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5
CIRCUIT DESCRIPTION
5.11 Microprocessor Controller
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
Micr opr ocessor Contr oller
The microprocessor controller circuit uses an 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.
The analogue to digital converter is used to measure the forward and reverse power,
tuning voltage and dc supply voltage.
If the processor detects that the PTT_WIRE_OR signal is asserted low, it will attempt to
key the exciter up. It will first attempt to key the VCO through Q10, and if the LD pin
goes high, it will switch the 9.2 Volt transmit line through Q14 and Q16.
Asserting
Q16 has the effect of also asserting the yellow Tx LED (D12) on the front panel,
enabling the local 25W power amplifier, and causing the T/R Relay output to be pulled
low. D24 is 30 volt zener which protects Q25 from both excessive voltages or reverse
voltages.
Should there be a problem with either the tuning volts, or the battery voltage, the VCO
locking, the forward power, or the reverse power, the microprocessor will assert the
ALARM LED through Q1.
Depending on the setting of Jumper JP19, the ALARM
signal can be brought out on pin 7 of P3.
RF Technology T220
Page 17
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