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The Microprocessor Board
Mounted on the microprocessor board are the microprocessor, its clock, a PROM,
the power-on-reset circuitry, address decoding and 32 lines of parallel input/output
(I/O).
The microprocessor used in the 9814 is the Motorola MC 6802. A 3.2768MHz crystal,
divided down internally by the 6802, supplies the 8192 kHz clock.
Parallel I/O lines is provided with a pull-up resistor.
An on board link provides for the installation of either a 16K (2516), 32K (2532) or
64K (2764) PROM.
The power-on circuitry is a low frequency oscillator held off by the microprocessor
stop, the oscillator does not receive a pulse and automatically resets.
Auto-cal models are fitted with a non-volatile RAM (type X2816).
The IEEE Interface Board
The IEEE interface board is constructed around the Motorola MC68488 general pur-
pose interface adaptor (GPIA). The I/O lines of the 68488 are driven through
MC3448 drivers. The address select switches are mounted directly on the board and
their condition is gated onto the 6802 bus via 74LS244 tristate buffer.
The Reference Board
The encapsulated reference circuitry consists of two precision, aged zener diodes
running at their optimum current. The output from these diodes is averaged and am-
plified supply the master reference voltage of + 13.1072V. This voltage is inverted by
a precision inverter to give –13.1072V. Either the positive or negative reference volt-
age is applied to the output by a low thermal FET switch controlled by a logic input.
The reference voltage is split to drive two D/A convertors:
a) The 12 bit monolithic convertor (MPS 7623) mounted on this board via a buffer,
an inverter and a calibrator. This convertor makes up the least significant bits
and hence is termed the LS bits D/A. A trimmer (LS bits gain) adjusts the mag-
nitude of reference fed to this convertor, while another trimmer (balance) ad-
justs any offset between the positive and negative reference voltages.
b) The precision 6-bit D/A convertor on the D/A board.
D/A Convertor Board
This board consists of an encapsulated module and a R-2R resistor ladder network.
This encapsulated module contains logic, buffer amplifiers and switching.
The R-2R resistor ladder consists of high quality low temperature coefficient resistors
which have been carefully chosen, both in value and type, to give optimum stability.
The value of each resistor can be adjusted by a multi-turn trimmer to give very high
linearity. The output from the ladder is buffered and returned to the reference board.
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