5-4 THEORY OF OPERATION MDS 05-2415A01, Rev. A
If J1-Pin 12 is grounded U13 and U38 are disabled. Since U38 supplies current to nearly all
of the transceiver circuits, current consumption in the transceiver is reduced essentially to
zero. This eliminates the need to externally switch the +13 volts applied to the transceiver.
AUDIO/DATA MODULATION INPUT SWITCHING
One section of U15, switch U15X, controls data appearing at the RXD terminal, Pin 3 of J1,
switching between modem data and microcontroller data from U16. Another section, U15Z,
switches the transmit audio path between the modem transmit audio output and the external
transmit audio input to the transceiver from J1.
U15Z is controlled by means of a modem enable line which is tied to +10 volts when the
modem option is installed.
Without the modem installed, the normal state of U15Z is such that transmit audio from J1
Pin 9 modulates the transmitter. With the modem installed, the external transmit audio from
J1 is cut off, and modem transmit audio is selected.
Should the positive-going push-to-talk (PTT) input on J1-14 be driven high (to key the
transmitter—such as when the external order wire option is used), U19B pulls the control line
for U15Z low. This allows external transmit audio in from J1 to modulate the transmitter, and
cuts off any audio coming from the modem.
MICROCONTROLLER/EEPROM
The microcontroller, U16, controls many of the on-board functions of the transceiver. Some
of the control functions are:
• Frequency Programming and Control of the Synthesizer
• Modem RTS/CTS Delay
• Transmitter Time-out Timer
• Transmit Soft Carrier Dekey Delay
• Transmit Squelch Tail Eliminator Delay
• Loopback/Diagnostic Functions
U16 runs a predetermined routine that controls its functions; this routine is permanently
programmed within the IC and cannot be altered. All programmable functions and values are
stored by the microcontroller in an electrically erasable, programmable, read-only memory
(EEPROM) IC, U18. These include operating parameters such as frequency, time-out timer
limits, soft carrier dekey times, and CTS delay time, as well as model and factory serial
numbers. U16 and U18 share a common clock and exchange data through data lines.
The microcontroller can be reset by several different means. An error output from U13 drives
the reset line low causing the microcontroller to reset or the internal modem can drive the
reset line from J8. The error signal from U13 resets U16 allowing a stable initialization when
13 Vdc is first applied to the transceiver.
The inhibit line on U13 is controlled indirectly by the radio enable line available at J1-12.
The inhibit line on U13 will force a microcontroller reset through the error output line.
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