GAI-Tronics L3149B C200 Advanced Deskset - Manual (Page 48)

Theory of Operation L3149B (C200) Advanced Tone Remote Desk Set
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In addition to the transmit audio, data is combined with the transmit audio prior to line-out compensation
at U11-3. U10-1 is the line-driver enable for the transmit audio. This provides for a 560-ohm transmit
output impedance.
Microphone Audio
There are three sources for mic audio. These are the internal, handset and audio accessory mic. The
microcontroller controls which mic is used based on the source of the PTT. The selected mic audio is
gated between the handset/internal and the audio accessory mic by U14 using the outputs of U5-C6.
The selection of the handset and internal mic is controlled by U5-C5. When U5-C5 is high, U25
provides a high level of VMB or mic-bias voltage to the internal mic, thus, enabling the internal mic and
disabling the handset mic. If U5-C5 is low, the internal mic will not have mic bias, but the handset mic
will and, thus, the handset mic is selected.
The audio accessory port can be configured to provide a mic bias to the audio accessory. This is
controlled by the microcontroller using U5-C7. Each mic audio is gain compensated by U22 so that the
same level is presented to the mic gain-stage consisting U12 and U22. The audio is then presented to the
A/D section of U17 where it is converted to digital for use by the DSP. The DSP compresses the mic
audio using U13 and, if tone control is in use, notches the mic audio prior to combining it with the
transmit audio.
Guard Tone Detection
When configured for tone control operation, the C200 must detect guard-tone from parallel units to
restrict transmissions. This is done by first auto-leveling the transmit audio input from line 1 using U12.
The audio is then converted to digital using the A/D section of U16. The DSP this processes this audio
to determine if guard tone is present.
Relay Control Module
The reference designators listed in this section refer to the relay control schematics unless otherwise
stated. The relay control module is used to provide closures out via relays and sense closures via
closures to ground. The microcontroller provides closure-out information and reads closure-in
information via serial data. Closure outputs are controlled by the data in U701. Closure senses inputs
are read from U702.
To provide a closure on relay 1, the microcontroller asserts U701-15. This, in turn, drives the
corresponding Q701 that provides the ground for the coil of K701. To provide a closure sense input on
input 1, J701-5 must be grounded. This, in turn, is latched into U702 by the microcontroller where the
microcontroller takes the appropriate action.
Reset Circuit
Both the DSP and the microcontroller use the EPROM, U2. When coming out of reset, the DSP must
boot its software from the EPROM. In doing so, it asserts U1-21, BMS, low, which causes the
microcontroller to remain reset.
After the DSP has completed loading its software from the EPROM, it no longer requires access to the
EPROM. The DSP then disasserts BMS, which allows the microcontroller to come out of reset. The
microcontroller then executes its software from the EPROM.