User's Manual

MOBILE_MIC_HI (P102-12) - This audio path contains microphone audio from the
mobile during In-Car-Monitor (ICM) transmissions. The microphone audio passes
through op amp circuit U104-4 and can be level adjusted via potentiometer R148. During
an ICM transmission, both MIC_FIL_SEL line and TONES-MOBILE_AUD line are set
to a Low state to configure both mux U101-3 and mux U101-1 to route the mobile mic
audio to the HT750’s transmitter input (EXT_MIC, J30-19).
Single Tone Encoder :
The VRS750 Single Tone Encoder (U100-1, U124-3, U124-2, and U124-4) is used to
produce a tone which is transmitted by the HT750 transceiver when the VRS750 is
activated. Other VRS750 repeaters that are on in the operating area receive this Single
Tone transmission. The other repeaters will then increment their priority counters by one
while the transmitting VRS750 goes to delay state zero and becomes the priority repeater.
When the VRS750 is in transmit mode for Single Tone, the HT750 microctonroller
generates a square wave at a set frequency of 847.5Hz to the SNGL_TX_DATA (J30-6)
pin. The control signal, TONES-MOBILE_AUD, is set to a High state to allow the
Single Tone signal to be routed to the input of the Single Tone Filter. The Single Tone
Filter is a band pass filter that allows frequencies between 300Hz and 1.7 kHz. U124-4,
R126 and R127 are for gain adjustment. The resulting sine wave is then sent to the
HT750 transceiver to be transmitted on EXT_MIC (J30-19).
This circuit is also used when the HT750 generates various VRS750 acknowledgment
tones.
Single Tone Decoder:
The VRS750 Single Tone Decoder (U100-1, U124-3, U124-2, and U124-1 detects other
Single Tone transmissions, then increments the priority counter.
VRS_TX*_RX from the HT750 microprocessor is set to a High state when the VRS750
is in the Single Tone Decode mode (receive mode). When this signal is high, U101-2
allows HT750 receive audio (URX_SND, J30-11) to pass to buffer U100-1. The output
of the buffer drives the low-pass filter that is also used in the encoder circuit. U124-1 and
the associated diodes, D100 and D101, produce a square wave which is divided down to
3.3V by R172 and R173. This signal, SNGL_DET(J30-12), is then sampled by the
HT750 microprocessor to determine if Single Tone is present.
Boot Control Circuitry:
The firmware for the VRS750 is stored in the HT750’s Flash memory. This firmware
can be programmed using the VRS750 Upgrade Tool (refer to section ?). Prior to
programming the firmware, the HT750 must be placed into bootstrap mode.