Thread: Serial Port - Receive Packet

Hi guys,

I have a program that currently opens a serial com port on WinXP machine and sends data in a packet to an LCD screen. But some of the commands for the LCD screen require the program to receive packets from the LCD to WinXP.

Here is my send_packet() code. It store relevant data such as command, data_length, data, crc. The packet is called outgoing_response, as defined by the LCD screen manufacturers

Code:

void send_packet(COMMAND_PACKET *packet)
{
     ubyte i;
     unsigned char buffer[20];
     int index;
     index = 0;
     buffer[index++] = packet->command;
     buffer[index++] = packet->data_length;
     for(i = 0 ; i < packet->data_length ; i++)
             buffer[index++] = packet->data[i];
     
     packet->CRC.as_word = 
     get_crc((ubyte *)&outgoing_response,outgoing_response.data_length+2,0xFFFF);
     buffer[index++] = packet->CRC.as_bytes[0];
     buffer[index++] = packet->CRC.as_bytes[1];
     
     DWORD bytes_written;
     bytes_written = 0;
     
     if(handle)
       if(TRUE != WriteFile(handle,
                            buffer,
                            index,
                            &bytes_written,
                            NULL))
       printf("WriteFile() failed!\n");
     }

With this information can anyone help me to write afunction to receive packets from the LCD screen. the receive packet name is incoming_command as defined by the LCD manufacturer.

Thanks

Sure thing Mats,

this is what I got from the data sheet:

0 (0x00): Ping Command
The LCD will return the Ping Command to the host.
type = 0x00 = 010
valid data_length is 0 to 16
data[0-(data_length-1)] can be filled with any arbitrary data

The return packet is identical to the packet sent, except the type will be 0x40 (normal response, Ping Command):
type = 0x40 | 0x00 = 0x40 = 6410
data_length = (identical to received packet)
data[0-(data_length-1)] = (identical to received packet)

Hope this helps

Thanks Mats, should something like this do the trick? But it doesn't work? Do you know where I have gone wrong?

Code:

void receive_packet(COMMAND_PACKET *packet)
{
     ubyte i;
     unsigned char buffer[20];
     int index;
     index = 0;
     buffer[index++] = packet->command;
     buffer[index++] = packet->data_length;
     for(i = 0 ; i < packet->data_length ; i++)
           buffer[index++] = packet->data[i];
     
     packet->CRC.as_word =
     get_crc((ubyte *)&incoming_command,incoming_command.data_length+2,0x0000);
     buffer[index++] = packet->CRC.as_bytes[0];
     buffer[index++] = packet->CRC.as_bytes[1];
     
     DWORD bytes_written;
     bytes_written = 0;
     
     if(handle)
       if(TRUE != ReadFile(handle,
                           buffer,
                           index,
                           &bytes_written,
                           NULL))
       printf("ReadFile() failed!\n");
       printf("%s\n", buffer);
       getchar();
       getchar();
       }

I still don't fully know how to do it?
Do I call the readFile() function first?
How do I get and store the information of command and legth from the packet?

Code:

void receive_packet(COMMAND_PACKET *packet)
{
     ubyte i;
     unsigned char readbuf[20];
     int index;
     index = 0;
     DWORD bytes_read;
     bytes_read = 0;
     if(handle)
       if(TRUE != ReadFile(handle,
                           &readbuf,
                           MAXBUFFERSIZE,
                           &bytes_read,
                           NULL))
       printf("ReadFile() failed!\n");
     readbuf[index++] = packet->command;
     readbuf[index++] = packet->data_length;
     for(i = 0 ; i < packet->data_length ; i++)
           readbuf[index++] = packet->data[i];
     
     packet->CRC.as_word =
     get_crc((ubyte *)&incoming_command,incoming_command.data_length+2,0xFFFF);
     readbuf[index++] = packet->CRC.as_bytes[0];
     readbuf[index++] = packet->CRC.as_bytes[1];
     
   
     

       printf("%s\n", readbuf);
       }

How about this?

Code:

void receive_packet(COMMAND_PACKET *packet)
{
     ubyte i;
     unsigned char readbuf[20];
     unsigned char received[20];
     int index;
     index = 0;
     DWORD bytes_read;
     bytes_read = 0;
     if(handle)
        received[index++] = ReadFile(handle,
                           &readbuf,
                           index,
                           &bytes_read,
                           NULL);
       printf("ReadFile() failed!\n");
     //readbuf[index++] = packet->command;
     //readbuf[index++] = packet->data_length;
     for(i = 0 ; i < packet->data_length ; i++)
           readbuf[index++] = packet->data[i];
     
     packet->CRC.as_word =
     get_crc((ubyte *)&incoming_command,incoming_command.data_length+2,0xFFFF);
     readbuf[index++] = packet->CRC.as_bytes[0];
     readbuf[index++] = packet->CRC.as_bytes[1];
     
   
     

       printf("%s\n", received);
       getchar();
       getchar();
       }

Yes you are right, I don't really know what I am doing. Can you explain it to me please?

At this stage I know how to write a file to the Com port. One of the commands write a ping request to the LCD screen but I don't know how to read the data back from the LCD screen?

This is the latest readFile() code that I have

Code:

void receive_packet(COMMAND_PACKET *packet)
{
     int ret;
     unsigned char buf[2];
     unsigned long len;
     
     ret = ReadFile(handle,
                    buf,
                    1,
                    &len,
                    NULL);
     if(len > 0)
     {
            //if(trace == 1)
            printf("read char %d\n", (int)buf[0]);
            }
}

How does that look?

But when I open a hyperterminal I know that I am not reading anything?

these are the list of commands that I call in the main()

Code:

outgoing_response.command = 0; // coomand to send ping
outgoing_response.data_length = 0;
send_packet(&outgoing_response);
receive_packet(&incoming_request);

Can you tell me what I am doing wrong?

I really dont know how to intercept or catch the response from the command?

Yes the first thing I get back should be the command.

The packet type for this particular packet as far as I know is 0x40 as defined in the data sheet

0 (0x00): Ping Command
The LCD will return the Ping Command to the host.
type = 0x00 = 010
valid data_length is 0 to 16
data[0-(data_length-1)] can be filled with any arbitrary data

The return packet is identical to the packet sent, except the type will be 0x40 (normal response, Ping Command):
type = 0x40 | 0x00 = 0x40 = 6410
data_length = (identical to received packet)
data[0-(data_length-1)] = (identical to received packet)

which is a ubyte command as defined in the data structure

Code:

typedef struct
  {
  ubyte
    command;
  ubyte
    data_length;
  ubyte
    data[MAX_DATA_LENGTH];
  WORD_UNION
    CRC;
  }COMMAND_PACKET;

With this information and the previous readFile function would you be able to help me to receive the dat please? Do I need to set up a thread or process to wait for bytes to be read? Could you help me write this please?

Hi Mats

here is my attempt at understanding what you mean.

Code:

void receive_packet(COMMAND_PACKET *packet)
{
     int ret;
     unsigned char buf[2];
     unsigned long len;
     unsigned long length;
     len = 0;
     char cmd[2];
     int d_len;
     
     while(len != NULL) {
     ret = ReadFile(handle,
                    buf,
                    1,
                    &len,
                    NULL);
     if(cmd == NULL) {
     cmd = buf;
     }
     if((cmd != NULL) && (d_len == NULL)) {
             d_len = buf;
             }
             }
     if(len > 0)
     {
            //if(trace == 1)
            printf("read char %d\n", (int)buf[0]);
            }
            
     
}

would you mind writing a small code snippet of your solution please. I cant fathom this at all.