Fluke 5720A service manual Control Display Circuitry, êêêêêêê

Theory of Operation 2

Digital Section Detailed Circuit Description

	Table 2-6. Front Panel Memory Map
Name		Read/Write
OTDCS*		R/W
DMDCS*		R/W
ENCODERCS*		R
ENCODERRESET*		W
LED_OUTPUT_CNTRL		R
LED_LATCH_EN		W
KEYBOARDCS*		R/W

Address

D02000 to D027FF

D02800 to D02FFF

D03000 to D033FF

D03000 to D033FF

D03400 to D037FF

D03400 to D037FF

D03800 to D03BFF

2-50. Control Display Circuitry

Control display circuitry consists of a 26-row by 256-column vacuum-fluorescent dot matrix display under the control of PLD U4, four high voltage grid drivers (U20-U23), four high voltage anode drivers (U16-U19), a filament switching circuit, and 1K x 8 (1 KB) dual-port RAM U1.

This display is divided into 129 grids; alternate grids contain two anode columns lettered B C or D A. Grid G129 and column C in grid G128 are not used. Each column contains 26 individual anodes.

IC U4 is an EP900 Programmable Logic Device (PLD). It provides the timing and control signals for control display circuitry. Display data written by the microprocessor into the Control Display’s dual port RAM (U1) is read by U4 and sent serially to the high voltage anode drivers. Both the anode and grid drivers are serial TTL-level input, 32-bit parallel high voltage output devices. IC U4 also controls the grid timing and display refresh. ê

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B C	D A	B C
G4	G5	G6

Adjacent columns in adjacent grids are driven, while the opposite columns are turned off. For instance, grid G4 contains columns B and C, and grid G5 contains columns D and A. G4 and G5 are driven simultaneously while anode columns G4-C and G5-D are activated, and G4-B and G5-A are driven off. Next, grids G5 and G6 are driven simultaneously, while columns G5-A and G6-B are activated, and G5-D and G6-C are driven off. This pattern is repeated for all 128 grids at a refresh rate of about 75 Hz.

This particular scheme was selected because of the way the anode drivers are loaded with display data. Both the A and C (U16 and U18), and B and D (U17 and U19) anode drivers’ input registers are latched with the same data, while the output drivers are appropriately enabled and displaying the data previously strobed to the driver outputs from the input registers. The input register data is strobed to the output drivers while all of the drivers are disabled, or blanked. Following this, either the A and B drivers are enabled to display the A-B data, when the C and D drivers, latched with A-B data, are disabled, or the C and D drivers are enabled to display the C-D data, when the A and B drivers, latched with C-D data, are disabled.

Control display filament driver circuitry consists of transistors Q1 through Q6 and zener diodes VR1 and VR2, with associated resistors. The transistors are driven by 7406 open collector drivers U13B and U13A. These drivers are controlled by AOUT and BOUT.

AOUT and BOUT are synchronous, non-overlapping, three-eighths duty cycle, 57.6 kHz timing signals generated by U3. Each signal is alternately active high for 6.51 us, with a dead time between active signals of about 2.17 us to allow for turn-off times of the drive

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