3012—Technical description
304
NGT Transceiver System Technical Service Manual
DSP Modem PCB
The DSP Modem PCB comprises a 32-bit DSP and associated memory, interfacing and
power supply components.
Digital signal processor and CoDec
A Texas Instruments TMS320C31 32-bit floating-point DSP (IC101) forms the heart of
the data modem. It is interfaced to the transceiver transmit and receive audio paths via a
CoDec (IC108), input differential amplifier (stages IC109/A and IC109/B), and audio
isolation transformers (T101 and T102).
The CoDec (IC108) is an ADC/DAC, which interfaces to the DSP via a serial interface,
using frame strobe and clock signals.
A watchdog/power on/reset IC (IC102) ensures that the DSP is held in reset until the
power supply rails have stabilised, and also serves to reset the processor if unexpected
code run-away stops the strobe line from being regularly toggled.
Memory
The program code for the data modem is stored in two 2 Mbit EPROMs (IC202/204),
each organised as 128 k by 16-bit words. These are paralleled to create an effective 128 k
of 32-bit memory.
The data modem also requires external SRAM, which is provided by devices IC201 and
IC203. Each 128 k by 16-bit device is paralleled to provide a 128 k by 32-bit memory.
A small amount of non-volatile data storage is provided by a 2 kbit (256 by 8-bit)
EEPROM (IC205), which is accessed serially using a 2-wire I
2
C-compatible serial
interface.
Logic and interfacing
A 32-Macrocell programmable logic device (IC106) performs the ‘glue logic’ required
to perform several functions, including:
•
decoding the memory span of IC101 into the format required by the firmware
•
generating read, write and chip-select signals for the memory devices
•
inverting the interrupt request signals produced by a dual asynchronous
communications element (IC107)
IC107 is a complex device, which features two complete serial communication interfaces
with 16-byte FIFOs and an additional parallel port. The primary role of IC107 is to
provide the serial interface between the RS232 interface of the data terminal equipment
and the DSP.
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