Fluke 2635A, 2625A service manual Display/Keyboard Interface

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Service Manual

phototransistor base discharges through A1R16. With this arrangement, the rise and fall times of the phototransistor collector signal are nearly symmetrical.

The transmission of data from the Microcontroller (A3U9) to the Microprocessor (A1U4) is accomplished via the circuit made up of A3Q1, A3R7, A1U5, A1R7, and A1R3. The transmit output from the Microcontroller (A3U9-14) is inverted by A3Q1, which drives the optocoupler LED (A1U5-2) through resistor A3R7. The current through the LED is limited by resistor A3R7. The phototransistor in A1U5 responds to the light emitted by the LED when A1U5-2 is driven low; the emitter of the phototransistor (A1U5-4) goes high. The phototransistor collector (A1U5-5) is pulled up by VCC, and the emitter is pulled down by resistor A1R3. When turning off, the phototransistor base discharges through A1R7. With this arrangement, the rise and fall times of the phototransistor collector signal are nearly symmetrical.

2-36. Display/Keyboard Interface

The Microcontroller sends information to the Display Processor via a three-wire synchronous communication interface. The detailed description of the DISTX, DISRX, and DSCLK signals may be found in the detailed description of the Display PCA. Note that the DISRX signal is pulled down by resistor A1R1 so that Microprocessor input A1U4-15 is not floating at any time. The Display PCA also provides the system reset circuitry and watchdog timer.

The Keyboard interface is made up of six bidirectional port lines from the Microcontroller. SWR1 through SWR6 (A1U4-21 through A1U4-26, respectively) are pulled up by A2Z1 on the Display PCA. The detailed description of the Display PCA describes how the Microprocessor interfaces to the Keyboard.

2-37. ROM

The ROM provides the instruction storage for the Microprocessor. The chip select for this device (A1U8-20) goes low for any memory cycle between hexadecimal addresses 2000 and FFFF (accessing 56 kbytes). Whenever this device is chip selected for read, the instruction in the addressed location is output to the data bus and read by the Microprocessor.

2-38. NVRAM/Clock

The NVRAM/Clock (A1U3) provides the data storage and real-time clock for the instrument. A lithium battery, a crystal, and an automatic power-fail control circuit are also integrated into this single package. When the RAM* chip select signal (A1U3-20) is low, the Microprocessor is accessing one of the 8192 bytes in the NVRAM/Clock. The RD* (A1U3-22) and WR* (A1U3-27) signals go low to indicate a read or write cycle, respectively.

The internal power-fail control circuit disables access to this device and drives the INT* output (A1U3-1) low when the VCC power supply is below approximately +4.5V dc. This action keeps locations in the NVRAM/Clock from being modified while the instrument is powering up and down. When the INT* output is low, the reset circuit on the Display PCA is discharged, and a system reset occurs. Therefore, the Microprocessor is reset on power failure as soon as it can no longer access the NVRAM/Clock.

The NVRAM contains 8184 bytes of nonvolatile data storage. The nonvolatile instrument configuration information, the nonvolatile measurement data, and the Microprocessor temporary data are stored in this area.

The Clock is composed of 8-byte wide registers that allow access to the real-time clock counters. The Microprocessor accesses these registers in the same way as the NVRAM.

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