Fluke 2680A, Service Manual

Page: 186 / 254

268XA
Service Manual
6-40
PCMCIA PCA Troubleshooting
The PCMCIA interface board mainly consists of a card holder and a set of bi-directional
level shifting buffers. The buffers on this board are driven off the same supply as the
card, which can be either 3.3 V or 5 V. The buffers on A4 board are always powered
from Vcc (3.3 V). This board also has a 5 V to 12 V boost topology switching regulator
that is only turned on if the installed card says that it needs 12 V for programming. This
12 V supply was designed to only provide enough power for the PCMCIA card, no other
devices should be powered from this supply. This board plugs into P4 on the 2680A-4004
PCA. It is a daughter card that is only installed if the device is a 2686 chassis, as opposed
to a 2680 chassis that has no PCMCIA option.
A/D Power Supply PCA Troubleshooting
To access this card for test, the user may need the 2680A-7001K Extender kit, p.n.
1619174. This module is used to isolate module communication and power signals from
the earth referenced SYS/CTRL PCA. It is located inside each module’s housing, as a
daughter card that plugs into a 2 row, 10 pin header on every module. The power supply
design uses a differential 393 KHz clock source generated on the SYS/CTRL card, U25,
and arrives to the PCA on P1 pins 9 and 19. There are 3 regulated o5.2 V (Vdd),
-5.2 V (Vss) and +5.6 V (Vddr), each one is derived from a dedicated linear low dropout
regulator (U5, U6 and U4, respectively).
The other circuitry on the A31 board are the optoisolators (U2 and U3) and the
calibration EEPROM (U7). U2 and U3 on the A31 PCA are used in the communication
between U17, FPGA, on the SYS/CTRL PCA and the module’s in-guard micro
controller. U7 is a 2Kbit serial EEPROM addressable from the out-guard microprocessor
(SYS/CTRL PCA) via an SPI bus that runs on the Backplane PCA . Communication with
any A/D power module only occurs after a module has been initialized. There after,
communication occurs every scan cycle.
DIO Module Troubleshooting
To access this card for test, the user may need the 2680A-7001K Extender kit,
PN 1619174. No calibration is required for this module. Problems can occur either with
communication to the SYS/CTRL card or with individual DIO, relay, or totalizer inputs.
A Texas Instrument MSP430P337A micro controller, U4, is the heart of the DIO PCA. A
quartz crystal operating at 32.8 kHz provides the main clock signal for the micro
controller. This signal is internally multiplied to 3.6925 MHz for clocking of the micro
controller functions and is divided down to 8kHz (XBUF U4 Pin 97) providing a control
signal for the debounce PLD; U3.
Internal to the micro controller is a watchdog timer that is set to a 1 second interval.
Should the micro controller become inoperative for a period of 1 second, the watchdog
timer will activate and perform a Power Up Clear (PUC) operation that resets all outputs
and restarts the micro controller. C50 controls the duration of the reset signal.
During power up, U16 provides a reset pulse of approximately 2 seconds to insure that
the crystal oscillator (Y1) is given sufficient time to start. U16 also has undervoltage
detection such that if the supply voltage to pin 2 drops below 4.5V, the output will reset.
Depressing S1 can manually reset the micro controller. This applies a ground signal to
the reset (RST U4 pin 96) pin of the micro controller.
U3 is a Programmable Logic Device (PLD) and is configured as a signal debouncer. The
Totalizer signal is applied to U3 Pin 13 from the Totalizer Input Protection Buffer circuit
(U1 Pin 1 and U18). See "Totalizer Input" for more details.