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Remote Access of Multiple Processors - Chris Clement, MSCS, CDP 9/27/2008

Past methods have utilized modems and separate PBX lines. A simple firmware code change turned a Type I processor into a cascadeable 1-of-16 selector. Legacy data ports for serial and parallel data were easy to use and quite reliable under several versions of DOS. Rapid changes in telecommunications technology, pc software, and pc hardware have made this functionality very difficult to support beyond any given four-year period. Choosing one of four com ports is still possible but quite iffy on many pcs. We are often lucky to get one operational. USB adaptors are notorious for reassigning physical connections between virtual com ports and physical USB slots which can scramble data collected into the wrong folders for different processors. As a general rule it would be wise to externalize as much of the remote selection as possible. For example, using different com ports is limited to 1 through 4 by the Microsoft architecture and, as mentioned above, we are lucky to get one working. Bi-directional use of a single com port was used for modem AT commands as well as the cascadeable switch (above) but the increased complexity of Windows drivers has sometimes resulted in leaving a port "contaminated" and an unconfirmed selection status when data is to be received following a transmission.

A more robust means of remote switching by running a program on a remote desktop could cause a circuit switching device at the remote location to act without relying on any traditional pc data ports. One well-supported pc feature these days is the sound card. If we can produce DTMF tones from the speaker, we can then control an external switch that can decode the sounds. There are freely downloadable programs that provide a simple touchtone (TM) pad and the user can operate it by pointing and clicking. There are numerous sources and if one fails to work, another can be tried at little or no cost. The following links yielded free desktop touchpads that worked under MS Vista. No other warranties are implied, however.



A web page with all 16 single tones is available /micromtr/dtmf/dtmf.htm . This is less intrusive and will make no changes to your pc. Tones were derived from the second link.



This will control a CM8870-based device which responds to DTMF tones produced by speakers. A remotely-controlled desktop application can use the tone generator to switch processors to be read with this circuit within audio range of the remote pc. Verify proper operation with moderate volume settings using the indicator LED's. The "D" code is low-value nybble and "C" is high-value nybble. A customized version of this device is available for any remotely-controlled multi-processor installation.

4/16/2009 - Coming soon. A Rabbit Core 3720-based controller completely web-enabled.

New Rabbit uPC software - access mM from cellphone! (see Downloads).