what equipment is being tested. The configuration in
which the POFA was run determines some of the
equipment being tested. The POFA can be run in two
configurations, as shown in figure 5-1.
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
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 randomly picked up or
randomly 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 5-2 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.
5 -4
