I went the opposite way. I invented my "DMX-lator I" to get my
microplex dimmers onto the same
DMX board I use for my intels. Running dimmers and intels through two separate boards made no real sense when most scenes were combinations of intel and
dimmer lighting... it just made extra work for me and increased the chance that I'd screw up a
cue.
That said, if you insist on running two separate boards, it's probably better to go with two separate cables.
DMX combiners are available, but they aren't cheap.
A combiner is absolutely necessary to get two boards on one cable (unless one of the boards has combiner functions built-in). DMX-512 was designed for a single transmitter (the board) and multiple receivers (dimmers and intels). There is no provision for multiple transmitters. Because each transmitter thinks it's the only transmitter in the
DMX universe, it sends whenever it wants. Both transmitters not only could, but
would transmit at the same time. They'd interfere with each other and nothing would get through properly.
A combiner consists of two or more
DMX receivers, one transmitter and some memory shared between them. Assume a 24-channel board for the dimmers and a 192-channel controller for the intels. On the output side, to keep things simple, we'll put the dimmers on channels 1-24 and the intels from
channel 25 up. Receiver A takes the
DMX signal from the
dimmer board, strips off the packet
header and puts the data into memory locations 1-24. Receiver B takes the
DMX signal from the intel board and puts the data into memory locations 25-216 -
channel 1 from the intel board goes into location 25,
channel 2 into location 26 and so on. The transmitter creates a new packet
header, then sends the data from memory locations 1-216 as a brand new
DMX packet.
There's some really critical timing stuff going on in the combiner, because this is all happening simultaneously - the receivers don't "take turns" because each board's transmitter thinks it's the only one - it doesn't know there's somebody to take turns with. The transmitter in the combiner can't wait for the receivers to stop, because chances are they won't.
Getting the snaky timing right isn't easy, either in terms of the hardware or the software to do it, so combiners can get kind-of pricey. Unless you're running a few miles from the booth to the
stage, a second cable run is much more cost-effective.
John