Keithley 486 Instruction Manual page 155

SECTION 6
Theo y of
Operation
' 16LSH-15
SHIFT DONE
741 HCT393
u507
COUNTER
G
A
u502
>
ADDRESS
A15-A10
DISPLAY CLOCK
DISPLAY DATA
I I
Figure 6-15. Simplified Block Diagram
of
Serial Port
3. After all eight bits are shifted out, the shift counter
generates the SHIFT DONE signal to indicate that
theentirebytehasbeenshiftedout.Thissignalisfed
back to the PAL, U507, which halts the GCLK signal
to stop the shifting process.
4.
The process in steps 1 through 3 repeats for seven
successive bytes until all the necessary information
is imnsmitted to the display and keyboard circuits.
5. The DISPLAY LEFT and DISPLAY RIGHT lime.,
whichre controlled by the PA6 and PA7 terminals
of U504, are brought low to enable the information to
be displayed on the display board,
6.5.3 Display/Keyboard
The display and keyboard circuits are shown on drawing
486-116 for the Model 486 or 487-116 for the Model 487,
located at the end of Section 8. Figure 6-16 shows a block
diagram of the display and keyboard circuits.
Display Multiplexing
The digits of the X-digit left display, and the group of
separate LEDs are multiplexed, which means that only
one digit (or the group of separate LEDs) is actually on at
any given time. Because of the rapid multiplexing rate,
display flicker is not visible to the user.
For each digit, display data is docked in via the DISPLAY
DATA line using the DISPLAY CLOCK and the tians-
mission sequence discussed in the previous paragraphs.
This data is shifted into shift registers located on the dis-
play board. U401 and U409 hold segment data, and U406
and two bits of U411 store selected digit data. One bit of
U411 is used to contiol the separate LEDs, while five bits
of U411 are used to strobe the five rows of the keyboard
matrix.
After sll data are shifted in, the DISPLAY LEFI line is
brought low to enable into the LX01 and LJ409 outputs.
This action causes the pertinent segments of the selected
digit (or the selected separate LEDs) to turn on. The proc-
ess repeats for a total of 11 times per display update cycle
so that each display digit or LED can be turned on as ap-
propriate.
Segment drive capabilities are an integral part of U401
and 409, and external current-limiting resistors are re-
quired and provided by R405 and R406. Drive capability
for U406 and U411 are provided by U405 and U410 re-
spectively.
The 8-d@ right display operates in a similar fashion.
Display data is clocked into three shift registers. U408
and U412 hold segment data, while U404 stores selected
digit data. After all data are shifted in, the DISPLAY
RIGHT line is brought low to enable the U408 and U412
6-12