Fluke 2680A, Service Manual

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Theory of Operation
A/D Theory of Operations
2
2-33
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If the Next bit is 1, the inguard does the next housekeeping reading indicated by its
internal schedule. This is the same schedule used for a housekeeping timeout.
•
If the Do2 bit is 1, the inguard does the two reference balance readings.
•
If the PS bit is 1, the inguard presets the function relays for the first defined channel.
These bits may be set or cleared independently. Note that the actions described above are
carried out regardless of whether scheduled housekeeping is “enabled” by the global
configuration command.
Response Packets Returned Always returns a single response packet. This packet is not
returned until the inguard completes all indicated housekeeping measurements.
Response Packet Format Returns a single ACK packet.
Checksums
The last byte of each command and response packet is its checksum.
Any time a packet that fails its checksum test is received, it is treated as a communication
error. The inguard transmits a break and waits to be reset. The outguard resets the
inguard.
Errors
Whenever the inguard encounters an unrecoverable error or a guard-crossing
communications error (e.g., parity error, overrun), it attempts to send a break character to
the outguard and then goes into a loop, ignoring all subsequent commands from the
outguard, and waits to be reset by the outguard. This insures that all measurement
hardware is properly reset. This type of error could be caused by a glitch in the inguard
hardware, which is conceivable but rare.
The inguard returns a NAK whenever it receives an illegal command or a command with
illegal parameters. Such an error should never occur and probably indicates a software
defect. The exception to this is that an error in a scan command returns a break (instead
of a NAK).
Power-Up Protocol
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
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
The major functional blocks of the inguard are given in Figure 2-1. 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.