5-6 THEORY OF OPERATION MDS 05-2415A01, Rev. A
compatible output. Transient protection for the five RS-232 I/O lines from J1 is
accomplished by means of zener protection diodes. Static discharge or over-voltage condition
appearing on J1 will be shunted to ground with these devices before reaching U31.
In the data signal interface between the plug-in modem assembly and the main transceiver
board (i.e., the plug in modem option), the signals are inverted from standard RS-232 signal
polarity. The inverted signals, namely RXD(L), TXD(L), DCD(L), RTS(L), and CTS(L), are
fed directly to U31.
Header J11 allows the TTL option to be selected. When the pins 1 and 2 of J11 are connected
together, U31 is disabled. This effectively removes U31 from the circuit.
LED INDICATORS
U29 and U5 drive the LEDs. U29 is a non-inverting buffer. A jumper across J11, pins 3 and 4
is required to enable the LEDs.
PWR
This LED, CR31B, is illuminated if more than 10 volts is applied to the radio. It flashes if
the VCO is Out-of-Lock. U5B is a flasher circuit.
RTS
This LED, CR31A, is illuminated if any of the Push-To-Talk lines is active.
TX
This LED, CR32B, is illuminated if the transmitter power control is active.
TXD
This LED, CR32A, is illuminated if the RS-232 transmit data line is low.
CD
This LED, CR33B, is illuminated if the receiver detects a carrier strong enough to break
squelch.
RXD
This LED, CR33A, is illuminated if the RS-232 receive data line is low.
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