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A MICROCOMPUTER AIDED MICROWAVE SPECTROMETER
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CONTROLLED BY A LARGER COMPUTER
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N.W. LARSEN#h1 and J. OXENBØLL#h2*nl1*
#h1Chemical Laboratory V, University of Copenhagen
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the H.C. Ørsted Institute, Universitetsparken 5
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DK-2100 Copenhagen Ø (Denmark)
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#h2Computer Department, the H.C. Ørsted Institute

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ABSTRACT
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*ld12*
A Hewlett-Packard 8460A microwave spectrometer
has been connected to a microcomputer based on the
Motorola MC6800 microprocessor. The microcomputer
is programmed to control frequency and Stark voltage
and to perform A/D conversion, storage and
accumulation of intensities.
*np6*
The microcomputer is connected to a larger local
computer, the RC4000, from which it may be loaded
with a program, started and stopped, and from which
data transfer between the storages of the two computers
is controlled. This transfer may be performed, while the
microcomputer is running a spectrum.
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*nl2*
INTRODUCTION
*np6*
Some of the reasons for digitalizing a spectrometer
are common to most forms of spectroscopy:
The possibility of improving the S/N ratio by
accumulation of spectra, and the easy subsequent
treatment of the spectra with smoothing procedures and
line measuring procedures etc.
For a Stark modulated microwave spectrometer it is desirable to
control, not only the frequency, but also the Stark voltages
from the computer, with the aim of eliminating some
of the disadvantages associated with the use of
Stark modulation. The HP 8460 microwave spectrometer
is contructed in a way that makes it easy to computerize
and originally an option equipped with a minicomputer
was available.
Instead of choosing the traditional set-up with a
minicomputer connected to the spectrometer,
we found that a better solution, which was
moreover given as a service from the computer department,
was to connect a microcomputer directly to the
spectrometer and to let the microcomputer be
controlled from a larger general purpose computer
(the RC4000). In this way we benefit from the extensive
facilities of the larger computer, although the experiment
can run on the microcomputer without interference
from other users of the large computer.
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THE MICROCOMPUTER
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_«bs»H_«bs»a_«bs»r_«bs»d_«bs»w_«bs»a_«bs»r_«bs»e_«bs». !The microcomputer, called HC6800,
was designed in 1975 at the H.C. Ørsted Institute,
with the explicit purpose of controlling
laboratory equipment (ref.1).
It consists of four different types of modules,
each placed on a single printed circuit board:
1) a cpu module, based on the M6800 microprocessor
from Motorola, with interrupt logic, decoding of
address lines for selecting
memory and input/output, and with a simple
multiprogrammed monitor program placed in a 4 kbyte EPROM
and a 2 kbyte RAM for descriptions and variables.
2) a memory card with 32 kbyte of RAM memory, used for
application programs and data. 3) a system input/output card,
which controls three asyncronous full-duplex serial lines
for connection to the RC4000 via the local area network
and a 24 bit system timer. 4) a number of user input/output
cards, designed for the specific experiment, but using standard
Motorola microprocessor peripherals,
in a way which is common for all the HC6800 microcomputers.
*np6*
Only two different kinds of external devices are connected
to the microcomputer, namely the RC4000 computer and
the equipment to be controlled. This simple configuration
makes the hardware and software construction simple and cheap.
All other external devices, such as terminals, discs,
magnetic tape stations, printers, etc, are connected to
the RC4000 computer.
*np6*
_«bs»S_«bs»o_«bs»f_«bs»t_«bs»w_«bs»a_«bs»r_«bs»e_«bs». ! The monitor in the HC6800 takes care of upstart,
communication with the network, and handling of
peripherals. The monitor is multiprogrammed and runs several
tasks in parallel, eg. the user program and the
communication process, and further provides a number of
utility procedures used by most application programs.
*np6*
An Algol program, acting as an operating system for the microcomputer,
runs in the RC4000, which has full control over the microcomputer.
It starts and stops it, loads it with a user program, and extracts data
from it when appropriate.
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*np6*
The microcomputer running the HP8460 spectrometer is equipped with
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three user i/o-cards (Fig.1). The first contains a dual 8 bit
parallel interface adapter (pia), where one port
outputs four 8 bit words into four 8 bit registers, storing
8 bcd digits for the frequency. At the other port two bit
are used to control the local/remote inputs on the HP8460
spectrometer for frequency and Stark voltage. The card also contains a
programmable timer module (ptm),
that controls the experiment by means of a pulse train
in which each pulse has the effect of 1) initiating the transfer
of the frequency from the four registers to the spectrometer,
2) initiating the A/D conversion (se below), 3) sending an
interrupt
*nl1*
Fig. 1. Diagram
showing in a scematic way the HC6800 microcomputer with
the interface to the microwave spectrometer and to the
RC4000 computer.

*nl2*
to the microprocessor.
The second i/o-card contains a pia and four 8 bit registers,
for storing the digital information for two 12 bit D/A converters,
also placed on the card. The D/A-converters produce the
control voltages for
the ground to base voltage
and the base to peak voltage respectively.
The last i/o-card contains one pia and two
8 bit registers for storing the digital information from
a 14 bit A/D-converter, placed on the same card. The A/D-converter
is used to convert the intensity signal.
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APPLICATIONS
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*np6*
Two programs are used in the experiment.
The first is an Algol program that runs
interactively in the RC4000 and
controls communication with the microcomputer.
It also performs the further treatment
of the spectra, such as drawing,
smoothing, differentation, line measurement, least squares
fitting to Lorenzian and Gaussian line shapes, etc.
The second program is written in assembler code for the
microcomputer and controls the experiment in accordance with
a given parameter set.
*np6*
The parameter set required by the last program includes
the frequency limits (upper and lower frequency),
the step length (!!1 kHz), the time between two steps
(20 ms - 10 s), the number of spectra to be recorded
and added in the microcomputer (!!100), and for each
of these spectra the ground to base and the ground to peak
values of the Stark voltage.
All other parameters defining
the experiment must be set manually on the spectrometer.
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*np6*
When the parameters have been read by RC4000, they
are, together with the microcomputer program, transferred
to the microcomputer, which is then started.
In the most
common experiment the microcomputer is now able to
work independently of the RC4000 until the spectra have
all been recorded and their sum has been stored.
Finally the spectrum
is transfered to the RC4000
initiated by an order from the terminal.
*np6*
The Stark voltages are normally chosen according to one
of several standard procedures.
The most obvious choice is to let the base voltage
be zero and let the peak voltage assume the same value for all
spectra, with a usual time averaged spectrum as the result.
*np6*
We have found it very useful however to
use Stark averaging instead, which implies that the
peak voltage is changed from one scan to the next.
The peak voltages may eather be distributed
between zero and a chosen maximum value
in an equidistant way,
or alternatively
in such a way that the difference between the squares of
two settings is constant.
If the first distribution is used whenever the spectral lines
have first-order Stark effect and the second distribution is used
when the lines have quadratic Stark effect, then each individual
Stark component will be shifted uniformly from scan to scan
and the result, with an appropriate number of scans and a sufficiently
high value of the maximum peak voltage, will be an average spectrum
corresponding to zero field, in which the only reminiscence of
the Stark components is a shift of the base line.
*np6*
Stark averaging can also be used with a non zero base voltage,
and in this case one ideally obtains an average spectrum corresponding
to one particular non zero field, defined by the base voltage.
This type of spectrum is very useful for
determination of dipole moments (Fig. 2).
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Fig. 2. Three spectra of the 5#l0#l,#l5!!6#l0#l,#l6 transition of
methylene oxalate. (a) Is a usual Stark spectrum
at 500 V peak voltage. The other two are Stark averaged
spectra with 200 V base voltage (b), and 400 V
base voltage (c). The quantum number M, is indicated
on (c).
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Since the space available for the spectrum in the storage of the microcomputer
is limited to 10000 points, each containing 24 bit, very long spectra
must be treated by a different procedure: Only one scan can be recorded
and the data must be transferred to the RC4000 during the run.
To accomplish this, the storage of the microcomputer is
organized as 39 cyclically ordered segments each containing 255
points. ▶17◀Whenever a particular segment is filled, it is marked
as ready for transfer, and the filling of the next segment
is undertaken. ▶17◀With small time intervals during the run,
the Algol program
asks the microcomputer whether a segment is ready to be
transferred, and whenever the answer is affirmative, a transport
is performed. In this way the transfer of data may without problems, be
dalayed by the time it takes to fill 38 segments.
*np6*
The latter type of experiment with direct transfer to RC4000 is mainly
being used for survey spectra, but with the automatic line measuring
procedure even such spectra may often provide a quite good
basis for assignments, calculation of the rotational constants etc. The accuracy
of the measurements is mainly limited by the step length of
the scan and not by the number of MHz pr. centimeter as
for a paper recorded spectrum.
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*np6*
All kinds of original spectra, whether they have been
transferred to the RC4000
during the scan or after the experiment,
are as a routine permanently stored
on a magnetic tape, before they are altered in any way
by smoothing routines, subtraction of background, etc.
This means that at any time one can choose an alternative
approch to extract the information from the spectrum.
*nl2*
CONCLUSION
*np5*
We believe that in general a configuration with several
parallel experiments connected to a large central computer
offers several advantages to the user. First the setting up of
a control system for
a new experiment is a relatively simple and cheap task, and secondly
the exchange of experiences and programs between the users
is easy. With respect to the microwave experiment we feel
that the present set-up makes the microwave
spectrometer much more flexible and provides several new
posibilities, and our experience from the last several months is,
that with very few exceptions all of the work on the spectrometer
is made by means of the computer system.
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REFERENCES
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1 A. Lindgård, J. Oxenbøll and E. Sørensen,
Hierarchical Multi-Level Computer Network for
Laboratory Automation.
Software - Practice and Experience. In press.
2. N.W. Larsen and B. Bak, The Microwave Spectrum
of Methylene Oxalate. To be published.
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