CTCSS decoding
It’s quite easy to place a CTCSS decoder into the RS9044, the only
problem is the availability of these chips.
For a variable CTCSS, selectable by software the FX335 and FX365 can be
used. When it’s only nescesary to decode one fixed tone, a XR221 can be used
instead.
The RS9044 is able to control the FX365 directly, via the serial data
used for the PLL IC’s. Because there are only a few free digital outputs, the
communcation must be serial, when only a FX335 is available, the data must be
shifted into a HEF4094 first.
On the CPU board aren’t any unused digital inputs also, only 3 analoque
inputs via the 32 ADC’s. One of these analogue inputs will be used to read the
output of the decoder IC. The decoder output of the FX3x5 must be connected to
pin7 of P4, the pin next to the CTCSS output of the main processor.
Most of the pins of the FX3x5 can be left open, they are partly pull-ed
up and used for an unused block of the device.
FX365 Pin: FX335 Pin:
1MHz clock in, cristal or external oscillator 2 1
Serial Strobe, to a 4k7 pull-up and to pin 7 of P4 4 1 (of 4094)
Serial Data, directly to pin 4
of the CPU 7 2 (of 4094)
Serial Clock, directly to pin 5 of the CPU 8 3 (of 4094)
Toon geldig, directy to pin 7
of P3 15 16
Audio output after filtering, from pin 12 of P3 19 20
Audio in of the detector, zie tekst- 23 23
\ 24
5V 1 24, 3
GND 11,
6 11
Pin 13 of the FX3x5 is normally used for switching the audio path, this is the output of the hysteresis
circuit. To make the modification simple, this hysteresis isdone by the
software of the main processor. The software reads the output of the detector
(Pin 15) directly. In this case some passive components can be left away.
The active filters of the RS9044 don’t remove the CTCSS tones from the
audio signal, this can be done by the FX3x5. Therefore the connection to pin 12
of P3 must be cut, this is the signal from the interfrequent part. The signal
from pin 12 of P3 goes to pin 24 (Tone in) and 23 (Audio in) of the CTCSS
decoder. The filtered audio signal comes out of the decoder at pin 19, this pin
must be connected to the other side of the cut wire in the RS9044 via a
capacitor of 0.22uF...
The audio path is opened by software, the FX3x5 will normally block
while the hysteresis ciruit is not used.