MPX

from The Free On-line Dictionary of Computing (8 July 2008)

Multiplexor Channel
MPX

   (MPX) {mainframe} terminology for a slow peripheral device
   connection, e.g. for a {printer}, operator console, or card
   reader.

   (1997-06-30)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

MPX
       MultiProgramming eXecutive (OS)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

MPX
       MultiProgramming eXecutive (IBM, OS, IBM 1800)