Srams (U5,U6,U7); Eprom (U4); Eeprom (U11); Serial Eeprom (U22) - Fluke 39 Service Manual

2-34. SRAMs (U5,U6,U7)

The SRAM is used for two main functions: operating code storage and sampled and
calculated data storage. After the Fluke 39/41B has completed its initialization and is
running, program execution is directly from one program section of the SRAM. Storage
of all sampled and calculated data is stored in the X and Y data spaces.
The 96 kilobytes (32 kilowords x 24 bits) of static RAM is divided into three sections:
Program, X-data, and Y-data. The Y-data space uses 8K words, X-data space uses 8K
words, and Program space uses the remaining 16K words. The first 0.5K word
(P:0000..1FF) of Program space is on the DSP chip (U2).

2-35. EPROM (U4)

The EPROM contains the instrument software. During start-up 512 words (1536 bytes)
are read from the EPROM and then executed from within the internal DSP program
RAM. The remaining portion of the 16K words is downloaded into the program space of
the SRAM. After boot-up, the EPROM is not accessed again until the unit is powered up
again.
Although U4 is an EPROM, it cannot be reprogrammed, since it is not a windowed part.
This type of EPROM is know as an OTP or One Time Programmable.

2-36. EEPROM (U11)

The EEPROM (U11) provides a nonvolatile storage of waveform and calculated data for
the Model 41B's memory feature.

2-37. Serial EEPROM (U22)

The serial EEPROM (U22) provides non-volatile storage of calibration constants, power-
up defaults, and display contrast constants.

2-38. Keypad

The keypad consists of 15 individual keys in a 4x4 matrix. When a key is pressed, one of
the 16 possible points in the matrix is shorted together. The rows of the matrix are pulled
up to Vcc with 30 kΩ resistors in Z2. The columns are normally at a low state, which is
controlled by the DSP. The total resistance of the switch as seen at the pca should be less
than 3 kΩ. The contact resistance itself is only about 200Ω, but there are several other
virtual resistors in the circuit, including the elastomeric connector that contacts the
keypad to the pca. The normal voltage at the four row lines is Vcc, when no keys are
pressed. When a key is pressed, the corresponding row is pulled down to about 0.4V dc.
The four row signals are also connected to U3. If U3 senses that any of the row signals is
in a low state, it generates an interrupt to the DSP. By applying a logic low to the
columns one by one, the DSP can determine the pressed key.

2-39. Display

The Fluke 39/41B display consists of two circuits, the LCD Module and the Contrast
Control.

2-40. LCD Module

The LCD module is a complete system. It contains the liquid crystal, LCD drivers, con-
troller, display RAM, backlight, and hardware to hold it together as a module. The LCD
module connects to the rest of the system with a 24 conductor flat cable. Extra logic was
required to interface the display control lines to a fast DSP. This logic resides in the PLD
(U3).
Theory of Operation
Circuit Descriptions
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