Tektronix 7623, Instruction Manual

Page: 55 / 266

Whenever this instrument is turned on, the Vertical
Chopped Blanking pulses are being produced at a two-
megahertz rate. However, these pulses are available as an
output at pin 4 only when the remaining inputs to U55B
are at the correct levels. The following discussions give the
operating conditions which produce Vertical Chopped
Blanking pulses to blank the CRT during vertical trace
switching. Fig. 3-5A identifies the functions of the pins of
U55B.
1. CHOP VERTICAL MODE
When the VERT MODE switch is set to CHOP, Vertical
Chopped Blanking pulses are available at pin 4 at all times.
The input conditions necessary are:
PIN 3 HI-VERT MODE switch set to CHOP.
Pin 7 LO-VERT MODE switch set to any position
except ADD.
Pin 10 LO-Delay Ramp more negative than about 0
volts.
2. LEFT VERTICAL UNIT SET FOR CHOPPED
OPERATION
If the Left Vertical unit is set for chopped operation, the
setting of the VERT MODE switch determines whether the
Vertical Chopped Blanking pulses are available. If the
VERT MODE switch is set to the CHOP position, con­
ditions are as described in No. 1 above. Operation in the
ADD position of the VERT MODE switch is given later.
For the LEFT position of the VERT MODE switch, or
when the left vertical unit is to be displayed in the ALT
mode, Vertical Chopped Blanking pulses are available at all
times (two-megahertz rate). The input conditions are:
Pin 3 LO-VERT MODE switch set to any position
except CHOP.
Pin 5 LO-Left vertical unit set to chopped mode.
Pin 6 LO-Left vertical unit to be displayed (Vertical
Mode Command LO).
Pin 7 LO -VERT MODE switch set to any position
except ADD.
Pin 10 LO-Delay Ramp more negative than about 0
volts.
Notice that the Vertical Mode Command at pin 6 must
be LO for output pulses to be available at pin 4. This means
that when the VERT MODE switch is set to ALT, Vertical
Chopped Blanking pulses are produced only during the time
that the left vertical unit is to be displayed (unless right
vertical unit is also set for chopped operation).
Circuit Description-7623/R7623 Service
3. RIGHT VERTICAL UNIT SET FOR CHOPPED
OPERATION
If the right vertical unit is set for chopped mode,
operation is the same as described previously for the left
vertical unit except that Vertical Chopped Blanking pulses
are produced when the VERT MODE switch is set to
RIGHT or when the Vertical Mode Command is HI in the
ALT mode. The input conditions are:
Pin 3 LO—VERT MODE switch set to any position
except CHOP.
Pin 6 HI-Right vertical unit to be displayed (Vertical
Mode Command HI).
Pin 7 LO-VERT MODE switch set to any position
except ADD.
Pin 8 LO -Right vertical unit set to chopped mode.
Pin 10 LO-Delay Ramp more negative than about 0
volts.
4. ADD VERTICAL MODE
When the VERT MODE switch is in the ADD position
and either or both of the vertical units are operating in the
chopped mode. Vertical Chopped Blanking pulses must be
available to block out the transition between traces of the
vertical units. The input conditions are:
Pin 3 LO-VERT MODE switch set to any position
except CHOP.
Pin 5 LO—Left vertical unit set to chopped mode (can
be HI if pin 8 is LO).
Pin 7 HI-VERT MODE switch set to ADD.
Pin 8 LO-Right vertical unit set to chopped mode (can
be HI if pin 5 is LO).
Pin 10 LO—Delay Ramp more negative than about 0
volt.
Fig. 3-6A shows a logic diagram of the Vertical Chopped
Blanking stage. Notice the comparator block on this
diagram (one input connected to pin 10). The output of
this comparator is determined by the relationship between
the levels at its inputs. If pin 10 is more positive (HI) than
the grounded input, the output is HI also; if it is more
negative (LO), the output is LO. An input/output table for
this stage is given in Fig. 3-6B.
Chop Counter
The Chop Counter stage produces the Mainframe Chop
Signal and the Vertical Plug-In Chop Signal. The Clock
3-11