Tektronix ECO 422, Instruction Manual

Page: 58 / 177

Theory of Operation
4–4
ECO 422
Figure 4–3: Block Diagram for the Control and Remote Circuit
Keyboard Encoder. The front panel is connected to the Main board through J16.
The input from the front panel is scanned by the Keyboard encoder, U11. The
outputs are active high and chosen so that only one can be active at a time. The
KEY_VALID signal is ANDed with each output inside the controller to provide
switch debouncing.
Front-panel LED Driver.
U12 outputs the signals to drive the LEDs on the front
panel. It drives the LEDs to +5 V.
U12 outputs the signals to driver the LEDs on the front panel. It drives the LEDs
to +5 V. The timer, U13, provides the pulsing necessary to flash the front-panel
fault LEDs and clock the front panel Time Out circuit in the Controller. When
(TIMER_DIS) is low, the timer is prevented from pulsing. When the input goes
high, the timer output goes low, turning on and allowing the Controller to flash
the LEDs.
Remote.
The Remote port, J17, provides three inputs and four outputs from the
Controller. (R_AUTO) controls auto switching, (R_SOURCE) is a manual
source switch, and (R_RESET) resets the fault indicator LEDs. These signals are
buffered by U18B that is always enabled. The input has six diodes for ESD
protection.
The buffer drives the two input source light outputs through resistors to ground
because there is no remote power on the rear-panel connector. The inversion of