Mainpowersupply - Keithley 263 Instruction Manual

The display is updated at 1.56msec intervals. Timing for
this process is performed by a 64OHz clock which controls
the segment latches, U22 and U23, located on the mother
board. Each time an interrupt is generated, the micro-
processor unit writes segment data for two digits to the
segment latches. The two latches outputs are then enabled
in sequence by the 64OHz clock. When a latch output is
enabled, segments in the selected digit are turned on.
When a latch output is enabled, segments in the selected
digit are turned on. Digit selection is performed by data
from the PA3, PA4, and PA5 outputs of the microprocessor
unit to control IJ4. A single U4 output, corresponding to
the selected digit, will go low when the correct data ap-
pears on its inputs. For example, if 010, appears on the in-
puts of U4, DSl will be turned on.
Selection of the display annunciators is performed in a
similar manner. The data stored in the corresponding seg-
ment latches will then turn on the appropriate annunciator
LEDs.
Front panel switches are read in a somewhat similar man-
ner by using a row-column matrix along with U3. To select
a particular row, data is transmitted out the microprocessor
unit PA3, P&I and PAS lines to U3. This action selects one
of the rows by placing the corresponding output of U3
high. Matrix columns are then read by reading data in
through PAO, PAl, and PA2, of the microprocessor unit.
If a particular switch contact is closed, the data bit will be
high; if the switch is open, the data bit will be low.
7.10 MAIN POWER SUPPLY
The diagram for the main power power supply is located
on drawing 263-106, sheet 2. AC power is applied to Jl,
which is the power connector located on the rear panel
of the instrument. Sl is the POWER switch located on the
front panel, while 52 is the line voltage selection switch.
When 52 is in the ll5V position, the primary windings
are connected in parallel; when 52 in in the 230V position,
the Tl windings are placed in series.
Tl has three secondary windings which are used to supp-
ly the +V, +l7V, -24V, and +5V supplies. The +V supp-
ly is then used to supply the +6V supply while the -24V
supply is used to supply the -5V supply. The +l7V, -24V
+6V and the -5V supplies are used to supply the voltage
source. The +l7V supply is also used to generate the
+10.6V reference for the PWM circuitry. The +5V supply
is used for the ditital circuitq and the *V supplies are used
for the analog circuitry.
The &V supplies (- +2OV) consist of a full-wave rectifier
(CR2) and filter capacitors (C38 and C39). Rectification for
the +l7V supply is provided by CR5 and CR8 Filtering
is provided by C44, and regulation is provided by U49. Rec-
tification of the -24V supply is provided by CR4 and Cl77
Filtering is provided by C45, and regulation is provided
by U42. Rectification of the +5V supply is done by
elements of CRl, while filtering is performed by Cl. The
supply voltage is regulated by U33.
The +6V supply is derived by applying the unregulated
+V supply to U5. Resistors R4 and R5 adjust the regulator
to output the proper voltage. The -5V supply is derived
by simply applying the -24V supply to a -5V regulator
(U50).
The +10.6V reference is supplied by the +l7V supply. U2
provides the reference, while U?, Ql5, R3, R9 and RlO pro-
vide a constant current to minimize zener voltage variation.