An
Introduction to IEEE-488 Measurements
Introduction
A
Typical Sequence of Operations
Introduction
IEEE488 is a standard - written by IEEE - for communication between computers
and instruments. The standard covers both hardware and software aspects
of computer-instrumentation communication. Here we will examine the
basic things you need to do to make a typical IEEE488 instrument communicate
with a computer.
Although you can write programs in many environments including C (Console
and Visual applications), Visual Basic and LabView, the basic concepts
of what transpires are all the same. We will examine a simple task
- a single measurement - and examine how to do that in general, and then
examine several specific environments.
Hardware
Issues
There are some things that you need to take care of before you can address
any software issues. Here are the things you need to check.
-
IEEE488 requires a special
cable that connects the instrument to the computer. Actually, it
connects to a special card in the computer, and if that card is not present
you will not be able to use IEEE488 instruments with that computer.
All of the computers in the EE department have that card, but your own
PC probably does not have it. The card we use is manufactured by
National Instruments (NI). Check to be sure that the "fat" cable
is connected between the computer and the instrument you want to use.
-
The instrument and the
computer must both be turned on. That's not as funny as it seems.
IEEE488 connections cannot turn on an instrument via software, so, despite
any assumptions you may have made, you have to turn it on.
-
Finally, when the IEEE488
card was installed in the computer, software should also have been installed
so that you can interface between your programming environment (C/C++,
Visual Basic, LabView, etc.) and the IEEE488 card. We will discuss
what is necessary in each of these environments when appropriate.
Once you have all of the above taken care of, you are ready to use software
to control an instrument.
The
Sequence Of Operations
If you want to use an IEEE488 instrument to take measurements, the following
sequence of operations is typical.
-
First, you will need to
know the address of the instrument. The IEEE488 system uses a bus
to which various instruments may all be connected. Each instrument
must have a unique address on that bus.
-
Our example instrument
will be the Fluke/Hydra Data Acquisition Unit (DAU), and in our labs we
have set those instruments to have an address of 3.
-
Note also that some software
environments use an "alias" or "nickname" for the address. The default
for address 3 is "DEV3", but we have tried to set the alias to "DAU" or
"DAU1".
-
Secondly, you may need
to get a handle for the device using the alias. That happens in C,
but not in LabView, for example.
-
Next, you use the address
or handle to reset the instrument.
-
When you reset the instrument
you clear out old measurements and any "start-up noise" that may exist
in the memory of the instrument. You also restore the instrument
to a defined initial state.
-
In the Fluke/Hydra you
can use a command string *RST.
-
After resetting the instrment
you need to send command strings to the instruments. These command
strings are acted upon by the instrument to set scales and take the measurement.
-
For the Fluke/Hydra these
strings are sent to the instrument.
-
FUNC 0,VDC,AUTO
- which sets the function on Channel 0 to be VDC (DC Voltage measurement)
with AUTO-ranging.
-
*TRG
- which triggers a mesurement when the instrument receives this command.
-
LAST? 0
- which requests the instrument to prepare the last measurement on Channel
0 for transmission to the computer.
-
After setting up the instrument,
taking a measurement and preparing the data, you must get the instrument
to set the string to the computer. Different programming environments
do that in different ways.
Here are links to various
programming options.