At one point while trying to set up my rig to use a JamMan and a sequencer live, I encountered the need to get multiple MIDI outputs who's input could be derived from one of two outputs. The deal was, I wanted to sometimes slave my JamMan to the sequencer MIDI clock and sometimes slave my JamMan to the sequencer's clock. All of the off the shelf switchers with programmable control were quite expensive so I decided to just build one.
It turns out that there really isn't much to it. You need an opto-isolator to convert the MIDI in (current) to TTL (voltage) then a bunch of transistors to drive the outputs (one for each output). For simplicity, I decided to use a relay to select between the two MIDI inputs (the alternative would have been 2 opto-isolators and a TTL switch. The relay just required less parts).
The biggest issue was power and control to select which input to use. I really wanted the box itself to be simple so I decided to handle both of these issues externally. As it turns out, a hidden feature I slipped into the Lexicon Reflex proved to be just the ticket. When I was developing the software for the Reflex I couldn't help but notice that there were two unused TTL control lines coming off of the UART chip (we actually used them to debug sometimes). In the system software, I hard wired continuous controllers 94 and 95 (I think) to these two control lines. When the controller was at 127, the line was hi. When the controller was at 0, the line was low (for more information about this hidden feature, check out my Lexicon Reflex page).
Another little known aspect of the Reflex is that it's hardware design is almost identical to the JamMan. The idea was that if JamMan was successful, we could do a second version using the same board as the Reflex with different front panel paint and a BOM change. But thats a different story. One of the things a JamMan has that a Reflex doesn't is a second footswitch jack. Installed, that is. We actually laid out the board for the extra switch but never installed the jack and never provided any software support. This is where my MIDI Thru box comes in. What I did was cut the etches coming from the unused jack and ran a wire from one of the hidden control pins to the pc board pad connected to the tip of the jack. I then ran a wire from the +5v supply of the Reflex to the pad for the ring of the connector. Back on my MIDI Thru box, I added a similar connector wired the same way. Presto. Power and control.
If you end up building something like this, be cautioned that the Reflex needs to be connected directly to whatever is going to change its controller values. I set it up connected directly to the output of the sequencer. I configured the system so that the default connection was from the JamMan. The sequence would change the controller to the appropriate connection. Again, refer to my Reflex page for details.
| If you are attempting to build one of these units, I'll gladly answer any questions. Just email me at BobSellon@StecRecords.com. |
