This is not easy to do with the browser technology. Since you need to do it on each client, not on the server, the browser will have to run some software to send the signal to the hardware port. It can be this DOS utility if you can copy this utility to each client.
The problem is to run this software from the browser. There are two technologies that can do this: Java applets and ActiveX controls. There are problems with both: Java applets probably won't let you run any software on the client because of their strict security policy. ActiveX controls will most probably only run in the Internet Explorer (I could be wrong here). If the latter is not a problem you can make an ActiveX control to do this (how to make an ActiveX control is beyond the scope of this forum). Then you will need to embed this ActiveX control in some Aware IM form. If you get to this stage let us know and we will advise you how to do this.