FTDI cable / RS232 Data Modem question

I have been playing with some wireless modems that are RS232.  I was thinking they should should be plug and play with my FTDI cable right?  The cable is 5V and the modems are 5V and so are the rx/tx so I dont need a level converter right?  I am trying to get them to communicate with the software used to program them with no luck.  I swapped the rx/tx lines with no luck there either.  Do I need to buy a USB to RS232 converter?

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Replies

  • Neil -

    Its been a couple of years - but were you ever successful with this integration effort?  I am considering a similar type solution.

  • If the modems are 5V then they are not RS-232. 

    RS-232 is +/- 12v, and a "1" is when the TX line is negative (reverse the standard TTL sense).

     

    If the modems are some form of bastardized "+5v RS-232" then you can _safely_ connect them but you may need to invert the logic sense to read the data. In this case you will need a hardware inverter; just reversing the incoming bits won't work because the start bit will have the wrong sense too -- the UART won't know when to begin. (If the modems are "5v RS-232 compatible" then they are *not* safe; they will use +/-5v and the -5v will harm Ardu circuits.)

     

     

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