RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
1070
3-23
3.2.6 FSK 7E Flag Detector
The FSK 7E flag detector can be used to detect the presence of the energy produced by the T.30 FSK 7E flag preamble
while the modem is configured in any high speed receiver mode except for V.27 ter short train. FSKFLS and FSK7E bits
indicate the status of the detection process (see Table 3-1 for bits description). The detection process will always start after
the modem enters its receiver idle mode and a waiting period of 8 bauds after FED turns on (Figure 3-4). The modem
receiver normally enters the idle mode after any of the following events:
1. The host setting of SETUP bit.
2. After FED turns off with enough lapse time (about 30 ms in V.29) in data mode to return to the idle mode.
3. After the modem detects a significant gain hit in the data mode (RLSD on). The presence of an FSK signal when noise
is above turn-on threshold will generate such a hit.
After the modem enters the idle mode, the modem will reset FSKFLS and FSK7E bits to a 0. If FED is on, then the modem
will set FSKFLS to a 1 and start the detection process. After the completion of the detection process, if the FSK 7E signal is
present the modem will set FSK7E bit to a 1 and reset the FSKFLS bit to a 0. The FSK7E bit will remain at this state until the
modem enters the idle mode again or the host resets the bit to a 0.
The detector will not work properly if the modem is not allowed to complete the detection process, which lasts about 106.67
ms after FSKFLS goes to a 1.
In the event that the host sets the control bit TDIS to a 1 after the modem enters its idle mode, the modem will quickly enter
the data mode after FED turns on and, hence, not allow enough time for the detector to complete its detection process.
Therefore, the TDIS bit must be reset to a 0 to assure the proper function of the FSK 7E flag detector.
A simple example usage of the FSK 7E flag detector in a signal recognition algorithm is illustrated in Figure 9-26.
FED
SIDLE
FSKFLS
8 baud times
~106.67 ms
1070F3-4 FSK 7E Timing
FSK7E
Figure 3-4. FSK 7E Flag Detector Timing
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