Monday, April 11, 2011

An introduction to Intercom

Intercom was originally written in C code for inclusion in C programs. For C programmers this was quite acceptable. Unfortunately the vast majority of non-professional programmers do not work in C, so something needed to be done.

I've taken the original C code and placed it into a plain C DLL. Most higher level languages like C#, VB.Net, VB6 and Delphi can utilize the subroutines in a C DLL. When I originally wrote about Intercom (way back in 1999 – You can tell, because the title of the example application reads: “Intercom with CC2), I was a professional Visual Basic programmer I used VB6 as my language of choice in the following example.

First, what is intercom? Intercom is code that creates a memory-mapped file that both CC3 and your program can use. What this means is that in memory a chuck of space is set aside, half for you and half for CC3. You can only read your half and you can only write to CC3's half. CC3 on the other hand can do just the opposite.

Whenever a new message is written, a 'New mail flag' is raised. CC3 looks for a raised flag automatically. Your program needs to check periodically.

Second, what is it that is sent back and forth between CC3 and a program? Intercom sends an integer and a string back and forth. To CC3 these have very strict meanings. The integer is part of the name of the macro it will now run and the string is used as input data.

If you sent CC3 via intercom the number 123 then the macro CC3 will attempt to run in named RCV123 and the string will now reside inside the variable RCVDATA.

CC3 ships with an important macro: RCV20.  If you run the exe in the VBIntercom zip file (Intercom.exe) and enter 20 and LINE 0,0 1000,1000 you will see.


See? The macro RCV20 contains nothing but RCVDATA in it. This macro will run whatever text is sent to it, so this means that the calling program can dynamically build and run scripts/control CC3.

To connect to the DDL, you need to use Declare Statements. See the code in the VB6 form for the exact syntax, later l will post C# examples.

Here is a link to the original C code for Intercom.

Here is a link to the Intercom code wrapped into a plain C DLL plus the old VB6 example.