Encoder Pca (A2); Ohms Pca (A5) - Fluke 5080A Service Manual

Encoder PCA (A2)

The Encoder PCA (A2) controls the front panel keyboard, knob, and displays. It
has its own microprocessor and communicates with the Main CPU PCA (A9) on
the Rear Panel through a serial link. Memory for the Encoder PCA is contained in
EPROM. The Encoder PCA is the interface to the Keyboard PCA (A1).

Ohms PCA (A5)

The Ohms PCA (A5) sources 1x and 1.9x fixed value resistances, provides
compensation, and generates an active guard. The board can source ohms in
one of several ways: two-wire, two-wire with compensation, and 4-wire ohms.
Proprietary resistor networks are used in the ohms board. The values are not
exact, but the resistors have excellent stability and low temperature coefficients.
The resistors are made for 4-wire operation but may be used as 2-wire devices
with degraded specifications. In four-wire mode, the resistors are connected to
the NORM_HI, NORM_LO, AUX_HI, and AUX_LO terminals without additional
active circuitry.
The relay switch matrix is used to switch in the resistors, compensation, and
other circuits that provide all of the A5 Ohms functionality.
Besides 4-wire ohms and uncompensated 2-wire ohms, the user can also select
to have two-wire compensated ohms where circuitry is used to negate most of
the effects of path loss to a two terminal resistance measurement instrument.
Compensation circuits are protected by clamping diodes. However, the resistors
have maximum peak currents that cannot be exceeded. See Figure 4.
NRM_HI
AUX_HI
AUX_LO
NRM_LO
HIGUARD
Relay Switch
Matrix and
Switch
Control
Figure 4. Ohms Function
Other Control
and Monitor
Circuits
Resistor Networks
Compensation
Circuits
Active Guard and
Protection Circuits
Calibrator
Theory of Operation
Floating
Supply
(FCOM)
gru117f.eps
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