RFL 9300 RFL Electronics Inc.
October 29, 2001 2 - 7 (973) 334-3100
A hardware receiver in the RFL 93B CC module counts the bits and tests each word for a valid CRC code and
framing bit. In normal operation, the receiver hardware is framed correctly and tests the word at a count of fif-
teen (8 data bits + 6 CRC bits + 1 frame bit). If the framing is not correct, the microcontroller goes into a special
sub-routine that resets the counter after five good words are observed.
After the counter is initialized, the burden of counting the bits and strobing each word into the microcontroller
falls on the hardware receiver. When a word is received, the hardware signals the microcontroller and indicates
whether the word passed the CRC test. The software in the microcontroller then applies squelch rules. These
rules determine if the channel is good enough to give messages formed from the words to the RFL 93B SV
module. To enhance security and dependability, a two-out-of-three voting scheme is used. Each word is sent
three times; the receiver groups the three word segments and looks for two out of the three word segments to
agree. If two segments are identical, the word is accepted as valid.
2.4.1.2 SQUELCH RULES
The microcontroller counts the number of good and bad word segments. It uses this information to decide if the
channel is good or bad. The word segments are binary, and the errors that are counted yield a bit error rate
(BER) which is a measure of communication quality. The noise on a channel may be long-term average noise,
or the noise may come in pulses for a short duration. The RFL 93B CC uses three squelch rules to determine
whether communication quality is below that required by the RFL 9300. They are identified as Squelch Rule 1,
Squelch Rule 2, and Squelch Rule 3 (SQ1, SQ2, and SQ3):
SQ1 SQ1 blocks messages from going to the RFL 93B SV module, but does not alarm the module. SQ1 is
entered if 16 bad word segments in a row are received, and turned off if 32 good word segments in a
row are received.
SQ2 SQ2 relates to short error pulses, and is the trigger that frames the local receiver hardware. SQ2 will
also alarm the RFL 93B SV module and block messages from being sent to the rest of the terminal. If
30 bad word segments are received out of a total of 250, the RFL 93B CC enters SQ2. It takes 100
good word segments in a row to exit SQ2.
SQ3 Under SQ3, the RFL 93B CC module will squelch the channel if the long-term noise produces a BER of
-3 or worse. This represents one bad bit in 1,000 bits. When SQ3 is on, the RFL 93B CC module alarms
the RFL 93B SV module and blocks all communication messages from going to the RFL 93B SV mod-
ule.
To come out of SQ3, the BER must be better than -4 (one bad bit in 10,000). It takes 540 ms to meas-
ure the 2000 word segments used to calculate SQ3; a fast exit from SQ3 occurs if 1,000 error-free word
segments in a row are received within 270 ms.
To enter SQ3, 30 bad word segments out of a total of 2000 word segments were measured. If 30 bad
word segments are received in a row, SQ3 is entered without waiting for the full 2000 word segments.
After the RFL 93B CC has entered SQ3, less than 3 bad segments out of 2000 segments are required
to drop SQ3.
2.4.1.3 FRAMING
The RFL 93B CC module frames the receiver hardware when SQ2 is entered. If the framing is not successful,
another frame attempt is made after two seconds. This continues every two seconds until a successful frame is
made. A pin on the RFL 93 DD I/O module's DC-37 connector acts as a remote channel condition input. If the
DCE knows that the channel is bad, this pin is held low. When the channel is good and the logic level on the pin
is raised, the channel is framed after two seconds.
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