28022

599 Menlo Drive, Suite 100 Rocklin, California 95765, USA Office: (916) 624-8333 Fax: (916) 624-8003 General: info@parallax.com Technical: support@parallax.com Web Site: www.parallax.com Educational: www.stampsinclass.com DS2760 Thermocouple Kit (#28022) 1-Wire® Thermocouple Interface Introduction Thermocouples provide a low-cost, reliable means of measuring temperature over a wide range. The challenge when using a thermocouple is accurately measuring the very low Seebeck output voltage (fractional to low millivolts) from the element, and providing for cold junction temperature compensation. The Dallas/Maxim DS2760 High Precision Li+ Battery Monitor is very easily configured into an effective thermocouple interface. The Parallax DS2760 Thermocouple Module capitalizes on this application and provides a complete connection between the BASIC Stamp and a standard thermocouple element. Features    ® 1-Wire interface allows multiple devices with just one Stamp IO pin Cold Junction measurement: 0°C to +127°C (0.125°C resolution) Low power consumption: -- Active current: 90 A max -- Sleep current: 2 A max Packing List Verify that your DS2760 kit is complete in accordance with the list below:    DS2760 Thermocouple Module (3) Thermocouple elements: -- (1) K-type (Chromel / Alumel) -- (1) J-type (Iron / Constantan) -- (1) T-type (Copper / Constantan) This documentation #550-28022 #800-00011 #800-00012 #800-00010 Note: DS2760 demonstration software may be downloaded from www.parallax.com. Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 1 Connections Before connecting the DS2760 Thermocouple Module to the BASIC Stamp you will need to prepare a thermocouple element, and then connect it to the cold junction port of the module. Start by carefully removing about one inch (250 mm) of the outer sleeve from each end of the element. From each lead on the temperature measurement end, remove about ½ inch (125 mm) of insulation and then carefully twist together (using pliers if necessary) and trim as shown in Figure 1. Figure 1: Thermocouple Junction On the cold junction (DS2760 module) end of the element, remove only ¼ inch (60 mm) of insulation from each lead. Route these leads through the bottom of the thermocouple module PCB and insert snuggly into the pin sockets as shown in Figure 2. Figure 2: Cold Junction Connection to DS2760 PCB Use this table to ensure that you make the proper thermocouple connections to the module. If the leads are reversed, the measured temperature will be incorrect. Type K J T Materials Chromel / Alumel Iron / Constantan Copper / Constantan SNS Red Red Red Vss Yellow White Blue Finally, the DS2760 Thermocouple Module is connected to the BASIC Stamp as shown in Figure 3 below ® (Note that the module includes a 4.7 K pull-up on the 1-Wire data line). Figure 3: DS2760 Connections to BASIC Stamp Vdd Data IO Vss 2 Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 BASIC Stamp Application The following BASIC Stamp application will run on either the BS2p or BS2pe and demonstrates how easy measuring wide-range temperatures can be when using the DS2760 Thermocouple Module. Other Stamps will require a Serial-to-1-Wire protocol converter, as well as code to manage the large tables across program slots, and are not covered in this document. A little background: When two dissimilar metal wires are joined, a voltage will be developed across the open end that is proportional to the temperature difference between the joined and open ends. This effect was discovered by Thomas Seebeck in 1821. Through empirical testing, voltage tables have been established that correspond to the thermocouple junction temperature. These tables, however, use a cold junction (voltage measurement point) reference of zero degrees Celsius, forcing electronic devices to employ cold junction compensation. Using the DS2760 we can measure the Seebeck voltage from the thermocouple with a resolution of 15.625 microvolts, then measure the cold junction temperature with a resolution of 0.125 degrees Celsius. A simple table look-up using the cold junction temperature will give us the cold junction compensation voltage. This is combined with the Seebeck voltage and, using a modified binary search algorithm, we can determine the compensated temperature from the thermocouple data table. ' ========================================================================= ' ' File...... DS2760TC_Demo.BPE ' Purpose... Thermocouple temperature measurement using the DS2760 ' Author.... Parallax, Inc. (Copyright 2004, All Rights Reserved) ' E-mail.... support@parallax.com ' Started... ' Updated... 19 JAN 2004 ' ' {$STAMP BS2pe, KTablePos.BPE, JTablePos.BPE, TTablePos.BPE} ' {$PBASIC 2.5} ' ' ========================================================================= ' ' ' ' ' -----[ Program Description ]--------------------------------------------This program lets a BS2p or BS2pe read the temperature from the Parallax DS2760 thermocouple module. User input of thermocouple type (K, J, or T) and temperature display is via the DEBUG window. ' -----[ Revision History ]------------------------------------------------ ' -----[ I/O Definitions ]------------------------------------------------OW PIN 8 Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 ' 1-Wire buss pin 3 ' -----[ Constants ]------------------------------------------------------ReadNet SkipNet RdReg CON CON CON $33 $CC $69 ' read OW net address ' skip OW net address ' read register ' -----[ Variables ]------------------------------------------------------idx type char VAR VAR VAR Nib Nib Byte ' loop counter ' device type ' display byte/char vIn tmpCJ tCuV sign VAR VAR VAR VAR Word Word Word Word ' ' ' ' cjComp tempC tempF VAR VAR VAR Word Word Word ' temp compensation ' temp in Celsius ' temp in Fahrenheit tblLo tblHi eePntr testVal error VAR VAR VAR VAR VAR Word Word Word Word Bit ' table pointers in millivolts device temp in C thermocouple millivolts TC sign bit ' test value from table ' 1 = out of range ' -----[ EEPROM Data ]----------------------------------------------------- ' -----[ Initialization ]-------------------------------------------------Stamp_Check: #IF ($stamp < BS2P) #THEN #ERROR "This program requires BS2p or BS2pe" #ENDIF Check_Device: OWOUT OW, %0001, [ReadNet] OWIN OW, %0010, [SPSTR 8] GET idx, char IF (char $30) THEN DEBUG "No DS2760 found." STOP ENDIF 4 ' ' ' ' get serial number store in SPRAM read device type if not $30, wrong device ' stop program Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 Menu: DEBUG CLS, "===============================", CR, " DS2760 Thermocouple Interface ", CR, "===============================", CR, CR, "Select TC Type (1 - 3)", CR, CR, "(1) K - Chromel/Alumel", CR, "(2) J - Iron/Constantan", CR, "(3) T - Copper/Constantan", CR, CR, ">>> " DEBUGIN DEC1 type IF (type < 1) OR (type > 3) THEN Menu DEBUG CRSRXY, 0, 3, CLRDN STORE type ' ' ' ' get selection validate selection remove selections point READ to table Show_SN: DEBUG CRSRXY, 0, 4, "Device SN... " FOR idx = 0 TO 7 GET idx, char DEBUG HEX2 char NEXT Show_Type: DEBUG CRSRXY, 0, 6, "TC Type..... " LOOKUP (type - 1), ["KJT"], char DEBUG char ' -----[ Program Code ]---------------------------------------------------Main: DO GOSUB Read_TC_Volts GOSUB Read_CJ_Temp READ (tmpCJ * 2), Word cjComp ' combine cjComp and tCuV ' IF sign THEN ' TC below cold junction IF (tCuV < cjComp) THEN cjComp = cjComp - tCuV ELSE cjComp = 0 ENDIF Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 ' read Seebeck voltage ' read cold junction temp ' get compensation voltage ' limit to 0C 5 ELSE ' TC above cold junction cjComp = cjComp + tCuV ENDIF LOOKUP type, [1023, 1023, 400], tblHi GOSUB TC_Lookup tempF = tempC * 9 / 5 + 32 IF (error = 0) THEN DEBUG CRSRXY, 0, 7, "Temp °C..... DEBUG CRSRXY, 0, 8, "Temp °F..... ELSE DEBUG CRSRXY, 0, 7, "Temp °C..... DEBUG CRSRXY, 0, 8, "Temp °F..... ENDIF ' set high end of search ' reverse lookup of table ' x 1.8 + 32 ", SDEC tempC, CLREOL ", SDEC tempF, CLREOL Out of Range", CLREOL Out of Range", CLREOL PAUSE 1000 LOOP END ' -----[ Subroutines ]----------------------------------------------------' Reads device input voltage (Vin pin) ' -- mV in millivolts (max reading is 4.75 volts) Read_Vin: OWOUT OW, %0001, [SkipNet, RdReg, $0C] OWIN OW, %0010, [vIn.BYTE1, vIn.BYTE0] IF (vIn.BIT15) THEN vIn = 0 ELSE vIn = vIn >> 5 */ $4E1 ENDIF RETURN ' check sign ' disallow negative ' x 4.88 millivolts ' Reads current register to get TC voltage ' -- each raw bit = 15.625 uV ' -- tCuV in microvolts Read_TC_Volts: OWOUT OW, %0001, [SkipNet, RdReg, $0E] OWIN OW, %0010, [tCuV.BYTE1, tCuV.BYTE0] 6 ' read current register Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 sign = tCuV.BIT15 tCuV = tCuV >> 3 IF sign THEN tCuV = tCuV | $F000 ENDIF tCuV = ABS tCuV */ 4000 RETURN ' save sign bit ' correct alignment ' pad 2's-compliment bits ' x 15.625 uV ' Reads cold junction (device) temperature ' -- each raw bit = 0.125 degrees C ' -- returns tmpCJ in whole degrees C Read_CJ_Temp: OWOUT OW, %0001, [SkipNet, RdReg, $18] OWIN OW, %0010, [tmpCJ.BYTE1, tmpCJ.BYTE0] IF (tmpCJ.BIT15) THEN tmpCJ = 0 ELSE tmpCJ = tmpCJ.HIGHBYTE ENDIF RETURN ' ' ' ' ' check sign ' disallow negative ' >> 5 x 0.125 (>> 3) Search currently selected TC table for nearest entry -- uses modified binary algorithm to find cjComp -- high end of search set before calling (tblHi) -- successful search sets tempC TC_Lookup: tblLo = 0 tempC = 22 READ (tblHi * 2), Word testVal IF (cjComp > testVal) THEN error = 1 ELSE DO eePntr = (tblLo + tblHi) / 2 READ (eePntr * 2), Word testVal IF (cjComp = testVal) THEN EXIT ELSEIF (cjComp < testVal) THEN tblHi = eePntr ELSE tblLo = eePntr ENDIF Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004 ' low entry of table ' default to room temp ' check max temp ' out of range ' midpoint of search span ' read value from midpoint ' found it! ' search lower half ' search upper half 7 IF ((tblHi - tblLo) < 2) THEN eePntr = tblLo EXIT ENDIF LOOP tempC = eePntr ENDIF RETURN ' span at minimum Additional Resources • • Advanced thermocouple interface software (download from Parallax) Web Links: -- www.maxim-ic.com/quick_view2.cfm/qv_pk/2931 -- www.capgo.com/Resources/Sensors/Temperature/Thermocouple/Thermocouple.html -- instserv.com/rmocoupl.htm -- instrumentation-central.com/pages/thermocouple_reference_table.htm DS2760 Module Schematic Vdd Vdd DS2760 4.7 k Data IO 1 k CC Vin PLS Vdd DC PIO SNS Vss SNS Vss SNS Vss DQ PS IS2 IS1 0.1 µF 8 Parallax, Inc. • DS2760 Thermocouple Kit (#28022) • 01/2004