Keithley 486 Instruction Manual page 156

SECTION 6
Theory of
Operation
Display Data
SR
=
Shii
Register
Display-Clock
to all
SW*
-
Ygure 6-16. Display Block Diagrmn
outputs causing the segments of the selected digit to turn
on. The process repeats for a total of eight times per dis-
play update cycle.
Current limit for U408 and U412 are provided by R407
and R408. Separate drive for U404 is not required.
Reading the Keyboard
The front panel keys are organized as a 5-row by 3-col-
umn matrix. In order to determine which key is closed,
each row in succession is strobed by setting the appropri-
ate output of U411 high. For each row, the three columns
are read through the VIA on the digital board. Knob data
is read through the VIA as well. The rotary knob outputs
a digital 2-bit gray-code (one line changes state for every
bob detent rotation).
6.5.4
IEEE-488
interface
The IEEE-l88 interface is located on drawing 487-106,
page 2 at the end of Seaion 8. A simplified circuit of the
interface is
shown in Figure 6-13.
The interface includes three ICs. U514 is a 9914A GPIA
(General Purpose Interface Adapter) that performs many
bus control functions, such as handshaking automati-
cally to minimize MPU overhead. Bus drive capabilities
are provided by U515 and U516. U516 supports the data
6-13