Therm devices may be daisy-chained together on a single serial port (up to 16 units). Each will respond to a unique address. Each therm can in turn support up to 15 directly-addressed probes, or an unlimited number in batch mode.

Since the therm uses a standard protocol (RS-232 at 1200 baud) communication with it is simple and universal: You can use a terminal to test operation, run the therm remotely using a modem, and write your own programs to control it.

• Hot Little Therm unit
• Wall transformer and serial port adapter
• RJ-11 "phone" serial cable
• Temperature probe
• This manual

The addressed therm sends a reply back in one of the following formats:

The temperature reading is always in the same format: A sign followed by 3.1 decimal digits. Pound signs indicate that the requested probe doesn't exist (may be disconnected), while stars indicate a checksum error while trying to read the probe, possibly because the cable is too long or wired incorrectly.

Note:

Make sure the computer serial port connected to Hot Little Therm is set for No flow control or Raw mode. Therms do not support hardware (RTS/CTS) or software (XON/XOFF) flow control. If the serial port is set for flow control, it may appear that no therm is connected, or the commands may be eaten by the device driver. If it's not possible to set the port flow control, you should still be able to connect to a therm, but you may need to experiment with looping back flow control signals (e.g. CTS -> RTS or DTR -> DSR or CD).

If you're still wondering why the probes start at 2 instead of 0, it's because: this avoids the use of control characters (0x00--0x1F) for the most common probe addresses. If you're wondering why the probe number is the top nybble: there's no particular reason for that.

If a command is sent to read probe number 1 (the "16^th" probe), then all probes connected to the therm are sampled and a long-format listing is produced instead of a single temperature value. The format of the listing is:

Each line of the listing starts with a letter:

• V -- Indicates the firmware version number and unit serial number.
• S -- Digit indicates the state of auxiliary digital input pin on J3 (0 = lo, 1 = hi). This input does not affect the operation of the therm, but is an extra digital input that can be read in batch mode.
• T -- Gives the id and temperature of a probe. The id is always 16 hexadecimal digits, and is unique across all probes manufactured. The temperature is in degrees Celsius, as always.

  One T line will appear for each probe connected. The first 15 probes listed this way are also directly addressable by using their individual addresses, as explained above.

• Z -- Indicates the end of the listing.

The order of the probe ids may not be obvious, but they are always found least-significant bit of the byte first, left to right.

Note:

Multiple therms may be daisy-chained on a serial port by plugging additional units into the END connector. Outgoing and incoming characters are passed through the therm between HOST and END ports. If a command doesn't address the therm directly connected to the serial port, it is passed on to the next unit, and so on down the line.

The command byte is not passed transparently through from HOST to END, instead the unit number part of the command is decremented once at each hop (the probe number is not changed, for example 0x21 becomes 0x20). The unit number of every therm is zero -- this allows built-in automatic addressing of therm units. The first therm attached to a serial port will respond as unit 0, the second as unit 1, etc.

Note:

This feature affects the programmed unit numbers of older (v.7) firmware. The effective unit number of a v.7 therm will be higher by the number of v.9 therms upstream of it toward the host, because each one will subtract one from the unit number. The following table shows an example of how a mixture of version 7 and version 9 therms might be addressed:

If a command doesn't address any therm connected to the serial port, no response will ever be sent. Any program that reads the therm device directly must handle this by setting a timeout when waiting for the response. A maximum delay of 1 second is sufficient to ensure the therm has enough time to respond if it is connected.

How long can the wire from the therm unit to the probe be? Quick answer: A total of up to 300 feet of cable should work fine, and tests have shown 500 feet to work reliably. If you use too much wire, the therm won't be able to read probes correctly. You'll get "******" or "######" messages because the CRC checksum will fail.

The main concerns with long probe wires or many probes are:

• Capacitive loading due to the cable
• Reflections from the end(s) of the cable
• Resistance of long wires
• Noise on long wires

When sending data to the probes, the therm drives the line both high and low, and line capacitance is not very important. When receiving, however, the probes can only pull the line low and must let it float high. To read a bit from a probe, the therm sends a 4us low pulse and then stops driving the line. The probe may either drive the line low for 30us nominal to send a logic-zero, or release it and let it float high to send a logic-one. The therm samples the line after 15us to determine the bit value. The most critical timing occurs when the probe sends a logic-one bit. The line must float high through the pullup resistor within 15us to be read correctly.

The line voltage approximately follows that of a capacitor charging through a resistor:

In this case, V_s is the applied voltage (5 volts), V_c is the line voltage, r is the pullup resistor (5k ohms) and c is the total line capacitance.

The capacitance of a single 10' probe (as measured for 10 probes connected in parallel on the bench) is about 130pF, and this is mostly due to the cable itself. If we solve the equation with V_c=2.0 and r=5000 and t=15E-6 we get a maximum allowable capacitance of around 5800pF. This translates to around 45 probes, or 450 feet of wire, e.g. any combination of probe wire lengths up to a total of 450 feet should work. However, 2.0 volts is marginal, since the capacitance is somewhat variable, and can depend on where and how the wire is run. I'd recommend a more conservative limit, say 300 feet. That is, you could have one probe 300 feet away, or 10 at a distance of 30 feet each.

Reflections from the far end of the probe cable are the second main concern. It is not possible to terminate the cable in its characteristic impedance, due to the open-collector nature of the bus and constraints on the minimum value of the pullup resistor. To obtain the ultimate performance, an active-pullup circuit is recommended for the driver. Hot Little Therm uses a simpler circuit, and limits the slew rate of the negative-going signal (the worst case) with a series resistor. This works well enough for moderate (hundreds of feet) cable lengths.

Here are some suggestions when using long probe cables:

• Use high quality wire, such as Cat-5 twisted pair. Leave unused wires in the bundle disconnected to reduce parasitic capacitance.

• It is preferable to daisy-chain the cable from probe to probe, instead of wiring each probe directly back to the therm, in order to reduce total wire length and reflections from cable ends.

• To compensate for capacitive loading, you can increase the drive on the probe port by reducing the built-in 5k ohm pullup resistance. This requires a wiring change on the therm circuit board. The minimum value allowed is 1.5k ohm, so if a resistor is added in parallel, its value must be greater than 2.2k ohm.

Almost any 4 or 6-conductor phone cable should work as a therm extension cable, as long as there is a half twist between the two ends (pin 1 on one end is connected to pin 4 on the other, pin 2 to pin 3, etc.). Stock phone extension cables will also work, if you maintain the correct number of half-twists. There is no hard limit on maximum cable length, you'll have to experiment. I've personally run 9600 baud serial cables the better part of a mile.

Caution:

As always when performing electrical wiring, be aware of the environment. Hot Little Therm does not generate or use any voltages normally considered dangerous, but you must be careful when extending the probe or serial port wires to not let them come in contact with high-voltage (e.g. power or telephone) lines. Remember when running wires outdoors that lightning may strike them and be conducted inside.

One suggested method is to put the probe into a picnic cooler with an accurate lab thermometer. Allow some time for the temperature to stabilize (maybe 30 min.), then quickly read the lab thermometer and note the difference between it and the Hot Little Therm probe. If you have several probes, each one must be calibrated separately (at the same time if you like). You can now add this offset to the probe reading (for Unix software, see the thermcap to# entry).

Note: The probe may have different error offsets at different temperatures, so you may also want to calibrate it at two or more points and add a variable correction depending on the reading (the standard software does not support this). For a single-point correction, try to calibrate the probe somewhere in the middle of the expected temperature range in use.

Hot Little Therm comes with unsupported example software. It is intended to help you get started using your Hot Little Therm right away. We expect you will want to make changes to it so we include the source code in the distribution. Explanation of the software in this manual is minimal because we expect it to change over time. Each program is documented more fully in README files and manual pages that accompany the source code.

Introduction

All the software is fairly portable, and should work on most any flavor of Unix. You may need to select compile-time options or tweak it a bit to get it to work. Check the comments near the top of a program or in the Makefile to see what can be specified.

Also, in keeping with the Unix tradition, the programs are not all-in-one applications, but rather building blocks that you can use to create other things.

Probem is a utility program that searches a TTY for Hot Little Therms, reporting any units and probes that it finds. It can be useful when installing or debugging therm wiring.

Thermplot reads temperature log files and plots a graph of time vs. temperature, and outputs it as a PPM image. It can draw on a blank background of any size, or accept a source image on which to overlay the plot.

Thermplot has lots of options to change the appearance of the graph. It makes a good building block for web pages.

hltware

The therm unit requires a D.C. supply between 8 and 20 volts (9 volts nominal), and draws about 10mA. Its operating temperature range is at least 0 - 70 C.

RJ-11 HOST (J1) / END (J2) Jacks

Probes use a C-Grid connector, Molex part number #50-57-9002

This table shows the hexadecimal value and ASCII character equivalent for probe addresses, by therm unit and probe number. Addresses 0x80 -- 0xFF are not shown; they are the same characters repeated, but with the highest bit = 1.

                 Probe number
                 0 (15th)  1 (batch)  2 (1st)  3 (2nd)  4     5     6     7
               + 0x00      0x10       0x20     0x30     0x40  0x50  0x60  0x70
             0x0   ^@        ^P        spc       0        @     P     `     p
             0x1   ^A        ^Q         !        1        A     Q     a     q
             0x2   ^B        ^R         "        2        B     R     b     r
             0x3   ^C        ^S         #        3        C     S     c     s
             0x4   ^D        ^T         $        4        D     T     d     t
             0x5   ^E        ^U         %        5        E     U     e     u
             0x6   ^F        ^V         &        6        F     V     f     v
Unit Number  0x7   ^G        ^W         '        7        G     W     g     w     ...
             0x8   ^H        ^X         (        8        H     X     h     x
             0x9   ^I        ^Y         )        9        I     Y     i     y
             0xA   ^J        ^Z         *        :        J     Z     j     z
             0xB   ^K        ^[         +        ;        K     [     k     {
             0xC   ^L        ^\         ,        <        L     \     l     |
             0xD   ^M        ^]         -        =        M     ]     m     }
             0xE   ^N        ^^         .        >        N     ^     n     ~
             0xF   ^O        ^_         /        ?        O     _     o    del

Note: This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more of the following measures:

• Reorient or relocate the receiving antenna.

• Increase the separation between the equipment and receiver.

• Connect the equipment into an outlet on a circuit different from that to which the receiver is connected.

• Consult the dealer or an experienced radio/TV technician for help.

Caution: Changes or modifications not expressly approved by the party responsible for compliance could void the user's authorization to operate the equipment.

Spiderplant reserves the right to make changes without further notice to Hot Little Therm or to this manual. Spiderplant makes no claim, express or implied, of the fitness or adequacy of Hot Little Therm or application software for any particular purpose or use, or of the accuracy or completeness of information contained in this manual. Spiderplant will not assume liability for any loss or damage caused by the use or misuse of Hot Little Therm or application software. Hot Little Therm is not a medical instrument.

Spiderplant
therm@spiderplant.com

June 6, 1999

Part Number: 101-0001