C-2
APPENDIX C
MDS 05-2415A01, Rev. A
Level shifting and wave shaping operations within U6 cause the data to resemble a smooth
audio waveform. This waveform feeds directly into the modulation input of the digital
transmitter.
With no scrambling, a mark (the normal resting state when no data is being sent), is sent by
an alternating 10101010 pattern at 9600 bits per second. This pattern swings the transmitter
carrier back and forth about the center frequency. If the TXD line goes high (continuous
space), a 0000 or 1111 pattern is transmitted. This causes the radio’s carrier frequency to be
1.6 kHz offset from the center frequency for a 12.5 kHz channel. While continuous data is
being sent, the frequency measures approximately the nominal channel frequency as it
toggles back and forth about the center frequency. In the normal configuration, the scrambler
is enabled, which causes a random bit pattern regardless of whether a mark or space is being
sent. The scrambled signal is detectable by any MDS 9600 bps modem and no operator
configuration is required other than setting it on or off.
Receive Data
While no RF carrier is detected, the squelch circuit in the receiver senses the lack of quieting
and forces the DCD low and RXD output to mark condition. The squelch output also biases
the “slicer” circuit on the interface board so that it recognizes the very first mark-to-space
transition correctly once a signal is received. When an RF carrier is detected, the receiver
squelch forces DCD true (high) and the receiver receives the FSK (Frequency Shift Keyed)
signal just as it would any FM signal. The recovered audio is fed to the modem board.
A sample of the detected receive audio is held by U3A after it passes through a low pass
filter, U3B/C. This sampling takes place after the receiver squelch opens and before the
receive data signaling begins. Two comparators in U8 use this stored voltage as a reference in
recovering the data from the received audio signal. The detected data signal from U8 is fed
into U6 for reconstruction and distribution to external interface equipment through U31, the
interface IC on the transceiver main board.
Synchronizing Data
For asynchronous operation, the modem synchronizes to the incoming transmit data. For
synchronous operation, the modem generates the transmit clock timing signal for use by
external equipment. Data changes on the rising edge of the TX clock signal, and the modem
samples the data on the falling edge of the clock signal.
Table C-1. Settings for the MDS 9600 bps Modem’s Configuration Switch S1
SYNCHRONIZATION
CHARACTER
SIZE*
MODEM SWITCHES
1
2
3
4
5
6
7
8
Synchronous w/internal clock
N/A
C
O
O
O
O
O
O
O
Synchronous w/external clock
N/A
C
C
O
O
O
O
O
O
Asynchronous #1
8
C
O
O
O
O
C
O
O
Asynchronous #2
9
C
O
O
O
O
C
O
C
Asynchronous #3
10
C
O
O
O
O
C
C
O
Asynchronous #4
11
C
O
O
O
O
C
C
C
* Includes start bit, stop bit, data bits & parity.
LEGEND: C = CLOSED (ON) O = OPEN (OFF)
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