The Blackmagic Tally box has a 25 pin output and our Datavideo TB-5 tally box has a 15 pin input:
https://www.markertek.com/product/bmd-swtalgp18/blackmagic-swtalgpi8-gpi-and-tally-interface
http://www.datavideo.com/us/Intercom+Systems/TB-5
Users would need to make a custom cable (we do not sell this accessory) to connect the two boxes
Alternately, they can connect the boxes to D-Sub breakout boards
Attached is the wiring for the TB-5 (same as our SE-500)
The wiring for the Blackmagic Tally is printed on the bottom of the unit.
These breakout boards with jumper wires allow you to use standard DB15 and DB25 male to male cables:
DB15 Female Breakout: http://www.gravitech.us/dbfevbrbo.html
DB25 Female Breakout: http://www.gravitech.us/dbfebrbo.html
Blackmagic DB25 pins 23, 22, 21, and 20 go to Datavideo DB15 pins 1, 6, 11, and 5 respectively.
An additional wire can go to one of the ground pins on both boards
For an additional 4 lights,
Blackmagic DB25 pins 19, 18, 17, and 16 go to a second TB-5 with Datavideo DB15 pins 1, 6, 11, and 5 respectively, plus ground