Fluke 2625A, 2635A service manual Address Decoding

Theory of Operation (2620A/2625A)	2
Detailed Circuit Description

		GRID/ANODE TIMING
5V
0V
GRID(X)		1 . 14 ms
-30V
5V		116 μs
0V
ANODE(14..0)
-30V	∝s	56 ∝s
19
60 ∝s		98 ∝s
5V
0V
GRID(X-1)
-30V
		s10f.eps

Figure 2-10. Grid-Anode Timing Relationships

2-71. Memory PCA (2625A Only)

The Memory PCA is a serially-accessed, byte-wide, nonvolatile 256K-byte memory that is capable of storing up to 2047 scans of data. The following paragraphs describe in detail the Main PCA Connector, Address Decoding, Page Register, Byte Counter, and Nonvolatile Memory blocks that make up this assembly.

2-72. Main PCA Connector

The Memory PCA interfaces to the Main PCA through a 26-pin, right angle connector (A6J1). This connector routes the eight-bit data bus, the lower three bits of the address bus, memory control and address decode signals from the Main PCA to the Memory PCA. The Memory PCA is powered by the +5.1V dc power supply (VCC). The Memory PCA is sensed by the Microprocessor on the Main PCA through the connection of A6J1- 11 to the option sense signal OPS* (A6J1-22).

2-73. Address Decoding

Circuitry on the Main PCA decodes the Microprocessor address bus and provided the MEM* select signal to the Memory PCA. The 3-line to 8-line decoder (A6U8) is used to decode the three least significant address bits to get register select signals for hexadecimal addresses 4, 5, and 6. When the MEM* signal drives A6U8-4 low and the RESET* signal (A6U8-6) is high, the A0 through A2 address bits are decoded to get the MEMORY, PAGEL, and PAGEH register select signals.Address decoding is disabled when RESET* is low so that the Nonvolatile Memory cannot be accidentally modified during power-up or power-down.

2-74. Page Register

The Page Register is an 11-bit register that is writable by the Microprocessor on the Main PCA. The outputs of this register control the most significant address bits of the nonvolatile memories (A6U6 and A6U7.) When register select PAGEL goes high and

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