Decabox: RS232 to MIDI

This not-so-obvious firmware revision was first commissioned by the production crew of a long-running (nearly 30 years) game show televised in the USA.  One of the show's main set pieces was controlled by custom software running under DOS. This worked perfectly for a long, long time.  However, eventually the DOS system needed to be updated, as spare parts were no longer available. When the dust settled, we ended up with two firmware variants: The first was a simple baud rate / protocol converter.  RS-232 data at 38,400 baud was received, then sent back out the door as MIDI information.  The DecaBox neatly handles the changing baud rates (MIDI runs at 31,250) and signal types (voltage level vs. current loop).  All data generated by the RS-232 source is retransmitted, with a system latency of about 300 microseconds per byte.  Pretty fast. This allowed the production crew to generate raw MIDI messages using their legacy equipment, while taking advantage of today's MIDI sequencers, tone generators and so forth. However, the crew soon realized that, in addition to the RS-232 messages generated by the legacy gear, they needed to receive 'live' MIDI data from other equipment, and have data from both sources re-transmitted through the same MIDI cable. At first glance, this is an easy task.  What happens, though, if messages are received simultaneously from both sources?  The resulting interleaved datastream could be very confusing. Our solutions was to edit the RS-232 sending protocol to include a start-of-packet byte and a size-of-packet byte:

<stx> <NUM> <data data data>

Thus, serial packets are held until they're completely received.  Then, they're transmitted and incoming MIDI data is buffered.  Once the serial data is sent, any buffered MIDI bytes go out the door.

It works perfectly, and no data is lost.

It might be a simple way to get MIDI information out of a Crestron / AMX automation system, or something similar.  Obviously, USB-MIDI interfaces are only a few dollars these days.  But, sometimes USB isn't available.

Update 2/27/2012 Due to customer requests, the MIDI/RS232 bridge is now bidirectional.  The MIDI in and out ports on the DecaBox are both fully functional.  This was inspired by a systems integration group who wanted to control a Yamaha mixing console from their AMX controller.  The standard Decabox was providing them with perfect RS-232 to MIDI conversion, but they needed to know what feedback & status information was also being generated by the console.  So we edited the firmware to convert incoming MIDI to outgoing RS232.  They just had to add a second MIDI cable to the installation.  Easy!