Are you sure that your
EDI MKVII doesn't have 120 dimmers, as this was the standard for that rack? The multilink has two
circuit boards a top and a bottom. Each board controls 60 dimmers. It is very common for one board to fail without the other board being affected. The most common failure is the communication chip.
The MKVII multilink was introduced when
AMX and
CMX were still very popular and
DMX was just coming into
play. Because of this the board uses a communication chip that was meant for RS422 instead of RS485. As both are a
balanced signal, RS422 is also suppose to work with
DMX as well as
AMX and
CMX. My MKVII was losing one of the communication chips almost monthly, probably because of
ground loops in a building built in 1975. Also the RS422 chip does not like to see a signal when it is not powered. So if you have the
console sending
DMX/RS485 when the rack is not powered, you can damage the chip. When the chip is damaged, it will very often work off of a
console or controller that is DC powered or has enough isolation as to keep the building
ground from getting into the
DMX output. such as a Battery operated
DMX tester. It will not work with an
express series
console and many others. It took me 2 years and many tries to solve this problem. I tried several corrections, and therefore, I am not sure which ones resolved the problem. But here they are in no particular order:
1) Installed
line protection on all three phases of
power that go into the multilink.
2) Installed a double pole
relay on the
DMX in wiring that was only closed when the multilink was powered.
3) Had the
DMX signal go through a Optoisolated splitter from the
console to the rack.
4) Modified the multilink boards to use an RS485 communication chip (75176 transceiver) This is in my mind is the most important step. I did this by removing the RS422 chip and inserting an 8 pin Dip
socket in it's place. I then built a small
circuit board that holds the 75176 chip and a few resistors for better isolation and plugged it into the
socket I placed on the multilink board. I then put a
connector on my small piggy back board, that would accept the
DMX connector that previously plugged into the multilink board. In this way, there is no permanent modification to the multilink board in case that it would have to be sent back to
EDI for service.
Since all of this has been done, there hasn't been a failure in the rack in over five years. Building the piggy back board will require somebody with a good knowlege of pin assignments on transceiver chips and a schematic of the multilink.
Tom Johnson
Master electrician Theater Winter Haven
[email protected]