TELEMTRY.txt 6.5 APRS TELEMETRY SYSTEM Using the Micro.Interface.Module (MIM) NOTE: THIS INFORMATION IS PRELIMINARY, AND ALTHOUGH A PROTOTYPE IS WORKING FINE, HE AND I ARE STILL FINE TUNING THE PROTOCOLS AND FINAL DESIGN SPEC. IF YOU ARE INTERESTED LET US KNOW. Carl Wick, N3MIM, has developed a simple, yet powerful Micro-Interface- Module that can be used as a single chip APRS telemetry system. Although the intent of his original design was to make a very simple, light-weight, throw- away module for experimental balloon flights, his design has now evolved into a very useful APRS packet tool. Using a single chip microprocessor, he has implemented a complete TNC (transmitter only) on a chip. This chip has four analog inputs, five digital bit inputs, a receive audio input (for propoer CSMA operation) and outputs PTT and AX.25 audio using digital synthesis. The only external components besides the sensors themselves, are a crystal and a transmitter. A 0.2 cu in. 500 mw xmtr is available too! The pin-out follows: _____________ Analog 3 --O| |O-- Analog 2 Analog 4 --O| M.I.M |O-- Analog 1 Rcv input --O| |O-- Xtal reset --O| AX.25 |O-- Xtal Gnd --O| |O-- 5 volts Input bit 1 --O| Telemetry |O-- AX.25 out input bit 2 --O| |O-- AX.25 out input bit 3 --O| Chip |O-- PTT input bit 4 --O|___________|O-- Input bit 5 APRS TELEMETRY RECEIVING SYSTEM: Any telemetry system requires a table of equations and labels to make the values useable at the receiver site. The problem with most experimental telemetry systems, is that the receiver system must be tailored for every new application. In the case of a one-time balloon launch, (or any experiment assembled to meet a schedule) the equations are not usually available until just hours or minutes before launch. This means that it is next to impossible to distribute the equations and parameter definitions to a large number of tracking stations and to be able to have receiving telemetry software ready to go. For this reason, APRS has been designed to serve as a general purpose telemetry tracking system for the M.I.M. Using APRS it is possible to transmit the telemetry equations, parameter definitions and channel units IN REAL TIME! Once any APRS station receives these parameter transmissions, it is then ready to receive and to display the real-time telemetry values in the proper engineering units. The TELEMETRY page is displayed using the alt-T command. Hitting this command causes APRS to scan the READ MAIL screen looking for the telemetry equations, and then to scan the ALL_BEACONS pages looking for TELEMETRY values. Up to 16 samples are displayed per page, for a total of 75 samples. The TELEMETRY samples are saved in the normal LOG files. A sketch of the APRS telemetry display is shown below: APRS TELEMETRY FOR XYZ BALLOON LAUNCH SER TIME Battery AirTemp BTemp Pres Altude Camera Chute Sun 10m ATV NUM volts deg.F deg.F Mbars K feet BIT BIT BIT BIT BIT --- ---- -------- -------- ------ ------ ------ ------ ------ ------ ---- ---- 101 1215 12.8 86 85 999 0 ... ... ... ... ... 102 1216 12.8 86 85 999 1000 ... ... ... ... ... 103 1217 12.6 87 87 998 2000 ... ... ... ... ... 104 1218 12.4 84 80 980 4000 clik ... on on high 105 1219 12.3 80 76 900 8000 ... ... ... on high 106 1220 12.1 75 70 850 16000 ... ... on on ... 107 1221 12.0 70 65 800 32000 clik ... ... ... ... 108 1222 12.0 65 60 730 64000 ... ... on ... high Notice that the M.I.M module transmits a value for each of its four analog channels and each of its five digital bits once every sample time. On receipt, the fourth analog channel is displayed along with a fifth relative channel which has its own separate equation for the 4th value, such as for Pressure and Altitude in a balloon experiment. The sample periodicity can be set from any value from 1 second to 16 minutes depending on the application. Each sample includes a unique serial number. In addition, not only can the parameter name, units and equations be specified for each of the analog channels, but the word to be associated with either the 0 or 1 value of each digital bit can also be specified. To configure all APRS stations to properly decode the telemetry from the M.I.M module, the net control station (or any other designated station in the APRS network) needs to transmit the proper parameter definition packets. These packets are transmitted as APRS messages TO the CALLSIGN of the M.I.M module. If the M.I.M module is using the callsign of N3MIM, then the parameter definition station would send the following four messages: To N3MIM:PARM.Battery,BTemp,AirTemp,Pres,Altude,Camra,Chute,Sun,10m,ATV To N3MIM:UNIT.Volts,deg.F,deg.F,Mbar,Kfeet,Clik,OPEN!,on,on,high To N3MIM:EQNS.0,2.6,0,0,.53,-32,3,4.39,49,-32,3,18,1,2,3 To N3MIM:BITS.10110,PROJECT TITLE... The PARM format specifies the name of each of the ten parameters. The UNITs format specifies what units are to be displayed, and for the digital bits, show what label is associated with the digital condition. The parameters and units for the first two can be up to 8 characters, the next 6 can be 6 characters, and final 2 can be four characters each. The EQNS format has three coeficients for each of the four analog channels, plus the fifth relative channel that uses a different equation related to the channel 4 value. The BITS format specifies either a 1 or a 0 for each of the five digital channels to indicate which state is associated with the indicated label. This permits the payload designer to use 1's or 0's as convenient with his circuity without being forced to always use 0 for OFF and 1 to mean ON. A title can also be included in the BITS definition which will be used by APRS to title the TELEMETRY page. The three values for each of the analog channels are the coeficients of a quadratic equation: Final value = A*X^2 + B*X + C Where X is the M.I.M transmitted value FORMAL SPECIFICATION: The specific format for the TITLE, PARM, UNIT, and EQNS message packets are shown below. They are entered as messages to the address of the MIM module: PARM.P1,P2,P3,P4,P5,B1,B2,B3,B4,B5 Where Pn and Bn are the parameter names UNIT,U1,U2,U3,U4,U5,L1,L2,L3,L4,L5 Where Un are the units for analog ports and Ln are the labels for the bits EQNS,A1,B1,C1,A2,B2,C2,A3,B3,C3,A4,B4,C4,A5,B5,C5 Where the An,Bn,Cn are the coeficients for each of the four analog channels, plus the 5th relative channel. BITS.XXXXX,Title-up-to-23-chars The x's specify the state of the bits that match the BIT Labels. T#sss,111,222,333,444,xxxxx This is the on-air format for the UI data frame, where sss is the serial number followed by the four 3 digit analog values and the five binary values. APPLICATIONS: 1) OBVIOUS Balloon payloads using only party balloons, not needing the big WX balloons and all the paraphanalia. 2) TRAFFIC monitoring MILE posts! This is a neat idea! Given that HAMS will be commuting with APRS moving Map displays, why not build a match box sized traffic SPEED detector (solar powered MIM module) that can be stuck on the side of a highway pole ? Via a $1.29 crystal MIC from radio shack, use DSP to figure out the speed of the traffic based on audio analysis! Beacon this SPEED once every two minutes at about 10 mW. The beacon will, of coure, include the LOCATION of the device. What the APRS commuter sees on his MAP is these MILE posts ahead of him showing traffic speeds! He can then decide on alternate routing! We have plenty of room in the MIM to add this DSP (maybe), IS THERE ANYONE OUT THERE THAT IS INTO DSP THAT CAN DETERMINE THE ALGORITHM TO DETERMINE SPEED FROM THE AUDIO OF TRAFFIC?????????? (or the amplitude fluctuations of a photo cell?) Even cheap X band doppler motion detectors are possible, since they only need to turn on briefly to get a speed measurement. This thing has to be VERY small and low power to be able to be SOLAR powered and able to be COVERTLY installed with out a lot of STATE HIGHWAY bureaucracy. LOW POWER TELEMETRY TRANSMITTERS: To complement this less than ONE-CUBIC inch MIM telemetry system, Agrelo Engineering in NY makes a 1.5 x 0.5 x 0.25 inch 2 meter transmitter for $99. It outputs 500 mW at 6 volts 140 ma and 120 mW at 3 volts 50 ma. See more cheap transmitters in the GPS.TXT file. ORDERING YOUR M.I.M. SYSTEM: The packets from a M.I.M. chip are sent as AX.25 UI frames only. There is no provision for any connected protocols which would make the device overly complex. Therefore you must specify the UNPROTO string for the device. The default periodicity is burned in to the single value specified, but there is an initialization process that can be used to modify this default at power-up by grounding a combination of the four analog lines. Using this process, you can select 1/4th, 1/2 or 2 or 4 times the default rate. In the table below, the defaults for all options are shown. Please be very accurate in specifying the following items when ordering: (DO NOT ORDER YET! THIS IS PRELIMINARY!) Callsign and SSID: (required!) __________________________________ TO Address: (APRTLM) __________________________________ Digipeater VIA path: (WIDE) __________________________________ Default periodicity: (64 sec) __________________________________ (other defaults supported 8,16,32,64,128,256,512 Since the PERIODICITY is frequently different for different applications, there is a mechanism for changing it in the field. The four ANALOG inputs are sensed during the power up initialization process and if they are at +5 volts, they are interpreted in the following manner: A1 - divide period by 4 } A1 and A2 => divide period by 8 A2 - divide period by 2 A3 - Multiply period by 2 A4 - Multiply period by 4 } A3 and A4 => Multiply period by 8 The analog inputs are used for this initialization process, since their attached circuits are usually high impedance and can be temporarily pulled high without problems. This means, for example, that A1 and A2 can be diode ORed to a single push-button to give a start up period of 8 seconds for a nominal 64 second device. Or a resistor/capacitor combination on one of these pins can be used to automatically select a periodicity at power up.