LabWindows/CVI - OpenComConfig usage

[mpb]

Hi all,

I do not understand how OpenCom[Config] works:

this routine takes 8 parameters. The first two are very confusing:

param_1: port number

param_2: com port name

Why do  I have to specify both? I understand that port 1 is not necessarily COM1, but on my PC COM1 is broken and I have a USB emulator with 4 ports that have got names COM4 COM5 COM6 and COM7. I seem to only be able to make this OpenComConfig work with port Num=1 and name="COM4". For example, using OpenComConfig(4, NULL, ...) does not work. If I write a DLL that should be deployed on potentially different PCs, how do I know what port is associated with which COM# name? On my PC it seems that port 1 is COM4, but on a PC with a working COM1 I suppose COM4 would be port 2... Very confusing! Why on Earth doesn't OpenComConfig just take a COM port name??

Thanks

Marco

[mpb · Accepted solution]

Ok,

this was simpler than I thought! Passing a port number of X is "asking" OpenComConfig to associate the "COM#" name with the handle X. This port is not the port number that the OS associates with the port name, but a "file descriptor" to be used for later calls. As if the current process had a table of "COM ports open" and we were requesting to associate the handle to that COM port with the entry X into that table. This is a "return" parameter, definitely not an "input parameter"! The documentation is a little confusing, but I was not helped by the fact that there was a bug in my code making it working only when apparently associating the "first available port" (COM4 in my case) with the "handle" 1.

For example, one may do

int comPort = 0;

openCom(name)

{

...

   OpenComConfig(comPort++, name, ...);

    return comPort-1;

}

and then

{

...

    int com = openCom("COM4");

    ComWrt(com, buf, len);

}

Regards

Marco