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Service Manual
2-20
6) low to indicate that it is busy writing the register, thereby controlling the timing of the
write cycle. The microprocessor waits for this signal to go high before performing other
EEROM operations. If the EEROM fails to drive this signal high, the microprocessor
waits indefinitely.
New data is written to a register only after old data in that register is erased. After each
such erase or write cycle, the microprocessor polls the status of EEROM by setting Chip
Select (A1U5-2) high and checking the state of the Data Out signal (A1U5-6). If Data
Out is low, the erase/write cycle is still in progress. If Data Out is high, the EEROM is
ready for another command.
2-38. RAM
The RAM is a 8192 x 8 bit device that provides the temporary data storage used by the
operating software of the meter. The chip select for this device (A1U10-20) goes low for
any memory cycle between hexadecimal addresses 2000 and 3FFF. The RD* signal from
the Microprocessor enables the reading of data when it is low, and the WR* signal writes
data into the RAM when it is low.
2-39. ROM
The ROM provides the instruction storage for the Microprocessor. The chip select for
this device (A1U8-20) goes low for any memory cycle between hexadecimal addresses
4000 and FFFF (accessing 48 kbytes.) Whenever this device is chip selected, the
instruction in the addressed location is output to the data bus and read by the
Microprocessor.
2-40. IEEE-488 Option Connections
The interconnection to the IEEE-488 option is implemented by two ribbon cables that
mount to the 14-position and 20-position connectors on the Main PCA. The 14-position
connector (A1J3) routes the 8-bit data bus, RD*, R/W*, E, RESET and OPTSW* signals
to the option. The 20-position connector (A1J2) routes the 16-bit address bus and the
WR* memory control signal to the option. This connector also routes the IEEE-488
interrupt and option sense signals from the option. See Chapter 8 for further information.
2-41. Display Assembly
Display Assembly operation classified into six functional circuit blocks: the Main
Assembly 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.
2-42. Main Assembly Connector
The Main Assembly Connector is a 20-pin connector (A2J1) that provides the interface
between the Main Assembly and the other functional blocks on the Display Assembly.
Seven of the connector pins provide the necessary connections to the four power supply
voltages (-30 V dc, -5 V dc, +5 V dc, and 5 V ac). Six pins are used to provide the
interface to the Front Panel Switches (A2SWR1 through A2SWR6). The other seven
signals interface the Microprocessor to the Display Controller and pass the reset signals
between the assemblies.
2-43. 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
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