8. User EnhancementsReverse-engineering cables and hacking powerd.c

Try to get documentation for the cables that your UPS seller supplies. In particular find out:

• What lines need to be kept high.
• What line(s) turn off the UPS.
• What lines the UPS toggles to indicate that:
  • Power is out.
  • Battery is low.

You then need to hack powerd.c appropriately.

If you have trouble getting the above information, or just want to check it (a good idea) the following program might help. It's a hacked version of powerd.c. It allows you to set the necessary port flags from the command line and then monitors the port, displaying the control lines every second. I used it as "upscheck /dev/cua1 2" (for example) to set the 2nd bit (DTR) and to clear the other bits. The number base 2 indicates which bits to set, so for example to set bits 1, 2 and 3, (and clear the others) use 7. See the code for details.

Here's the (untested) upscheck.c program. It's untested because I edited the version I originally used to make it clearer, and can't test the new version at the moment.

---

/*
 * upscheck     Check how UPS & computer communicate.
 *
 * Usage:       upscheck <device> <bits to set>
 *              For example, upscheck /dev/cua4 4 to set bit 3 &
 *              monitor /dev/cua4.
 *
 * Author:      Harvey J. Stein <hjstein@math.huji.ac.il>
 *              (but really just a minor modification of Miquel van
 *              Smoorenburg's <miquels@drinkel.nl.mugnet.org> powerd.c
 *
 * Version:     1.0 19940802
 *
 */
#include <sys/types.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>

/* Main program. */
int main(int argc, char **argv)
{
  int fd;

/*  These TIOCM_* parameters are defined in <linux/termios.h>, which  */
/*  is indirectly included here.                                      */
  int dtr_bit = TIOCM_DTR;
  int rts_bit = TIOCM_RTS;
  int set_bits;
  int flags;
  int status, oldstat = -1;
  int count = 0;
  int pc;

  if (argc < 2) {
        fprintf(stderr, "Usage: upscheck <device> <bits-to-set>\n");
        exit(1);
  }

  /* Open monitor device. */
  if ((fd = open(argv[1], O_RDWR | O_NDELAY)) < 0) {
    fprintf(stderr, "upscheck: %s: %s\n", argv[1], sys_errlist[errno]);
    exit(1);}

  /* Get the bits to set from the command line. */
  sscanf(argv[2], "%d", &set_bits);

  while (1) {
    /* Set the command line specified bits (& only the command line */
    /* specified bits).                                             */
    ioctl(fd, TIOCMSET, &set_bits);
    fprintf(stderr, "Setting %o.\n", set_bits);

    sleep(1);

    /* Get the current line bits */
    ioctl(fd, TIOCMGET, &flags);
    fprintf(stderr, "Flags are %o.\n", flags);

/*  Fiddle here by changing TIOCM_CTS to some other TIOCM until    */
/*  this program detects that the power goes out when you yank     */
/*  the plug on the UPS.  Then you'll know how to modify powerd.c. */
    if (flags & TIOCM_CTS) 
      {
        pc = 0 ;
        fprintf(stderr, "power is up.\n");
      }
    else 
      { 
        pc = pc + 1 ; 
        fprintf(stderr, "power is down.\n");
      }
    }

  close(fd);
}

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