Tektronix 2445, Instruction Manual

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Theory of Operation—2445 Service
U2134. The Row data placed on the buffered Data Bus by
the Microprocessor is clocked into the registers as two,
eight-bit words by clocks from the Address Decode cir­
cuitry (DAC LSB CLK for the lower eight bits and DAC
MSB CLK fo r the upper eight bits). A ll eight outputs of
register U2134 and two outputs o f register U2034 drive the
ten rows o f the front-panel switch m atrix (the eleventh
line is not used in the matrix). Series resistors in the lines
lim it current flo w and eliminate noise problems associated
w ith excessive current flow.
While each row is selected, the processor w ill scan each
o f the five lines in sequence. To scan the columns, the
processor increments the three data select bits to U2034
that define the column to be checked. Eight-line data
selector U2456 connects the associated column line to
Status Buffer U2108. As each line is selected, the Micro­
processor reads the Status Buffer to determine if the
associated switch is open or closed.
In addition to the front-panel switches, the CAL/NO
CAL jumper (P501) is checked to determine whether
the instrument should be allowed to execute the calibration
routines. The levels on U2456 pins 7 and 9 are read by
scanning tw o additional columns at power-up. If the jumper
is pulling the CAL b it LO, the operator w ill be allowed to
use the calibration routines stored in firmware. If the NO
CAL bit is pulled LO, the calibration routines may not be
performed. If the jumper is removed, and neither bit is
pulled LO, the Microprocessor is forced into a special
diagnostic mode (CYCLE) used to record certain operating
failures during long-term testing of the instrument. (See the
"Maintenance" section o f this manual fo r an explanation of
the diagnostic modes.) Removing P501 or switching it
between the CAL and NO CAL positions w ill not be
recognized by the Microprocessor until the instrument is
powered down and then turned back on.
The SI (scope identification) b it is checked at power-up
to determine if the instrument is a 2445. Some parts o f the
firmware are shared w ith a similar instrument, the 2465,
and the check is necessary fo r the Microprocessor to
distinguish between the two instruments. A HI on the SI
b it indicates that the instrument is a 2445.
The resistors in series w ith the input lines to U2456 are
current-lim iting resistors that protect the CMOS eight-line
data selector from static discharges. The resistors con­
nected from the input lines to the +5-V supply are pull-up
resistors fo r the front-panel column lines.
Digital-to-Analog Converter (DAC)
DAC U2234 is used to set the various analog references
in the instrument and is used to determine the settings of
the front-panel potentiometer. The 12-bit digital values to
be converted are w ritten to octal registers U2034 and
U2134 fo r application to the DAC input pins. The DAC
then outputs tw o complementary analog currents that are
proportional to the digital input data. (Complementary, in
this case, means that the sum of the tw o output currents is
always equal to a fixed value.)
The maximum range of the output currents is estab­
lished by a voltage-divider network composed o f R2127,
R2227, R2228, and R2229 connected to the positive and
negative reference current inputs o f the DAC (pins 14 and
15 respectively). A +10-V reference voltage applied to the
DAC through R2228 sets the basic reference current.
Resistor R2229 and potentiometer R2127 provide a means
to adjust this current over a small range fo r calibration
purposes. The nominal reference current is 1 mA, and the
DAC full-scale output current is 4 mA. The output currents
flow through series resistors R2324 and R2325, connected
to the +1.36-V reference, and proportional voltages result.
Pot Scanning
The Pot Scanning circuitry, in conjunction w ith the
DAC, derives digital values for each of the various front-
panel potentiometers. Scanning of the pots is accomplished
by data selectors U2408 and U2418. Three bits are written
to register U2208 and select the pot to be read. The bits are
latched in the register and keep the pot selected until the
register is reset. The Microprocessor writes a LO to the
inhibit input (pin 6) o f either U2408 or U2418via register
U2308 to enable the device. The enabled data selector
connects the analog voltage at the wiper o f the selected pot
to comparator U2214.
Comparator U2214 compares the analog voltage of each
pot to the output voltage from the DAC (pin 18). To
determine the potentiometer output voltage, the processor
performs a binary search routine that changes the output
voltage from the DAC in an orderly fashion until it most
closely approximates the voltage from the pot.
The conversion algorithm is similar to successive approx­
imation and generates an eight-bit representation of the
analog level. When the pot's value is determined, the Micro­
processor stores that value in memory. Once all the pots
have been read and the initial value of each has been stored,
the processor uses a shorter routine to determine if any pot
setting changes. To do this the DAC output is set to the last
known value o f the pot (plus and minus a small d rift value),
and the status b it is read to see that a HI and LO occurs. If
w ithin the limits, the processor assumes that the pot setting
has not changed and scans the next pot. When the processor
detects that a pot setting has changed, it does another
binary search routine to find the new value of that pot.