Keithley 708B Reference Manual page 160

Models 707B and 708B Switching Matrix Reference Manual
Display watchdog
Once the display tasks are started, if the microcontroller stops refreshing, the watchdog circuit shuts
off the column drivers to preserve the life of the LEDs.
The display watchdog is part of the standard display test (see
matrix (on page 8-9) for information).
If the watchdog shuts down the display, the first several row labels show "SLOT GRID LEDS OFF."
Description of basic display operation
This display board is a subordinate device to the microprocessor board.
The microprocessor board communicates commands to change the state of the matrix or update the
row and column labels.
The display board processes these commands independently. It also refreshes the display and
performs other tasks. When a display command is issued, the microcontroller processes it to update
the display. The task of refreshing rows and PWM generation is done in an interrupt-driven routine to
ensure predictable timing.
707B-901-01 Rev. A / August 2010
Section 4: Theory of operations
Testing the display, keys, and channel
4-9