Lake Shore 234, User Manual

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Lake Shore Model 234/234D User’s Manual
1-2 Introduction
The Model 234 employs an analog control circuit to
maintain a constant voltage signal across the sensor.
A series of reference resistors convert the resulting
sensor current to a voltage. A microcontroller reads
the voltage with an A/D converter, calculates sensor
resistance, and converts the resistance to temperature
by table interpolation (requires a CalCurve
™
for
temperature conversion). The sensor excitation
voltage is reversed each reading to compensate for
thermal voltages and offsets.
A current of 4 to 20 milliamps representing either
sensor resistance temperature or log transmits. The
transmitted current changes linearly with resistance
temperature or log. Output scale depends on the
selected temperature range. Several switch selected
ranges are available. Highest accuracy and sensitivity
are achieved when the output is set for a narrow
temperature band. A 0 to 20 mA output is also
available to convert output to a voltage scaled from
zero. A 500 ohm, ±0.02% output load resistor
produces the maximum full scale output of 10 volts.
A serial interface (operating at 9600 baud) can be
used for service purposes to read sensor resistance,
temperature, and configuration, install and review the
CalCurve, and calibrate the transmitter.
A 5 VDC supply powers Model 234 circuitry.
The outputs are isolated so several Model 234s can
run off the same supply without interference. +5 VDC
can also be supplied from the pins on the VME bus
connector.
The Model 234 is built on a standard A-Size VME
Card. It fits directly into a single height (3U) VME card
holder. The transmitter does not use the electrical bus
format, only its physical shape and power supply.
The Model 234 Temperature Transmitter is available
as a stand-alone unit or with one of two enclosures: a
Model 2308-1 single space enclosure (included with
the Model 234D), or Model 2308-12 rackmount case
that holds 12 units. See Figure 1-1, and Tables 1-1
and 1-2.
Figure 1-1. Typical
Model 234 Front Panel
(and Model 234D Rear Panel)