Service manual

RC2000 Service Manual Chapter 2 Theory of Operation 11
Research Concepts, Inc; 10679 Widmer; Lenexa, Kansas; USA 66215 WWW.RESEARCHCONCEPTS.COM
MODESEL2, MODESEL1, MODESEL0 ... these signals are generated by the micro-controller. These
signals are the commands for the state machine. Here are the various commands which may be present
on these lines ...
BRAKE (000) ... the two lower MOSFETs of the bridge are on. This mode is used to stop the motors by
means of dynamic braking.
IDLE (001) ... all MOSFETs off. This state is active when an alarm is detected.
SLOW MOVEMENT EAST or DOWN (010) ... In this mode the motors are pulsed on and off to achieve
slow speed movement. The source of the pulse is the TACHIN input to the PLD. The TACHIN input is
generated by the micro-controller.
SLOW MOVEMENT WEST or UP (011) ... In this mode the motors are pulsed off and on to obtain slow
speed movement in either the west or up direction.
FAST MOVEMENT EAST or DOWN (100) ... In this mode the motors are turned on to achieve fast
movement in either the east or down direction.
FAST MOVEMENT WEST or UP (101) ... In this mode the motors are turned on to achieve fast movement
in either the west or up direction.
RESET ... This mode commands the state machines to reset the alarm state.
ALARM ... This mode commands the state machine to enter the alarm state.
TACHIN ... This input is generated by the micro-controller. It is used to pulse the motors off and on during
slow speed movement. The RC2000 implements a digital servo system. When the user specifies a slow
speed code, he or she is indirectly specifying the desired rate at which position pulses will occur when the
antenna is moving slow. The micro-controller varies the pulse width modulated waveform on the TACHIN
pin to vary the speed of the motor so that position pulses arrive at the rate specified by the slow speed code.
ALMLATCH (alarm latch - pin 21) ... This output from the state machine drives the alarm relay on the
analog board. The alarm relay is opened whenever an over-current condition is detected by the state
machine or the state machine is commanded to enter the alarm state by the micro-controller.
ASTAT1, ASTAT0 ... These outputs (from the state machine) are used to inform the micro-controller of the
alarm status of the state machine. 00 means no alarm, any other combination means that the state machine
is in the alarm state.
The state machine clock is derived from the micro-controller CLKOUT output. Whenever the micro-
controller changes the mode bits the clock to the state machine is disabled. The clock to the state machine
is present on pins 1 and 13 of the PLD. The CLKOUT pin of the micro-controller is connected to the CLKIN
pin of the PLD. The microcontroller controls the clock via the CLKEN (clock enable) input to the PLD. The
PLD logically ANDs the CLKEN and CLKIN signals to produce the CLKOUT (clockout - pin 22) signal. The
clockout signal goes to the 74HC4060 pre-scaler. The input clock frequency is divided down and presented
to the CLK1 and CLK2 inputs of the PLD.
2.2.6 Pulse Position Sensors
The pulses from the azimuth and elevation position sensors are processed by a comparator circuit (U15 -
azimuth, U16 - elevation). Resistors R13 and R15 form a voltage divider which presents 2.5 volts to the
non-inverting input of the LM311 comparator. R17 and R14 are used to create hysteresis in the
comparator circuit and eliminate jitter in the output of the comparators.