The SOC003 Serial Output Controller is a PCM Simulator contained on an ISA card.  The SOC004 is the equivalent version on a 6U VME printed circuit card.  These two modules provide equal functionality. 

The Serial Output Controller generates a serial output data stream using either static or dynamically updated data loaded from the data-bus (ISA or VME). With ample memory buffers onboard, any of several modes of data generation may be used. Data is output repeatedly or in bursts. Data to be serialized may be continuously updated by way of the data-bus controller. A frequency synthesizer, resident on the module, allows precise bit rates to be generated.