Using the DTM in high voltage
or electrically noisy environments
When using the DTM in electrically
noisy environments, it is important to properly
ground the instrument to protect it against high
voltage transients. These transients can come from
many sources, and could cause any number of problems
ranging from overwriting the user and calibration
tables, to actual physical damage to the DTM. We
have prepaired a series of documents to guide in
the proper installation and grounding of the DTM.
Recovery when DTM
tables are overwritten
There are times when all the transient
protection in the world just does not stop the random
hit. If you are using the DTM with either of the
computer interfaces, it is possible to detect and
correct the error under automatic computer control,
minimizing down time. While this paper does
not provide computer code, it does outline the steps
required to make an automatic recovery.
Replacement
of the DTM Firmware
Periodically the firmware for
DTM is revised, and it is often advantageous to
upgrade the firmware. Here we have a list of the
firmware revisions for all of the Group3 DTMs, and
instructions on how to replace the EEPROM. Do
keep in mind that just because there is a new version,
it is not necessary to upgrade. The simple
rule is: Only upgrade if you require it to fix a
bug that may affect you, of if it offers a new feature
that you desire.
My DTM displays the
wrong field, over range, or overflow
There are many reasons why a DTM
may display a reading that is not what was expected.
It could be in the wrong operating mode (AC vs DC),
the 'users' modification table may have been changed,
the calibration table may have been overwritten,
or the probe or DTM has been damaged. In
most cases, and simple reset of the instrument will
solve the problem.
What is the 'users'
modification table?
The users modification table is
a series of values used to change the displayed
field reading. These factors include filtering,
zero, offset, scale, and others.
Why would I not want
the DTM to reset with power-on?
If you have programmed a series
of values in to the users modification table, or
want to retain the zero point on the probe, you
would not want the DTM to reset on power-on. The
power-on reset would reset the table to it's default
values, and you would then be forced to re-enter
all of your user factors.
How do I reset the DTM?
The behavior of the power-on reset
is controlled by a micro switch on the interface
board in the DTM. If the switch is in the
'off' position, no reset is executed during power-up.
If the switch is in the 'on' position, a reset
is performed, and all the users factors are defaulted.
The instrument may also be reset over
the computer interface by sending a CTRL-X charactor
to the DTM.
In all cases, the reset operation
is confirmed on the DTM display with the message
'RESET'.
Please refer to the users manual for detailed information
on the reset function.
What causes the tables
to be overwritten?
This is generally caused by a
voltage transient of high enough energy getting
in to the DTM, upsetting the CPU. The CPU may then
write random data in to the users modification table,
and on rare occasions in to the calibration table
itself.
The best way to avoid or reduce
the danger of this is to install a ferrite kit,
use the Power Supply, Fiber Optic
interface, and make sure the DTM is correctly grounded
and shielded.
When communicating with the DTM, after a
short time it displays 'Group3'
This is caused by polling the
instrument too quickly and overflowing the DTM's
recieve buffer. It can occur in DTMs with
either the IEEE-488 or RS-232 interfaces. The solution
is simple, introduce a delay loop in the software
to slow the polling rate.
Remember that the DTM-133 updates
the reading 30 times a second, and the DTM-151 updates
at 10 times a second. Polling any faster than this
will do nothing for you.
The
DTM displays 'Group3' all the time
This is most often caused by the
failure of the serial controller chip in the DTM.
When the
chip fails, it sends random charactors to the receive
buffer at it's maximum data rate, overflowing the
buffer, and causing a reboot. Note that this fault
may occur even if you are not using the interface.
