Sub-Audio Signaling Processor
Page 10 of 24
MX805A
4.2.3 NRZ Decoding
Input (NRZ type) sub-audio signals are filtered and the data clock extracted. Decoded data is serially loaded
into a shift register buffer. This data is output on 8-bit byte at a time as Reply Data from the NRZ Rx Data
Register to the microcontroller. The expected Rx baud rate is programmed as the NRZ Rx Baud Rate
(R ). See Table 8. Any codeword recognition can be carried out by software.
NRZRX
4.2.4 CTCSS Decoding
Received CTCSS signals are filtered, and coherence is increased by the digital noise filter. The quality of the
signal is assessed by measurement of the cycle–to-cycle period variance and, provided it is sufficiently good,
the frequency is measured over a period of 122.64ms (4.0MHz crystal).
If the average signal quality is consistently too low, NoTone is indicated; if not the input frequency is precisely
indicated in the CTCSS Rx Frequency Register in a binary form.
Any single sub-audio tone within the specified range may be selected, enabling a DCS turnoff tone of 134Hz)
to be decoded while in the NRZ Rx mode.
5 Controlling Protocol
Control of the MX805A Sub-Audio Signaling Processor’s operation is by communications between the
microcontroller and the MX805A internal registers on the C-BUS, using Address/Commands (A/Cs) and
appended instructions or data (See Figure 10). The use and content of these instructions is detailed in the
following paragraphs and tables. The Address Select Input enables the addressing of 2 separate MX805As
on the C-BUS to provide full-duplex signaling.
5.1 MX805A Internal Registers
5.1.1 Control Register (70 /78 )
H
H
Write only, control and configuration of the MX805A.
5.1.2 Status Register (71 /79 )
H
H
Read only, reporting of device functions.
5.1.3 CTCSS Rx Frequency Register (72 /7A )
H
H
Read only, a 2-byte binary word indicating the frequency of the received sub-audio input.
5.1.4 CTCSS Tx Frequency/NRZ Tx or Rx Baud Rate Register (73 /7B )
H
H
Write only, a 2-byte command to set the relevant parameters.
5.1.5 NRZ Rx Data Register (74 /7C )
H
H
Read only, a single byte f received NRZ data.
5.1.6 NRZ Tx Data Register (75 /7D )
H
H
Write only, to load a single byte of NRZ data for transmission.
5.1.7 Gain Set Register (76 /7E )
H
H
Write only, a single byte to set the gain of the Tx Lowpass Filter.
ã2001 MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
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