Nautel NX15, Troubleshooting Manual

Page: 92 / 274

NX25/NX15 Troubleshooting Manual Detailed Circuit Descriptions
Page 2-14 Issue 5.0 2016-05-06
Control, Monitoring and Protection
Microprocessor U22 performs all control, monitoring, and protection services required. It monitors
analog samples and alarm conditions and will report back to a rack master controller via a serial data
bus. Each RF power module is programmed with a unique serial number and when inserted into a
transmitter is assigned a serial address by the rack master.
CONTROL
Serial Bus:
The RF power module uses the RS485 serial protocol for communication with the rack
master. This allows many devices to communicate on the same serial bus without interfering with
each other. Each RF power module is capable of sending status and analog sample data to the rack
master for diagnosis and display purposes. Modules also receive control data from the rack master.
Microcontroller U22 sends and receives data through RS485 transceiver U20. U20 converts either the
RX or TX TTL data to a balanced RS485 signal that can interface with the serial bus. U22 determines
if it is transmitting or receiving data by controlling drive enable and receive enable pins (2 and 3) of
U20. Typically U22 only transmits data over the serial bus when requested by the rack master.
Module Enable:
A discrete enable control line is connected to each module through the front RJ45
connector (J1). The enable signal is received as a differential signal and converted to a TTL signal
through RS422 receiver U1:B.
When the mating cable is unplugged from J1, the output of U1:B defaults to a high state. This places
5 V on pin 1 (
INT1 ) of microprocessor U22, which causes an interrupt to occur within U22,
disabling the module. An RF Power Module #: External Disable Active alarm is generated.
Mod Drive Enable:
The Mod Drive Enable signal is a status line that is switched through Q6. It
enables and disables PDM MOSFET drivers U12, U14 and U15.
RF Drive Enable:
The RF Drive Enable signal is a status line that is switched through Q5. It enables
and disables RF MOSFET drivers U23, U24, U25 and U26.
RF Relay Close and Open:
The RF Relay Close and RF Relay Open lines control the RF shorting relay
connected across the primary of the RF output transformer at the output of each module. The RF
shorting relay is a latching relay with both an ‘operate’ and ‘release’ coil.
By default the RF shorting relay is forced into the closed position due to pullup resistor R65 on the
gate of Q3. This causes Q3 to conduct current through the ‘operate’ coil as soon as the 5 V supply is
present. During normal operation microcontroller U22 holds the gate of Q3 low, preventing it from
conducting current.