what equipment is being tested. The configuration in which  the  POFA  is  run  determines  some  of  the equipment being tested. The POFA can be run in two configurations, as shown in figure 3-1. In the full configuration, the single station POFA will test the following areas: CDS  computer I/O  channel  interrupt recognition and acceptance Security device I/O path Data terminal transmit and receive registers, multiplex and demultiplex, and transmit and receive sequence operations Switchboard integrity DTS-to-radio and radio-to-DTS audio path Capability of the HF radio set to develop and accept sidebands (both transmit and receive). By studying the above list, you can see that most normal  link  operations  are  tested  during  a  single station  POFA.  Certain  functions,  however,  are  not checked by running a single station POFA. The DTS uses the transmit timing as the reference for the entire test;  therefore,  the  receive  timing  circuitry  is  not checked. Also,  certain  other  functions,  such  as Doppler correction, are not checked. The  printout  generated  at  the  end  of  a  single station POFA lists interrupt status, illegal interrupts, parity,  and  bit-by-bit  word  errors.  A  single  station POFA  should  always  produce  a  totally  error-free printout.  However,  when  a  printout  with  errors  is received, the technician needs to be able to analyze the error package effectively. The  interrupts,  for  example,  must  occur  in  the following   sequence: Prepare  to  transmit Prepare  to  receive 3-4 End  of  receive. If you receive interrupts in any other order, such as two consecutive prepare to transmit interrupts or an end of receive before the prepare to receive, an error condition   exists. The  parity  should  always  equal  zero.  As  you learned in the previous chapter, the parity, or error detection  status  bits,  indicates  an  error  has  been detected  in  the  received  data.  When  errors  are detected, they are listed in the bit-by-bit section of the printout. Even if the printout indicates a few random bit errors, this condition should not be ignored. Random bit errors can be caused by several areas in the system, including the CDS computer, the data switchboard, or the  DTS.  You  can  narrow  down  to  the  exact  area causing the problem by running the POFA in several configurations. Changing  computers  and  crypto devices can aid you in determining the malfunction. Because  of  the  unique  function  of  the  crypto device, a single broken line in the switchboard could cause  all  the  bits  to  be  picked  up  randomly  or dropped. When the broken wire is on the encrypted side of the switchboard, the crypto device reads the state of that line during the decryption cycle and the entire  decryption  cycle  is  changed. MULTI-STATION  POFA The multi-station POFA is a test of the Link-11 system   that   involves   more   than   one   platform. Because this POFA most closely represents normal link  operations,  more  equipment  is  tested.  The  multi- station POFA is run in the Roll Call mode using a set of known data words. Figure 3-2 shows the data flow for a multi-station POFA. A designated unit transmits a block of 230 data words that are received by the other platforms involved in the multi-station POFA. The receiving computer(s) compare(s) the data against the known pattern, count(s) the words in error, and send(s)  this  count  back  to  the  original  ship.  This transmission  is  known  as  the  error  status  report. Ideally, the multi-station POFA should run error-free.