Fluke 5725A Fan Supply Section, Digital Assembly A5, Microcomputer, External RAM, External ROM

5725A

Instruction Manual

4-21. Fan Supply Section

The fan supply uses the same transformer secondary windings as the ±15 and ±20V supplies. Its own full-wave bridge rectifier diodes create a supply voltage of +25V. Transistor Q351 is operated near saturation, and provides the measurement point to tell the CPU whether the fan is running. The output of Q351 is sent to the analog multiplexer on line -FAN to accomplish this.

4-22. Digital Assembly (A5)

From a software standpoint, digital circuitry functions as a subset of the 5700A in-guard CPU. Digital circuitry resides on three assemblies within the 5725A: the Digital assembly (A5), the High Voltage Sense assembly (A6), and the Current Amplifier assembly (A2). All the 5725A hardware, including digital, is in-guard, except for the guard crossing and the CABLEOFF detection circuit.

Figure 4-2 is a block diagram of the Digital assembly. The Digital assembly contains most of the digital circuitry, including the following:

∙Microcomputer (Hitachi 6303Y CMOS)

∙External RAM (CMOS static)

∙External ROM

∙EEROM (2Kb X 8)

∙Break-detect circuitry

∙Power up and reset circuitry

∙Watchdog timer circuitry

∙Front panel LEDs

∙LED decoder and driver

The High Voltage Sense assembly (A6) contains the optoisolator link (guard crossing) to the 5700A Main CPU and the CABLEOFF circuit.

The Current Amplifier assembly (A2) contains the interface to the digital bus (two latches and three relay drivers) and a relay driver chip.

4-23. Microcomputer

The 6303Y CMOS microcomputer is configured for Mode 1 operation, with external RAM and external ROM. Port 3 provides a common data bus (D00-D07), while port 1 and bits 0 through 5 of port 4 provide the address bus (A0-A13). Bits 6 and 7 of port 4 are address lines A14 and A15.

4-24. External RAM.

External RAM is enabled whenever A15 is high, A14 is low (hex 8000-BFFF), and either RD* or WR* is true. Reading and writing is controlled by bit 2 of port 7.

4-25. External ROM

The external ROM (U515) is enabled via ROMSEL* whenever A15 and A14 are high (hex C000-FFFF). Bit 0 of Port 7 (RD*) is OUTPUT ENABLE*.

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