Datum Systems PSM-500L Manual do Utilizador descarregar pdf (Página 56)

Datum Systems, Inc. PSM-500 Modem Remote Control Protocol

Rev 0.93 12/12/10

B-56

Notes on Creating a Controller Mechanism

I get occasional questions on writing M&C or controllers to talk to and remotely control either the M5 class

or M500 class modems. Here are a few suggestions on making a working controller.

1. The modem returns a response very quickly, usually only a few milli-seconds after the end of your

response message. You have to be ready to accept the response immediately.

2. If you are using a PC running Windows, the COM port must be set to a binary mode and not do

any manipulation of the incoming data. There are no carriage returns or line feeds that would be

used to read a line at a time or an entire response packet.

Linux computers seem to be even more difficult to get into a pure binary character receive mode.

Here is a short snippet showing one way to set the COM1 equivalent in Linux which is “ttyS0”.

The variable “devicename” in this case is a string set to “/dev/ttyS0”. This snippet does not show

any actual operations other than setting up the port.

fd = open(devicename, O_RDWR | O_NOCTTY | O_NONBLOCK);

if (fd < 0) {

perror("Device could not be opened");

exit(-1);

}

tcgetattr(fd,&oldtio); // save current port settings

// set new port settings for canonical input processing

newtio.c_cflag = B9600 | CS8 | /*STOPBITS | */CLOCAL | CREAD;

newtio.c_iflag = IGNPAR;

newtio.c_oflag = 0;

newtio.c_lflag = 0; //ICANON;

newtio.c_cc[VMIN]=1;

newtio.c_cc[VTIME]=0;

tcflush(fd, TCIFLUSH);

tcsetattr(fd,TCSANOW,&newtio);

close(fd);

3. The modem’s processor is an Infineon C166 derivative, and it writes words in classic Intel or any

X86 type “Little-Endian” style. If you are using a controller that is “Big-Endian” like the

Motorola/Freescale processors then the byte order of a variable is reversed and will have to be re-

ordered before evaluating or sending back to the modem.

4. Calculating checksums is not difficult as long as a computer is doing it. Here is a simple “C”

routine to calculate checksums. It could be made considerably simpler, but it has the ability to start

and stop anywhere with a string of characters representing a packet.

/* -----------------------------------------------

Calculates the 8 bit checksum for modem packets

----------------------------------------------- */

static char checksum8 (char *s, char start, unsigned char len)

{

unsigned char i, sum8 = 0;

for (i = 0; i < start + len; i++) {

if (i >= start)

sum8 = (sum8 + s[i]) % 256;

}

return (256 - sum8);

}

5. If you want to define Records or Structures to hold the variables of a particular message, those

variables or the whole structure will normally have to be specified as “Packed”. This is because

most languages will use, for example, a 4 byte integer as their least standard variable size. Packing

should allow you to copy the message data directly into the instance of the structure.

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