Codan NGT 2010 RF Technical & Service Manual page 785

8571—Technical description
Signalling format
To enable the microprocessors to communicate with each other it is necessary to provide
a signalling format that is recognised by all microprocessors used within the remote
control network.
Serial data from the UART in the microprocessor is converted to FSK format in the
Bell 103 modem using tone frequencies of 1070 Hz and 1270 Hz at 550 baud. At the
receiving terminal, the FSK is restored to digital data by a second modem and is read by
the microprocessor from the UART input.
The following outlines the signalling format used in the system:
•
•
The signalling sequence is used for all data transmissions including PTT.
Transceiver control logic
Drawing
The transceiver is connected using a dual interface cable via the split-site interface
controller to the 25-way D-type Tcvr Audio and 8-way Tcvr Control connectors
located on the rear panel of the 8571.
The transceiver communicates with the 8571 via RS232 commands (CICS) to the
microprocessor IC103 on the General Purpose Input/Output PCB in the 8571.
RS232 port
Drawing
The RS232 port provides the control interface with the transceiver. This functionality is
provided through the Tcvr Control connector.
The RS232 interface is fitted with a microprocessor IC103 (80C552) that is a member of
the Intel 80C51 8-bit microprocessor family. It has 256 bytes of internal RAM, no
internal program memory, two counter/timers, eight 8-bit ADC inputs, a watchdog timer,
2
I C bus and an internal clock oscillator.
The microprocessor operates in a conventional 8-bit data, 16-bit address configuration.
To minimise the pin count, the low-order address bits are multiplexed with the data on
pins 50 to 57 onto a common 8-bit bus. IC104 is used to latch the low-order address bits
to facilitate access to the external non-multiplexed devices. The ALE signal (IC103
pin 48) indicates to the latch when the address is present on the bus. The high-order
address bits are provided directly on IC103 pins 39 to 46.
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The signalling sequence commences with sending the 300 Hz control tone. After
20 ms, under the control of the microprocessor, the modem sends FSK carrier (bit
one) for a period of 20 ms followed by the FSK data. At the end of the data, the
300 Hz continues for a further 2 ms before it is disabled.
At the receiving end, the microprocessor first detects the 300 Hz followed by the
FSK carrier, enabling the microprocessor to be ready to receive data. The data is
read, and on completion, the carrier and the 300 Hz detector are reset.
04-02852
04-02852
NGT Transceiver System Technical Service Manual