Hi all,

I am trying to use a RS232 port to test if a key switch is turned off/on so I can enable/disable the sending of E-Mails from my ZoneMinder system.

I have done a lot of research and some experimentation and I thought I had if fugured out, but it appears I burned out my UART on the motherboard.

Here is what I was doing, could you take a look and see if I am way off base, or maybe suggest another idea.

First some documentation...

  ___________                    
  \1 2 3 4 5/  Looking at pins   
   \6 7 8 9/  on male connector  
    ------ 
Pin #   Acronym  Full-Name   Direction  What-it-May-Do/Mean
9-pin   
 1      DCD     Data Carrier Detect<--  Modem connected to another
 2      RxD     Receive Data      <--   Receives bytes into PC
 3      TxD     Transmit Data     -->   Transmits bytes out of PC
 4      DTR     Data Terminal Ready-->  I'm ready to communicate
 5      SG      Signal Ground
 6      DSR     Data Set Ready    <--   I'm ready to communicate
 7      RTS     Request To Send   -->   RTS/CTS flow control
 8      CTS     Clear To Send     <--   RTS/CTS flow control
 9      RI      Ring Indicator    <--   Telephone line ringing

I used a Key Switch, a 6.8K Resistor, and a RadioShack (276-0041) 2.6V - 28 maA LED in this circuit.

When the Key Switch is open, the LED is lit and I consider the system "Armed".

RS232        6.8K       Key Switch
1 CD  ______/\/\/\___________/ ___________
                     |              |    |
      ^              |              |    |
      |              |_____>|_______|    |
      |4.49V              LED            |
      |                                  |
                                         |
4 DTR ___________________________________|

The current flow and voltages are:

                      Switch Open           Switch Closed
                ----------------------   --------------------
Resistor       :    2.67V  .373mA            3.75V    .524mA
LED            :    1.82V  .373mA            0.0V      0.0mA
Pin 4 to Pin 1 :    4.49V  .373mA            3.75V    .524mA

I am using Linux and have written the following program to test if the switch is Open/Closed (Armed/Disarmed).

#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <unistd.h>

main()
{
  int fd, status;
  while( 1 ){
    sleep( 2 );
    fd = open("/dev/ttyUSB0", O_RDONLY);
    if (ioctl(fd, TIOCMGET, &status) == -1) {
      printf("TIOCMGET failed: %s\n", strerror(errno));
    }
    else {
      if (status & TIOCM_CAR) {
        puts("TIOCM_CAR - Alarm off!!!");
      }
      else {
        puts("TIOCM_CAR - Alarm is armed!!!");
      }
    }
  close(fd);
  }
}

It works just fine. The LED flashes off every two seconds when the port is being read. The voltage and current readings seem to be well within tolerance.

However, as I said at the top, it ran for a while but now the serial port no longer works. So, can you spot something I have overlooked before I go and burn out all my COM ports?

Thanks, Keith

 

Hi again,

Well no body replied so I'm guessing one of three things:

1) I stumped everybody.
2) I didn't give enough information.
3) Nobody has any experience with RS232 ports.

Does anybody know another forum where I could ask?

Thanks, Keith