Download How (not) to Fry a Thermometer: Testing a slow monitoring and

How (not) to Fry a Thermometer:
Testing a slow monitoring and recording system for the Double Chooz
project
Jim E. Black
Faculty mentor:
Dr. Glenn Horton-Smith
Until recently, neutrinos were believed to have zero mass. Particles without mass travel
at the speed of light, and they do not change between the source and the detector unless
they interact with something in between. The recent discovery that one type of neutrino,
such as an electron neutrino, can change into another type, like a muon neutrino, in midflight has fundamentally changed our understanding of these particles, and was a
significant step in the ongoing quest to understand what the universe is made of, and the
laws that govern it. The Double Chooz experiment [1] aims to refine our knowledge of
these oscillations between different neutrino types, by analyzing the neutrinos that arrive
at two detectors located at different distances from a nuclear power plant in Chooz,
France. The primary advantage of the site is that one of the underground laboratories in
which the neutrino detectors will be housed has already been built for a previous neutrino
detection experiment.
My project this summer was a small part of this undertaking. Since the temperature,
humidity, magnetic fields, and other environmental variables could affect the workings of
the neutrino detector and its associated electronics, we need a way to monitor and record
them. My mentor, Dr. Glenn Horton-Smith, has proposed using 1-Wire devices from
Dallas Semiconductor for the job [2]. These devices are inexpensive, and one can hook
up many 1-Wire devices in parallel on just two wires. My task was to test a prototype of
this slow monitoring and recording system.
We plan to use more than the minimum two wires, to provide separate circuits for data
and electrical power, increasing the speed and reliability of the network. Specifically, we
currently plan to use CAT-5 cable. The orange-white, orange, blue-white, and blue
conductors are being used for the data, data ground, power, and power ground lines,
respectively. Data from each device will be recorded about once a minute in a
POSTGRESQL [3] database. [2]
Software installation and configuration
For the first few days of the project, we searched for Linux drivers for the DS9490R
USB-to-1-Wire adapter [4]. We were originally planning to use Dallas’s Java 1-Wire
API, but had difficulty finding a version that worked with both a Linux operating system
and a DS9490R adapter. We turned instead to Dallas’s 1-Wire Public Domain Kit [5].
When we first ran the test programs accompanying the software, we were greeted with an
error:
Error 119: libusbses.c line 178: Failed to set libusb configuration
This error was traced to a permissions issue. The software needs permission to write to
the USB port at /proc/bus/usb/xxx/yyy (where xxx is the number of the USB bus the
adapter is plugged into, and yyy is a device number assigned when the adapter is
connected, which can be determined with /sbin/lsusb). The solution was to use Hotplug
to recognize the DS9490R adapter upon plugin by its product and vendor ID numbers
(0x04fa and 0x2490, respectively) and run a script that gives non-root users permission to
write to the device.
Over the course of the summer, several disadvantages of the 1-Wire Public Domain Kit
were found. It lacks high-level functions for reading and writing to specific devices,
meaning that every time we want to add a new type of device to the network, someone
has to look up the commands in the datasheet for the device, and write some new code to
handle it. Fortunately, the commands are fairly simple, but writing code to detect and
report all the errors that could happen can be time-consuming.
In addition, the 1-Wire Public Domain Kit was poor at reporting errors; in particular, the
search functions didn't report errors in searches. When there were transmission errors,
the search functions would often report the same device twice without noticing that there
was a problem. Finally, the 1-Wire PDK lacked the option to adjust the timing of the
pulses created by the DS2490 chip inside the DS9490R. To add these functionalities,
new code was written to replace the 1-Wire PDK, communicating with the DS9490R
adapter via the libusb library [6].
Basic 1-Wire Network Operation
In the simplest master-end interfaces for a 1-wire network, the data line is connected to a
pulldown resistor in series with a 5-volt voltage source. To send a signal on the 1-wire
network, a device draws current from the data line, and the voltage drop across the
pulldown resistor causes the data line's voltage to decrease to nearly 0V. Zeros and ones
are indicated with long and short low-voltage pulses, respectively. Generally, devices do
not transmit unless instructed to do so by the master; the master creates a short "one"
pulse, called a read slot, and the device extends the pulse if it is sending back a zero.
Each 1-Wire device has a 64-bit serial number that enables the computer to address it
individually. Communication usually consists of a long “reset” pulse, followed by a
“presence” pulse from any devices that are on the network, followed by a command to
select one of the devices by serial number, and last, commands specific to the device
being addressed. The 1-Wire protocol also specifies a search algorithm that allows the
master to identify the serial numbers of all devices on the network. [7]
Figure 1 An illustration of the operation of a 1-Wire network. Shown in the graph of voltage versus time
are a reset pulse, a presence pulse, and the beginning of a “Skip ROM” command (0xCC), which indicates
that the commands that follow it apply to all devices.
When there is a significant capacitance in parallel with the one-wire data line, either due
to long cables, a large number of devices, or other parasitic capacitances, the rise and fall
time of the pulses slows down. According to Dallas’s “Guidelines for Reliable 1-Wire
Networks,” [8] each additional 1-Wire chip is equivalent to 0.5 meters of cable in its
effects on network reliability, and 24 picofarads of capacitance is equivalent to 1 meter of
cable. The specifications for the DS9490R adapter we are using indicate it should be
reliable when the weight, meaning the total length of the wires connected to the network,
plus the contribution of devices and other parasitic capacitances, be less than 300 meters
[8, 9].
Tests with capacitors in parallel
One of the first tests done was reading a DS18S20 thermometer [7] in parallel with
capacitors of various values. The software in use at the time was a C program that called
functions in the 1-Wire Public Domain Kit libraries. Up to 30 nF, the network worked
correctly; in one test with 30 nF in parallel, the thermometer was read 197619 times over
a 66-hour period, and no errors were detected. Between 34 and 35 nF, the network failed
completely as thermometer stopped responding to read slots. The most notable
difference in the waveform of the pulses as capacitance was added was that the voltage
didn’t have time to get back up to 5 volts between a long “zero” pulse from the master
and the next pulse. At 34.7 nF, the voltage was only rising up to between 1.7 and 1.8
volts. Most likely, when the voltage reached this level, the thermometer was unable to
tell that a new pulse had begun, and thus failed to recognize commands from the master.
Success Rate
Rise of Voltage after Write-0
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0.00
10.00
20.00
30.00
40.00
50.00
Capacitance (nF)
Figure 2 Results of tests of a DS18S20 thermometer in parallel with a capacitor.
It should be noted that since the failure was caused by the device’s inability to interpret
its commands, other types of devices will probably fail at higher or lower capacitances.
It is believed that the DS2450 analog-to-digital converter fails at lower capacitances, but
tests of the two devices under identical conditions, which would be needed to confirm
this, have yet to be done. Preliminary tests showed that the choice of timing settings for
the DS2490 USB-to-1-Wire bridge chip has a large effect on the capacitance at which
devices fail.
Tests with long data lines
Connecting a long cable not only slows the rise and fall time of the pulses, but also
causes some overshoot and ringing. Reflections from the other end of the cable are
evident in the way the voltage rises in steps. If these problems are severe enough, they
could cause the signal to be misread.
Figure 3 Waveforms from the master writing zeros and ones, with and without an approximately 300meter cable connected.
To see if the 1-Wire adapter really could read data from a device at the other end of 300
meters of cable, I spooled out 305 meters of CAT-5 cable along sidewalks, to the ledge
above the book depository on the south side of Hale Library, where I left a 1-Wire
thermometer overnight. Despite the slowed-down rise and fall times of the voltage, I was
able to read the temperature from the opposite end of the cable, at a computer in Cardwell
Hall, the physics building. The only errors were some power failures at about the time of
the temperature spike (see Figure 4); it is likely that someone picked up the thermometer
incorrectly at that time, and disturbed the power cables.
Figure 4 Data from a thermometer at the library, at the other end of 305 meters of cable.
Further tests with the approximately 300-meter-long cable were done inside, with the
cable wrapped 15 ½ times around the room, hanging from the fluorescent lights. Aside
from two thermometers that stopped working, networks involving only thermometers
worked very well. In a three-day test with a DS18S20 thermometer at each end of the
cable, 173271 temperatures were read from each thermometer, and no errors were
reported (see Figure 5).
Figure 5 Data from DS18S20 thermometers, one at each end of a 300-meter cable.
To test how well the network would work the number of thermometers was increased, 19
DS18B20 thermometers [10] were connected in parallel on a breadboard for testing. One
of the thermometers stopped working, so it was removed, leaving 18 thermometers.
When the 18 thermometers were connected at a short distance from the computer, the
network worked perfectly, even if the 300-meter-long cable, with two more thermometers
and an analog-to-digital converter at the other end, was connected in parallel.
However, when the breadboard full of thermometers was moved to the far end of the 300meter cable, there were a few errors. The problem was traced back to the resistance of
the cable; specifically, when about 6 to 8 devices are performing an analog-to-digital
conversion, the power consumed by the devices is high, a lot of current flows through
both ground wires, and there is a voltage difference between ground at the far end of the
cable and ground at the near end. This decreases the voltage difference sampled by the
master and can turn ones into zeros. Fortunately, this is not likely to be a problem at
Double Chooz. The cable lengths used there will be on the order of 10 to 40 meters, and
since the devices will probably only be read about once a minute, they can perform
conversions one at a time.
In another test, the 300-meter cable was cut in half, and the breadboard with 18
thermometers was placed in the middle. In addition, there was a thermometer at the near
end of the cable, and a thermometer and ADC at the far end of the cable. Each device
was read 4124 times, and no transmission errors were detected.
A test of a single DS18S20 thermometer over 610 meters of cable was done by sending
the signal and the power for the thermometer through the cable twice, using the other four
wires. As expected, the network failed; the thermometer did not respond to read slots.
Thermometer Failures
One of our most serious concerns came from two thermometer failures during the
summer, one DS18S20 and one DS18B20. One of the thermometers (the DS18S20)
failed in the middle of the night while being read. When it stopped working, it was
connected to a 300-meter-long CAT-5 cable. At the far end of the cable was a
thermometer and ADC, and at the near end was the thermometer that failed. Figure 6
shows how the near thermometer’s failure rate worsened as the night progressed.
Eventually, the presence of the broken thermometer interfered with other devices.
Oscilloscope traces of a “zero” pulse from each thermometer (Figure 7) show under good
network conditions show that the damage to the thermometer is permanent.
Figure 6 Progressive failure of a DS18S20 thermometer.
Figure 7 Waveforms when a working and a broken thermometer attempt to signal back a zero.
Given that at least one of the thermometers was damaged during communication, Dallas
technical support suggested a possible mechanism, involving a feature of the DS2490
chip that speeds up the recharging of the data line during rising edges. When the voltage
reaches a critical level between 0.25 and 1.1 volts, the DS2490 turns on a strong current
to charge the line more quickly [9]. If the thermometer is writing a zero to the master,
and due to noise on the data line, the voltage reaches the critical level, then a large
amount of current can pass through the thermometer, possibly damaging it. This would
explain why it was the thermometer at the near end of the cable that stopped working; the
resistance of the long cable would have offered some protection to the thermometer at the
far end.
The solution suggested by Dallas technical support was to limit the current on the 1-Wire
data line by adding a small resistor in series with the data line. The usual resistance
recommended for short networks is 150 ohms; for larger networks, this may need to be
reduced. A Zener diode in parallel was also suggested to limit the voltage. Long-term
tests are needed to see whether this protection actually works, and how much resistance is
sufficient.
A different device, different transmission errors
Preliminary tests indicate that even at the far end of the 300meter cable, the DS2450 analog-to-digital converter [11] can
correctly report voltages applied to its input pins. When reading
the voltage across a resistor that was in series with an LED
connected to its 5-volt power supply (see Figure 8), the average
voltage read by the ADC agreed with the average voltage read by
a multimeter: 3.57 volts. There were fluctuations of 0.17 volts
RMS in the voltage reported, mostly due to actual fluctuations in
the power supply voltage, in particular, the power supply voltage
drops when large currents are drawn. Switching the places of the
resistor and LED reduced these fluctuations to 0.017 volts RMS;
after adding a 10-nF capacitor between the power supply and
Figure 8
ground, the fluctuation was reduced to 0.012 volts RMS (see
Figure 9). It is unclear as of yet whether the addition of the
capacitor improved the accuracy of the ADC or simply decreased the actual fluctuations
in the voltage across the capacitor.
Without capacitor
With 10-nF capacitor
400
350
300
250
200
150
100
50
0
1.5
1.52
1.54
1.56
1.58
Voltage Measured (volts)
Figure 9 Measurements of the voltage across an LED, with and without a 10-nF capacitor in parallel with
the power supply.
The current plan [2] calls for using the DS2450 ADC’s at Double Chooz to measure
magnetic field values and other slow-moving data from analog sensors. Tests of the
ADC in conjunction with a magnetometer were delayed due to difficulties getting a good
connection between the solid-core CAT-5 cable and the circuit board, but preliminary
tests have shown that the setup can detect a bar magnet when it is brought near the
sensor.
Although the ADC worked at the far end of the 300-meter cable, when the ADC was
connected to the computer at the near end, in parallel with the cable, new transmission
problems arose. In this configuration, the ADC does not show up in searches, but a
thermometer on the same circuit board often does. It is possible that the ADC has a
lower tolerance for noise on the line than the thermometers do. While the ADC did not
show up in searches, sometimes it did respond to a “Skip ROM” command followed by a
“CONVERT” command (see the descriptions of these commands in the DS2450
datasheet [11]). This was valuable because the response rate provides an indicator of
how much changes to this network improve it.
In many of the tests done with an ADC connected at the near end of the cable, there were
errors in the search other than missing the ADC. The programs accompanying the 1Wire PDK failed to notice many of these errors; thus, in search results, they listed certain
devices twice or not at all. It is often possible to determine that a search error occurred
by analyzing the “discrepancy” bits returned by the DS2490 USB-to-1-Wire bridge chip
[9], something the programs in the 1-Wire PDK did not do. A program that identifies
errors this way will not report a device twice in searches.
At first, it was suspected we were witnessing something warned about in Dallas’s
“Guidelines for Reliable 1-Wire Networks”; networks with multiple long branches of
cable perform poorly due to the impedance mismatch at the branch [8]. This view was
supported by the fact that when the ADC was moved to the far end of the 300-meter
cable, the search errors vanished. However, even when the cable to the ADC was
shortened from 2½ meters to 5 centimeters, the ADC did not show up in searches.
Shortening the cable did dramatically improve the number of responses to “Skip ROM”
and “Convert” commands. The response rate improved from less than 1% to as much as
69%, depending on the timing settings of the DS2490.
Terminating the cable with a 100-ohm resistor to avoid reflections was not helpful. In
fact, because it constantly drained current from the 1-Wire data line, it held the voltage
low, preventing any communication from occurring. The DS2490 chip reports a short.
Resistances of up to 600 ohms caused the network to fail. At 667 ohms, the thermometer
once again appeared in searches, but the ADC did not.
Various other ideas were tried to make it possible to make an ADC at the near end of the
cable appear in searches, including adding a 100-ohm or 200-ohm resistor in series with
the cable branching off toward the ADC, putting a 1-kiloohm resistor between the 5-volt
power supply and the data line to make the voltage rise faster during rising edges,
adjusting the timing settings of the DS2490 chip, putting 15-ohm resistors in series with
both the data line and the ground line coming out of the DS9490R adapter, and
terminating the cable with a series combination of a 100-ohm resistor and 10-nF
capacitor. None of these approaches were successful. What did allow the ADC at the
near end to be found in searches was increasing the impedance contributed by the 300-
meter cable, either by adding a 33-ohm or 50-ohm resistor in series, or by using a pair of
wires in the CAT-5 cable that were not twisted around each other. Both of these
solutions are inadvisable because they decrease the performance of the rest of the
network.
Timing settings of the DS2490 USB-to-1-Wire bridge chip
One feature of the DS2490 chip is that the timing of the pulses it generates can be
adjusted [9]. In Dallas’s “Guidelines for Reliable 1-Wire Networks,” [8] they discuss
these settings in the context of a similar chip, the DS2480B. They note that the default
settings are optimized for small networks, and recommend that the settings be adjusted
for medium or larger networks:
Pulldown Slew Rate: 1.37V/µs
Write-One Low Time: 11µs
Data Sample Offset/Recovery: 10µs
Oscilloscope observations of the waveforms from the DS2490 chip we were using
showed that the actual timing of the pulses varied markedly from the nominal values
given in the datasheet [9]:
Pulldown Time
Code Nominal Measured
0
0.3 µs
0.3 µs
1
2.3 µs
2.0 µs
2
3.0 µs
2.6 µs
3
3.6 µs
3.15 µs
4
4.5 µs
3.85 µs
5
6.0 µs
5.2 µs
6
7.1 µs
6.0 µs
7
9.1 µs
7.6 µs
Write-One Low Time
Code Nominal Measured
0
8 µs
3.8 µs
1
9 µs
4.8 µs
2
10 µs
5.7 µs
3
11 µs
6.8 µs
4
12 µs
7.7 µs
5
13 µs
8.8 µs
6
14 µs
9.7 µs
7
15 µs
11.0 µs
Write-Zero Recovery Time
Code Nominal Measured
0
3 µs
10.5 µs
1
4 µs
12.5 µs
2
5 µs
14.5 µs
3
6 µs
16.5 µs
4
7 µs
18.5 µs
5
8 µs
21 µs
6
9 µs
26 µs
7
10 µs
30 µs
It has yet to be determined what settings will work best for the networks at Double
Chooz. However, extensive testing of different combinations of timing settings was done
in an effort to reduce the transmission errors that occur when an ADC is connected to the
computer in parallel with a 300-meter cable. The settings that worked the best were:
Pulldown Slew Rate Code: 0
Write-One Low Time Code: 0
Data Sample Offset/Recovery Code: 0
It is unknown whether these settings are optimal for normal network operation. Further
testing will provide better information about what settings work best.
Conclusions and Future Work
With the exception of the two thermometer failures, the DS18S20 and DS18B20 digital
thermometers performed well with network lengths much longer than can be expected at
Double Chooz. Yet to be tested is a network with a number of thermometers comparable
to that expected: an estimated 84 thermometers and 172 analog-to-digital converters per
lab. [2] The high failure rate of thermometers this summer could be a problem; if 2 out
of 22 thermometers failed in two months of intermittent use, then with 168 thermometers
in use, then we would expect to need to replace at least two thermometers each week. If
it can be shown that resistors in series with the data line can prevent these failures, this
will no longer be a concern. An extended test of many thermometers on a noisy but
resistor-protected network is needed to verify this.
The DS2450 analog-to-digital converters were not as resistant to data transmission
problems as the thermometers, but the 300-meter-long cable used for most of the tests
this summer is far longer than the 10 to 40-meter total cable lengths expected at Double
Chooz. Tests with shorter cable lengths and more analog-to-digital converters are the
next step in evaluating whether the proposal will work. Such tests will need to be done
for every device we want to add to the network.
Acknowledgements
I would especially like to thank Dr. Glenn Horton-Smith, my mentor in this project, Dr.
Larry Weaver and Dr. Kristan Corwin, who made the REU program at Kansas State
University this year both educational and enjoyable, and the National Science
Foundation, which funded the program. I am also grateful for the advice and assistance
of many people in the KSU high-energy physics department, including Jasmine Foster,
Dr. Tim Bolton, fellow REU student Ashley Wheeler, and others.
References
1. "Double Chooz: A Search for the Neutrino Mixing Angle theta-13," F. Ardellier,
et al, http://arxiv.org/abs/hep-ex/0606025/
2. "Slow monitoring and recording system status summary", Glenn Horton-Smith,
http://neutrino.phys.ksu.edu/~gahs/doublechooz/DC_SlowMRS/
3. PostgreSQL website, http://www.postgresql.org/
4. Data sheet for DS9490R, Dallas Semiconductor,
http://datasheets.maxim-ic.com/en/ds/DS9490-DS9490R.pdf
5. "1-Wire Public Domain Kit," Dallas Semiconductor,
http://www.maxim-ic.com/products/ibutton/software/1wire/wirekit.cfm
6. libusb project home, http://libusb.sourceforge.net/
7. Data sheet for DS18S20, Dallas Semiconductor,
http://datasheets.maxim-ic.com/en/ds/DS9490-DS9490R.pdf
8. "Guidelines for Reliable 1-Wire Networks", Dallas Semiconductor application
note 148, http://www.maxim-ic.com/appnotes.cfm/appnote_number/148
9. Data sheet for DS2490, Dallas Semiconductor,
http://datasheets.maxim-ic.com/en/ds/DS2490.pdf
10. Data sheet for DS18B20, Dallas Semiconductor,
http://datasheets.maxim-ic.com/en/ds/DS18B20.pdf
11. Data sheet for DS2450, Dallas Semiconductor,
http://datasheets.maxim-ic.com/en/ds/DS2450.pdf