Tektronix 2440, User Manual

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Theory of Operation— 2440 Service
put of OR-gate U132A LO. This LO enables the upper-four
buffers of the Octal Buffer U120. With the Q-output of
Flip-Flop U894B connected to the input of one of the
enabled buffers, that reset-forced LO obtained at WR is
coupled to the DO line of the System
m
P Data Bus. The
System
m
P monitors the DO bit as long as RD and
SEQINCS are asserted.
The input to inverter U424E is normally pulled up to
+ 5 volts by R120. This HI is inverted LO by U424E and
routed to the positive-triggered clock input of Flip-Flop
U894B. If, during the time RD is asserted, SEQUENCE IN
steps LO at J1905, it drives the clock input of U894B HI
and the + 5 volts hardwired to the D-input of the flip-flop
latches to the Q output. With the Quad Buffer still enabled
by RD , the System ^P reads the transition via data bit DO
and restarts the temporarily-halted sequence.
W o rd T rig g e r a n d G P IB S ta tu s C o n tro l R e g is te r
The Word Trigger circuit provides interface and control
of the external Word Trigger Probe. Two bits from Control
Register U754 are used to set the recognition mode of the
Word Trigger Probe. Forty bits of serial data are applied to
the W DATA (word data) line and clocked into the serial
shift register in the word probe by toggling the W CLOCK
(word clock) line. Once loaded, the Word Trigger Probe
outputs a trigger pulse each time (and as long as) the set
conditions are met.
The W DTTL output is applied to the trigger circuits
where, if selected as the trigger source, it produces a
scope trigger event. The trigger signal is buffered to the
rear panel by U844D, Q720, and the • associated
components. Output levels are TTL compatible, with the
maximum HI level being set by R716 and VR717. Output
impedances are 47 ohms LO and 227 ohms HI. Diode
CR722, zener VR717, and resistors R717 and R718 pro­
vide protection of the output circuit should an out-of-range
voltage be applied to the output connector.
The remaining inputs and outputs of Control Register
U754 are used to control the GPIB Status LEDs and to
reset GPIB Controller U630.
B ell
The Bell circuit allows the scope to produce an audio
tone to draw the operator s attention to certain warning
and error conditions. The circuit consists of a free-running
oscillator whose signal is gated through the
output speaker.
The oscillator consists of timer U274, configured as an
astable multivibrator (oscillator), and output transistor
Q594, used to buffer the oscillator output. Current flowing
in R274 and R276 charges C372 up until it crosses the
trigger level at pin 2 of U274. This sets the output applied
to the base of Q594 LO, turning the transistor off, and
sets the discharge output at pin 7 to ground potential.
Capacitor C372 now discharges through R276 until the
threshold level at pin 6 is reached, at which time the out­
put at pin 3 goes HI and the discharge pin goes to a high-
impedance state. Capacitor C372 begins to charge
through R274 and R276 again, completing the cycle. The
cycle continues as long as instrument power is applied,
alternately turning Q594 off and on with an approximate
50% duty cycle.
The BELL line from the Miscellaneous Register (U760,
diagram 1) is used to gate this oscillator signal through the
speaker to produce the audio output. As long as BELL is
LO, transistors Q596, Q558, and Q592 are off, and current
is cut off to speaker LS498.
When BELL goes HI, transistor Q596 turns on, which in
turn, turns on Q588. With Q588 on, the base of Darlington
transistor Q592 is pulled HI. Now, whenever the oscillator
transistor Q594 is on, proper biasing conditions for Q592
are established and current flows from the + 5 V D supply
to ground through Darlington Q592, the speaker LS498,
and transistor Q594. When Q594 turns off, current flow is
interrupted until the oscillator turns Q594 back on.
Since LS498 is inductive, the current decay portion of
its cycle (Q594 off) tends to force pin 1 of the speaker
above the + 5 V D supply level. Diode CR594 becomes for­
ward biased in this case and shunts the decay current
back to the + 5 V D supply, protecting transistor Q594 from
overvoltage conditions.
As long as the BELL line remains HI, the speaker pro­
duces an approximate 2 kHz tone. In practice, the System
sets the BELL line HI for a short time ( = 4 ms), turning
Q588 on, starting the tone and rapidly charging C590.
When BELL returns LO, C590 gradually discharges
through R594. As the capacitor discharges, bias on Q592,
and thus current through the speaker, is reduced, causing
the sound to gradually fade out in a pleasing "bell-like”
tone.
P ro b e P o w e r
The Probe Power outputs on the rear panel provide
access to three of the instrument power-supply voltages
and may be used to power approved voltage- and
current-probe accessories. Contact your Tektronix sales
representative for a list of approved probe accessories.
V id e o O p tio n C o n tro l R e g is te r
The Video Option Control Register (U750 on
diagram 20) is written to by the System Processor
(address-decoded location 6012h) to control operational
setup of the Video Option. The Video Option Control
Register is initialized on power-up and provides for control
of the following functions:
3 -8 3