Writing 4 bytes to COM Port
 

Hello all,

I have a device that has 8 switches (relay). This device has controller which is linked to COM Port. In order to manipulate switches first the device needs to be intialized with 4 bytes: 1 1 0 and the fourth is XOR of these first three, (0). So I need to send 1 1 0 0 in order to intialize this device.

I've tried to open com port and write this as characthers:

"00000001 00000001 00000000 00000000" - (no space between)

There is also some kind of driver, a monitor tool that monitors traffic between PC and the device. This monitor after sending above string says that only 2 bytes are sent. The device will not respond until 4 bytes are sent.

Does anybody have any clues that can be useful? I've searched everywhere but nothing to solve it, so I am trying my luck here :)

Thanks in advance, asmith

[added]: tried with c and yes it is char type

What you mean by writing as characters? Do you mean C data type character, then make sure that you are not using string API



Visu

Hi,
You must make own program with following windows API func.
CreateFile(), SetCommTimeouts, SetCommMask, GetCommState, BuildCommDCB and finaly WriteFile to send data

If you must only to write these 4 bytes to COMx, this can be done as synchronous write (no need for timeouts e.t.c).
In MS SDK (section WIN32 development, Communications) are some samples in C++ - do needed modifications, build and try. Search MSDN for this.
Requierad parts are:
Create file "COM1" with expected attributes - without Overlapped in sync. op. case,
DCB modification (speed, parity ...) - GetCommState,SetCommState,
Write data to created file,
Close handle.
Output data must be formatted as byte array.

In your case, you can pass pointer to int (0x0101) variable, length =4 as data buffer for WriteFile.

DWORD l = 0; //return value for really sent bytes
DWORD buf = 0x101; // data block 01, 01, 00, 00
WriteFile(hCom, &buf, 4, &l, NULL); //send buffer to opened COM port

or, if response isn't expected
for (int i=3; i>=0; i--) TransmitCommChar(hCom, (BYTE) (i>>1));
this function works very well for comm init strings sending w/o response.

If you must handle some incoming packets too, better would be use async. operations mode with multithreading, else this all can hang up while waiting response ...