synchronization of device access

[chcu275]

  I am using SCO XENIX 2.1 to read and write data on an IEEE-488 GPIB
  interface.  The XENIX op. sys. runs on an AT which serves as a 
  supervisor to a XT slave.  The reads are set up to occur at a specific
  time interval.  The writes occur at a specific time interval but also
  can occur at arbitrary times.  

  What is the best way to synchronize this GPIB activity?  Should I use
  "waitsm" with shared memory to delay action until the current read or 
  write leaves the shared memory? ( I think this is similar to semaphores
  on Sys 3 which are not supported on  Sys V. according to my manuals. )

  I would appreciate any advice on this matter.  Also, are there any good
  references for process control on UNIX/XENIX systems that provide more
  info. than the SCO or UNIX Sys.V manuals.

  Thanks in advance for any suggestions provided.

  Noel Bell
  Department of Chemical Engineering
  Univ. of Texas at Austin