DIAGNOSTIC  PROGRAMS The  following  three  types  of  test  (diagnostic) programs  are  available  for  most  magnetic  tape units: l  Internal  diagnostics l   The   programmed   operational   functional appraisal (POFA) for the MTU in your system l The MTU peripheral equipment functional test (PEFT) Internal Diagnostics Internal diagnostic tests are controlled by the MPC ROM.  For  periodicity  and  procedures,  look  in  the  PMS MIP and MRCs. Programmed Operational Functional Appraisal (POFA) The  programmed  operational  functional  appraisal (POFA) tests are run under the control of a stand-alone computer.  That  is,  the  computer  running  the  POFA cannot be simultaneously running the operational program. The POFA consists of four separate tests that are discussed in the following paragraphs. THE  FUNCTION  AND  FORMAT  TEST.—  The function and format test checks the ability of the MTU to respond to computer commands and to provide status and error condition information. THE DUPLEX TEST.—  The duplex test checks MTU response to duplex control commands. THE   TRANSPORT   COMPATIBILITY TEST.— The transport compatibility test checks the compatibility between the MTTs. THE EXTENDED OPERATION TEST.—  The extended operation test checks the ability of the MTU and MTTs to operate for extended time periods. Periodicity of the POFA tests is determined by the PMS MIPs. You’ll find the operating procedures in the NAVSEA  POFA  manual. Peripheral Equipment Functional Test The peripheral equipment functional tests (PEFTs) run  under  the  control  of  the  operational  program computer. They are a subset of the dynamic combat system test (DCST) and are designed to be run online with and as part of the operational program. You can use the PEFT tests to check MTU write and read compatibility  and  to  perform  a  function  and  format  test of  the  MTU  without  interrupting  the  operational program. SUMMARY—MAGNETIC TAPE STORAGE This chapter has introduced you to magnetic tape storage concepts and atypical magnetic tape unit. The following  information  highlights  points  you  should have  learned. MAGNETIC  TAPE—  Magnetic tape is made of a ferrous oxide material glued to a thin plastic or metal strip. Magnetic tape comes in a variety of sizes and lengths  such  as  open  reels,  cartridges,  and  cas- settes. MAGNETIC TAPE HANDLING—  Magnetic tape  handling  includes  the  storage,  handling, maintenance, and control of tapes. By following the proper  procedures,  you  can  prevent  damage  to  your tapes and safeguard the information contained on the tapes. DATE   STORAGE   ON   MAGNETIC TAPE— Magnetic tape storage concepts introduced you to how data is written on and read from magnetic tape. It also introduced you to the different methods used   to   encode   data   on   the   tape.   These   are return-to-zero    (RZ),    non-return-to-zero    (NRZ), non-return-to-zero  indiscrete  (NRZI),  and  phase encoding (PE). Phase encoding allows for the highest densities of data, while retum-to-zero is limited to very low  densities. MAGNETIC TAPE UNIT—  The magnetic tape unit is the device for writing and reading magnetic tape. It has two major functional areas; the magnetic tape controller and one or more magnetic tape transports (MTTs). MAGNETIC TAPE CONTROLLER—  The magnetic tape controller receives and processes data and commands from the host computer and sends data and status to the host computer. It provides timing and control signals to the MTT for read, write, search, and rewind operations. MAGNETIC  TAPE  UNIT  OPERATIONS— Magnetic  tape  units  can  perform  the  following operations: read, write, search, space file, and re- wind. 9-20