An EMI3M electronics module is installed in the top left slot of the L86 dimmer chassis. It isolates the DMX512 input signal from each of the L86 EM64 control modules, and also provides isolation between EM64 modules.
The EMI3M is typically not used where each dimmer bank is controlled by an EM264. The EM264 provides optical isolation for the bank, therefore no longer relying on the EMI3M for opto-isolation.
In a rack equipped with EM264's, the top-left slot previously reserved for the EMI3M is now filled with an EMD, a blank module that ensures the proper movement of airflow within the rack, but contains no actual circuitry.