VMS Help  —  RMU72  COSI_ERRORS, SIGNAL21
  signal SIGTTIN, code <num>

  Explanation:  A 'background read from control  tty'  signal  was
  raised as an exception in response to the arithmetic or software
  condition specified by the given code.

  User Action:  Eliminate the cause of  the  error  condition  and
  retry the operation.
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