Theory of Operation—2465B/2467B Service
Input signals from the Channel 1 input connector are
routed through an attenuator network by four pairs of
magnetic-latch relay contacts. The position of the relays is
set by Microprocessor data placed into Auxiliary Control
Register U140. Relay buffer U110 provides the necessary
drive current to the relays.
Four input coupling modes (1Mfi AC, GND, 1MJ2 DC,
and 50 fl DC) and three attenuation factors (1X, -MO, and
-MOO) may be selected by closing different combinations
or relay contacts. The three attenuation factors, along with
the variable gain factors of the Vertical Preamplifier, are
used together to obtain the crt deflection factors. The
relays are magnetically latched and once set, remain in
position until new attenuator-relay-setting data and
strobes are generated. (See the "Auxiliary Control Regis-
ter" description for a discussion of the relay-latching pro-
cedure.)
The 50 SI termination resistor has a thermal sensor
associated with it that produces a dc voltage (CH 1 OVL)
proportional to the input power. Should the input power
exceed the normal safe-operating level for the 50 0 DC
input, the termination resistor temperature will exceed the
normal operating limit and change the output voltage of
the thermal sensor. The amplitude of this dc level is
periodically checked via comparator U2510 and DAC
U2101 (on diagram 2) and allows the Microprocessor to
detect when an overload condition is present. When an
overload occurs, the processor switches the input coupling
to the 1 Mfi position to prevent damage to the attenuator
and displays 50
Q
OVERLOAD on the crt.
Compensating capacitor C105 is adjusted at the time of
calibration to normalize input capacitance of the
preamplifier to the attenuator.
A probe-coding ring around the BNC input connector
passes probe coding information (a resistance to ground)
to the Analog Control circuitry for detection of probe
attenuation factors. The readout scale factors are set to
reflect the detected attenuation factor of the attached
probe.
Auxiliary Control Register
The Auxiliary Control Register allows the Microproces-
sor to control various mode and range dependent func-
tions of the instrument. Included in these functions are:
attenuation factors, input coupling, Channel 3 and Channel
4 gains, vertical-bandwidth limiting, the X-Y display mode,
and the state of the measurement PAL.
When the Microprocessor sets the input coupling mode
and attenuation factors for Channel 1 and Channel 2, a
series of eight, 16-bit control words are serially clocked
into shift registers U140 and U150 (eight bits in each regis-
ter).
Each control word is used to set the position of one
of the eight attenuator and coupling relays (four relays are
in each attenuator assembly). Each control word will have
one HI bit. This bit will correspond to the specific relay
contact to be closed. Relay buffers U110 and U130A (for
Channel 1) and U120 and U130B (for Channel 2) are Dar-
lington configurations that invert the polarities of all bits.
This results in a LO being applied to only the coil lead
associated with the contact to be closed; all other coil
leads are held HI.
To set a relay once the control word is loaded, the
Microprocessor generates a ATTN STRB (attenuator
strobe) to U130G pin 7 via R129 and C130. The strobe
pulses the output of U130G LO for a short time. This out-
put pulse attempts to turn on both Q130 and Q131 (relay
drivers) via their identical base-bias networks. Due to the
lower level from the turned on Darlington relay buffer
(cou-
pled through the associated coil diode and either CR130 or
CR131 to one of the bias networks), one transistor will
turn on harder as the ATTN STRB pulse begins to forward
bias the transistors. The more positive collector voltage of
the transistor turning on harder is fed through the bias
diode (again either CR130 or CR131) to further turn off the
opposite transistor. This action results in one transistor
being fully on and the other one being fully off. The
saturated transistor sources current through the two
stacked relay coils to the LO output of either U140 or
U150 (current sink) to close the selected contacts. Once
set, the magnetic-latch feature will hold the relay set to
this position until opposing data is clocked into the Aux-
iliary Control Register and strobed into the relay. All coil
leads for the remaining relays are set HI, and only the
selected relay will be set.
To set the seven remaining Attenuator and coupling
relays, the sequence just described is repeated seven
more times. Whenever the Microprocessor determines that
the attenuation factor or input coupling has changed, the
entire relay-setting procedure is repeated for all eight
relays.
After the coupling and attenuator relays have been
latched into position, the Auxiliary Control Register is free
to be used for further circuit-controlling tasks. Eight more
bits of control data are then clocked into U140 either to
enable or disable the following functions: vertical
bandwidth limiting (BWL), triggered X-Y mode (TXY), the A
and B Sweep Delay Comparators (BDCA and BDCA), and
slow-speed intensity limit (SIL); or to alter the Channel 3
and Channel 4 gain factors (GA3 and GA4). Four other
3a-14
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