Interface; Microprocessor; Eerom - Fluke 45 Service Manual

2-35. RS-232 Interface
The RS-232 interface is composed of connector A1J5, RS-232 level shifter A1U7 and
the hardware serial communication interface (SCI) in Microprocessor A1U6.
The transmit signal from the SCI (A1U6-14) goes to the RS-232 driver (A1U7-12),
where it is inverted and shifted to transition between approximately +5.0 and -5.0 V dc.
When nothing is being transmitted by the meter, the driver output A1U7-5 is -5.0 V dc.
The receive signal from A1J5 goes to the RS-232 receiver A1U7-4, where it is inverted
and shifted to transition between +5.0 and 0 V dc. When nothing is being transmitted to
the meter, the receiver output A1U7-13 is +5.0 V dc.
Data Terminal Ready (DTR) is a modem control signal controlled by the
Microprocessor. This signal is an RS-232 output generated by driver A1U7-7; it is at
+5.0 V dc when the meter is powered up.

2-36. Microprocessor

The Microprocessor utilizes an eight-bit data bus and a sixteen-bit address bus to access
memory locations in ROM (A1U8), RAM (A1U10), and the IEEE-488 option. The upper
three bits of the address bus are decoded by A1U9 to generate chip select signals for the
ROM (A1U9-6) and RAM (A1U9-8). The Microprocessor enables the reading of
memory by driving RD* (A1U6-67) low, and writing of memory by driving WR*
(A1U6-66) low. The IEEE-488 option also makes use of the signal R/W* (read when
high, write when low) that is generated by A1U6-65.
The Microprocessor operates with a memory cycle time of 1.085 us as determined by the
3.6864 MHz crystal A1Y2. The system clock signal (A1U6-68) is a square wave with a
frequency of 921.6 kHz. It is used by the Display Assembly and the IEEE-488 option
assembly after being damped by series resistor A1R57.
The Microprocessor uses synchronous communication to store and retrieve meter
configuration and calibration information in the EEROM (A1U5). See the EEROM
description for more detailed information.
The Microprocessor communicates to the Display Controller using a synchronous, three-
wire communication interface described in detail in the Display Controller Theory of
Operation.
The Microprocessor communicates to the Analog Measurement Processor (via the Serial
Communication circuit) using an asynchronous communication protocol.
Communication to the Analog Measurement Processor originates at A1U6-11 (which is
normally low when no communication is being done). Communication from the Analog
Measurement Processor to the Microprocessor appears at A1U6-10 and is normally low
(unless communication is in progress.)

2-37. EEROM

The EEROM contains 64 registers, each of which is 16 bits long. These registers are
used to provide non-volatile storage of meter configuration and calibration information.
When the Microprocessor is communicating to the EEROM, Chip Select (A1U5-2) goes
high to enable the EEROM interface.
When the Microprocessor is reading data from the EEROM, the data bits are serially
shifted out on the Data Out signal (A1U5-6) with each one-to-zero transition of the
Serial Clock (A1U5-3).
When the Microprocessor is writing commands and data to the EEROM, the bits are
serially shifted into the EEROM on the Data In signal (A1U5-5) with each zero-to-one
transition of the Serial Clock (A1U5-3). The EEROM drives the Data Out signal (A1U5-
Theory of Operation
Detailed Circuit Description
2
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