Theory of Operation (2620A/2625A)

2

Detailed Circuit Description

2-64. Display PCA

Display Assembly operation is classified into six functional circuit blocks: the Main PCA Connector, the Front Panel Switches, the Display, the Beeper Drive Circuit, the Watchdog Timer/Reset Circuit, and the Display Controller. These blocks are described in the following paragraphs.

2-65. Main PCA Connector

The 20-pin Main PCA Connector (A2J1) provides the interface between the Main PCA and the other functional blocks on the Display PCA. Seven of the connector pins provide the necessary connections to the four power supply voltages (-30V dc, -5V dc, +5.1V dc, and 5.4V ac filament voltage). Six pins are used to provide the interface to the Front Panel Switches (A2SWR1 through A2SWR6). The other seven signals interface the Microprocessor (A1U4) to the Display Controller (A2U1) and pass the reset signals between the assemblies.

2-66. Front Panel Switches

The Microprocessor scans the 19 Front Panel Switches (A2S1 through A2S18, and A2S21) using only six interface signals (plus the ground connection already available from the power supply). These six signals (SWR1 through SWR6) are connected to a bidirectional I/O port on the microprocessor. Each successive column has one less switch.

This arrangement allows the unused interface signals to function as strobe signals when their respective column is driven by the Microprocessor. The Microprocessor cycles through six steps to scan the complete Front Panel Switch matrix. Table 2-7 shows the interface signal state and, if the signal state is an output, the switches that may be detected as closed.

Table 2-7. Front Panel Switch Scanning

Interface Signal States or Key Sensed

Step

SWR6

SWR5

SWR4

SWR3

 

 

 

 

 

SWR2

SWR1

1

2

3

4

5

6

A2S8

A2S1

A2S7

A2S14

NA

A2S21

A2S17

A2S2

A2S9

A2S15

NA

0

A2S10

A2S3

A2S5

A2S16

0

Z

A2S12

A2S4

A2S6

0

Z

Z

A2S18

A2S11

0

Z

Z

Z

A2S13

0

Z

Z

Z

Z

A2Sn indicates switch closure sensed.

0 indicated strobe driven to logic 0

Z indicated high impedance input; state ignored.

In step 1, six port bits are set to input, and the interface signal values are read. In steps 2 through 6, the bit listed as O is set to output zero, the other bits are read, and bits indicated by a Z are ignored.

Each of the interface signals is pulled up to the +5V dc supply by a 10-kΩresistor in network A2Z1. Normally, the resistance between any two of the interface signals is

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Fluke 2625A, 2635A service manual Main PCA Connector
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2635A, 2625A specifications

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