First of all, thanks for you reply, I'm great appreciated.

Let me brief you what actually the requirement in PC site.
The PC is responsible for one major task only, which is talk to an external device (security PIN pad, a smart card reader) and send the response back to mainframe. The scenario is when mainframe received a payment instruction from their client in EDIFACT format, it need to authenticate the message, so the mainframe will send a so-called digital signature which is derived from the EDIFACT message to PC for verification purposes, and PC will pass the digital signature to the smart card reader, get the response and send back to mainframe. It's on line. The PC program must keep listening to the incomming messages and response it respectively.

The PC will be one of the workstation in their local network (SNA), the mainframe people dictated using 3270 as the communication bridge. They suggested put the string (messages) in a file and transfer the file using 3270 terminal emulation program. This is something back to ten years ago, most important, this method will not solve the problem, and inefficient if it's on line. I've done a project several years ago, using 3270 terminal emulation as a file transfer utility, which is in DOS environment, there are a few utilities come with 3270 for PC, like FTP, there are programs, run directly from DOS prompt, our program have to run the 'command.com' to execute it, and we would know what's happenning to the process when the control is pass to the external program, the only stupid way is wait for the external program finish, and check the predefined path if any files available.

Now the smart card reader come only with 32bit DLL for windows. PC program have to be developed using one of the Windows development tools. So is your suggestion?

>>Sorry, this question is not suppose to be here, but I coundn't find it elsewhere, my client is chasing me.
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>>I'd like to a send a string to IBM mainframe and vice-versa, any idea?
>>such as:
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>It's going to depend entirely on how you connect the PC to the mainframe (LAN, dial-up, any of several specialized serial conections) the operating system run on the mainframe, and the capability of the software on both the mainframe and PC. One rather ubiquitous package is AttachMate; you might look into that, but regardless, the mainframe site management people will dictate just about everything related to the connection and capabilities their system will support. You need to talk in detail to the client's technical people to determine what they'll let you do and specify the details of the connection.
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>This can range anywhere from being a completely trivial task, to one where you'll need a great deal of knowledge not just of a PC, but of the mainframe environment. You probably should pass this 'opportunity' by if the client needs a price before giving you the necessary information to see what's involved.