Fluke PM6690, Service Manual, Page: 65 / 200

Microprocessor Bus &
Interfaces
The instrument has a conventional 16-bit microprocessor bus
with 16 bits bidirectional data signals, 5 bits address signals,
Chip Selects and wrn and rdn signals. It connects the proces-
sor to the FPGA, the GPIB and the USB. See Figure 6-34.
The FPGA connection has 16 data bits, 5 address bits, chip
select, wrn and rdn. The FPGA is controlled by the processor
via the bus. Measuring functions are selected, for instance.
The FPGA is controlled between each measurement or block
of measurements. An interrupt signal from the FPGA is con-
nected to the processor. See Figure 6-36 for a typical timing
diagram.
The connection to the USB has 16 data bits, 1 address bit, chip
select, wrn and rdn. An interrupt signal from the USB IC is
connected to the processor. See Figure 6-38 for a typical tim-
ing diagram. The USB IC is a complete USB unit. It is not
powered from the USB bus. The USB IC (U34) has a 6 MHz
oscillator. Check at C416.
The connection to the GPIB has 8 bits, 5 address bits, chip se-
lect, wrn, rdn and a special control signal for the level shifting
IC (U38). U38 is a buffer between the logic level of +3.3 V for
the processor and the logic level of +5 V for the GPIB IC
(U37). An interrupt signal from the GPIB IC is connected to
the processor. See figure ee for a typical timing diagram. The
GPIB IC is a complete GPIB unit. The GPIB IC (U37) has a
40 MHz oscillator. Check at TP20.
Only the selected interface is involved in communication on
the microprocessor bus.
Troubleshooting 6-21
Figure 6-29
LCD control signals, oscillogram #6.
Figure 6-30
Keyboard interrupt.
LD
RESETN
RAN
FRM
D3
D1