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logic 1 level that is slightly higher and a logic 0 that is
slightly lower. This pulse shaping minimizes interfer-
ence between data bits when the data is filtered by the
low-pass filter.
The data from U111 is fed to buffer U126A and
Transmit Data Enable Gate U117B. Gate U117B is
controlled by A/D processor U111 directly. When
enabled this gate passes the data to EEPOT U151.
U151 is an electronically adjustable potentiometer that
adjusts the gain of transmit audio amplifier U126B.
The gain of U126B can only be adjusted through the
software. Therefore, a computer must be attached to
the MAC card. U126B provides the required signal
level at the output of the low-pass filter. A relatively
stable DC bias voltage for U126C/D is required
because these stages are DC coupled to the transmit
TCXO (see Section 6.2.2) and changes in bias voltage
can cause fluctuations in the transmit frequency.
U126C/D form a low-pass filter that attenuates
square-wave harmonics in the data signal above 150
Hz to prevent interference with the audio band. From
this filter the signal is fed to summing amplifier
U129B and combined with the transmit audio signal.
The output of U129B is fed to Transmit Modulation
Mute Gate U118D. This gate is controlled by A/D
processor U111/latch U106. When enabled, transmit
audio and data is passed to the Exciter modulation
input and the transmit TCXO.
When needed, the External Modulation input on
P100, pin 11 is fed to External Modulation Mute Gate
U118C. Gate U118C is controlled by A/D processor
U111/latch U106. When enabled, this gate passes the
modulation on pin 11 to the summing amplifier
U129B and gate U118D to the modulation input of the
Exciter.
The repeater on the lowest frequency channel in
each system must periodically transmit the station call
letters as a continuous-wave identification encoded by
Morse Code. This identification is programmed with
the Edit Parameters software.
The CWID output is controlled by A/D processor
U111/latch U107. This output is fed to CWID tone
generator U100B/A and turns the tone generator on
and off to create the Morse Code. From the tone gen-
erator the signal is fed to bandpass filter U129A. This
filter passes the 800 Hz fundamental present in the
signal. The output of the filter is jumpered by P106
on J106, pins 2/3 and P107 on J106, pins 4/5 to the
summing amplifier and applied to gate U118D, and to
the modulation input of the Exciter.
The input and output connectors for the MAC are
defined as follows.
6.12.11 P101 SIGNALING CONNECTOR
The signal interface connector P101 (64 pin) con-
nects the Address and Data buses and control lines to
the backplane connector. See Figures 6-21 and 6-22.
Pins 1-10
ADDRESS BUS
Pins 33-42
This provides a path between the MPC main pro-
cessor and the processor and memory of the MAC.
This bus retrieves information programmed into mem-
ory for the operation of the MAC.
Pins 11-14
DATA BUS
Pins 43-46
This data bus provides a means of transferring
data to and from the processor on the MAC with
peripheral devices in the MAC.
Pin 15
MREQ
A memory request line operates in conjunction
with the Read/Write lines. These provide the ability to
read from or write to the processor memory.
Pin 16
MSTB
This is a memory strobe line used during MAC
processor Read/Write operations to external memory.
Pins 17-20
UNUSED
Pin 21
LPTT
The Logic Push-To-Talk is not used on the MAC.
Pins 22-23
UNUSED
6-41
CIRCUIT DESCRIPTION
October 1999
Part No. 001-2009-201
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