Fluke 900 - Manual (Page 20)

Theory of Operation FLUKE 900 SERVICE MANUAL
The busses shown are as follows:
EBUS
External bus represents DUT signals after conversion to TTL levels using a single
threshold in the IB. EBUS is labelled on pages 2, 4, 6, 8, 9, 10, 11 of the H3
schematics.
RDBUS
Reference device bus carries EBUS input pin signals to the RD socket and RD
output pin signals to the DIFTEST circuit. RDBUS is labelled on pages 7, 8,9,10,
11 of the H3 schematics.
EBUS
Fault bus indicates on which pins the RDBUS and EBUS were different for longer
than the FMASK value. The FBUS consists of the op amp ouQ)ut signals labelled
Fxxon pages 8,9,10,11 of the H3 schematics.
SYNCBUS
Synchronizing bus is either the EBUS or a pattern from the device library,
depending on whether the Sync Vectors technique is employed to synchronize RD
and DUT. The SYNCBUS consists of the 74244 output signals found on the
extreme left side of pages 8,9,10,11 of the H3 schematics.
MONBUS
Monitor bus is either the same as EBUS or FBUS, depending on which is selected
for display on the monitor LEDs. The Mon bus may be found at connector J3 on
the right side of pages 8,9,10,11, of the H3 schematics and on page 13 of the MB
schematics.
The 18 circuit blocks shown in the diagram on the previous page are the same ones listed across
the top of the Selftest/Circuit Block Reference Table (Section 4.2). Fifteen of the blocks
described below are found on the H3 Board.
1 . PuUups Latch
On the Interface Buffer, signals from the test clips are pulled up to VCC or down
to Gnd through a lOK resistor from the output of this shift register latch. It is found
on page 1 of the IB schematic, Ul, U2, U3, U4. The register at U24 of the BB drives
a DAC which sets the threshold.
Inbuf
These buffers found on pages 8,9,10, 11, of the H3 schematics (U186, U140, U95,
U42), feed the EBUS through to the SYNCBUS. They are tristated when the Sync
Latch is driving.
Sync Latch
Synchronizing patterns from the library data are shifted ink) the registers shown on
pages 8,9,10,11, (U182, U136, U91, U45) and then latched to the output as a test
vector. The stimulus for the RDTest ftmction is provided in the same way.
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