True RS232 Voltages

I want to connect that decoder to the OOPIC R board's male DB9 connector  directly
(http://www.acroname.com/robotics/parts/R180-OOPIC-R.jpg)

However online there is a warning for using RS232 with the OOPIC R

Quote

The serial input and output signals are TTL level signals providing 0 and 5 volts. Conversion to RS232 signals can be done with a TTL to RS232 signal converter chip such as the SN75188 or the MAX203 which will provide the voltage conversion to the RS232 levels of +12 and -12 Volts as well as providing the required signal inversion.

CAUTION: do not connect RS232 signals lines directly to the ooPIC's TTL level I/O.  The higher voltage of the RS232 lines will damage the TTL level circuitry.

My main question is how come I can't connect the DTMF decoder directly to the OOPIC but I can connect the OOPIC directly to my computer's serial port for programming . Isn't it all RS232 voltage levels?

Thanks,
Eric

Title: Re: RS232 Interface Voltages
Post by: airman00 on April 25, 2008, 07:41:16 AM

So heres what I found out after skimming an ebook about OOPIC microcontrollers

Quote

The OOPIC R also has a DB9 serial connector port and the associated RS-232 level translation hardware required to connect it to any computer serial port.

and after careful analysis of the schematics , lo and behold ! , a RS232 shifter

So that solved my problem

The OOPIC manufacturers should really rewrite their statement that RS232 levels are not supported directly , it would have saved me some time.

Title: Re: RS232 Interface Voltages
Post by: Tsukubadaisei on April 25, 2008, 07:46:30 AM

Your answer is in the Caution line. They say not to connect the serial DIRECTLY to the TTL bus. When you connect the OOPIC to the computer you use that serial port soldered to the PCB, right? That serial port is problably connect to a RS-232/TTL converter that is connected to the OOPIC chip's TTL port. However two of those pins soldered around the PCB are also  connected DIRECTLY to the OOPICs TTL port. So, if you connect you decoder by using that serial port(the same one that you use to connect to the computer) then I am pretty sure it will be fine. But if you connect the decoder Output directly to the TTL pin the you will burn the controller. I have never used neither the oopic nor the decoder but I have seen already lots of parts that work similary, including my first micro controller.

Title: Re: RS232 Interface Voltages
Post by: Tsukubadaisei on April 25, 2008, 07:47:56 AM

Crap!!! You just solved your problem while I was solving it! Next time gime some extra time!

Title: Re: RS232 Interface Voltages
Post by: airman00 on April 25, 2008, 07:48:44 AM

 :D :D :D

thanks for your reply anyways!