Operating instructions

RF Technology T350/T500 Page 15
5 CIRCUIT DESCRIPTION 5.10 PTT & DC Remote Control
When a DMTX board option is required, jumper JP23 allows the audio paths to be re-
routed. The DMTX board provides for an external digital modulation input signal.
When the two links on JP23 are positioned in the middle of the 6 pin header, the audio
from the exciter is passed to the DMTX board via pin5 of JP5, where the signal is
conditioned and then returned from the DMTX board via pin 2 of JP15, and passed to
the two modulation points.
RV2 adjusts level of the audio used to modulate the VCO. This primarily effects the
deviation of audio frequencies above 500Hz. RV2 is used to balance the high and low
frequency deviation to obtain a flat frequency response relative to the desired
characteristic.
5.10 PTT and DC Remote Control
Two main PTT inputs are provided. The first, a direct logic level input, is connected to
pin 3 of the system connector. The transmitter can be keyed by applying a logic low or
ground on pin 3. Pin 3 connects to the PTT logic and microprocessor through D10.
DC current loop control can be used for remote PTT operation. The current loop can be
configured by JP9, JP10 and JP11 for use with either a remote free switch or a remote
switched source.
Opto-isolator ISO1 is used to isolate the loop current signal from the transmitter PTT
logic. The loop current passes through the input of ISO1 and the output of ISO1
connects to the PTT logic.
A bridge consisting of diodes D6, D8, D9 and D14 ensures correct operation regardless
of the current polarity. Q17 limits the current and D7 limits the voltage input to ISO1.
Any low voltage current source capable of providing 2 mA at 4 V or switching circuit
with less than 4.8kΩ loop resistance can be used to switch the DC loop.
The test PTT button on the front panel and the local microphone PTT button will also
key the transmitter. Both of these also mute the line audio input. The microphone line
also enables that audio input.
A DMTX board can also cause the exciter to key up. When a TX(or TTL_TX) signal
is received by the DMTX board, it pulls pin 6 of JP15 low, which, in turn asserts the
PTT_WIRE_OR signal, causing the microprocessor (U13) to key the exciter up.
5.11 Microprocessor Controller
The microprocessor controller circuit uses a single-chip eight bit processor and several
support chips. The processor U13 includes non-volatile EE memory for channel
frequencies, tones, and other information. It also has an asynchronous serial port, a
synchronous serial port and an eight bit analogue to digital converter.
The program is stored in U5, a CMOS EPROM. U4 is an address latch for the low order
address bits. U2 is used to read the channel select lines onto the data bus. U11 is an
address decoder for U5 and U2. U3 is a supervisory chip which keeps the processor
reset unless the +5 Volt supply is within operating limits. U1 translates the
asynchronous serial port data to standard RS232 levels.