Fluke NetDAQ 2640A, Service Manual

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Theory of Operation
Inguard Software Description
2
2-49
Power-Up Protocol 2-89.
The inguard powers up silently, without sending any kind of unsolicited information to
the outguard. The outguard, after powering up, waits 3.5 seconds before attempting to
communicate with the inguard, to allow it to complete its initialization procedure and
power-on self-tests. The inguard performs only limited self-tests automatically on
power-up. The full set of self-tests is performed only in response to a self-test command
from the outguard.
Inguard Unresponsive 2-90.
The inguard does not contain any kind of watchdog timer. If, for whatever reason, the
inguard fails to respond after the expected length of time, the outguard should reset the
inguard by sending a series of break characters. The “expected length of time” for a scan
command is variable depending on the number and types of channels defined, and is
calculated by the outguard at run-time.
Inguard Software Description 2-91.
The major functional blocks of the inguard are given in Figure 2-7. The arrows show the
flow of measurement information. There is a control interface (not shown) between the
A3U5 A/D microprocessor and every other functional block.
The channel scanner relays select the desired channel to be measured and route it to the
function relays. The function relays route the signal to the appropriate portion of the
Signal Conditioning circuitry, depending on the function being measured (VAC, VDC,
ohms, etc.). The Signal Conditioning circuitry converts the signal into a form that can be
measured by the A/D (i.e., a DC voltage with a range of -3 to 3V).
The A/D converts the analog voltage to a digital value, which is then read by the A3U5
A/D microprocessor. The box labeled A/D microprocessor represents the microcontroller
and its associated memory and glue logic, upon which the inguard software runs. It
controls all of the other hardware elements on the inguard and handles communication
with the outguard.
The primary task of the inguard software is to interpret configuration information and
scan requests from the outguard, manipulate the hardware in the appropriate way to
obtain the requested measurements, and return the measurement data to the outguard.
Hardware Elements 2-92.
This section contains information about the various hardware subsystems on the inguard
board.
Channel MUX 2-93.
The channel multiplexing consists of treeing and channel switches, implemented with
either FET switches (2645A) or reed relays (2640A). There are two sets of bits
associated with these switches. The tree bits must be set to indicate which bank of
channels is being used where bank 0 is channels 1 to 10, and bank 1 is channels 11-20.
For four-wire ohms measurements, both banks are selected. The position of the tree
switches is also a function of the channel function and range being measured.