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The DB-889D have two 37-pin D-sub connector and two 20-pin header. the one D-sub connector connected to A/D Board's analog input connector and another one can be cascaded to another sceondery DB-889D.The 20-pin header is DB-889D' control logic circute. It is connected to A/D board's digital output port from 20-pin flat cable. then the digital output can control multi-plexer and amplifer.
   
DB-889D provides 16-channel analog input and CJC circuit for thermocouple input. THERMO-COUPLE with CJC we would like the A/D card to read only V1, but by connecting the DB-889D in attempt to measure the output of junction J1, we have created two more metallic junction: J2 and J3, since J3 is a copper to copper junction, It creates no thermal EMF (V3=0) but J2 is a copper-to-constantan junction which will add an EMF(V2) In oppositional to the temperature dif-frewnce between J1 and J2. This says that we can't find the temperature at J1 unless we first find the temperature of J2.
   
Example:
Input T-type thermo-couple; Setup DB-889D Gain: 500

reading thermocouple voltage : Vt
reading CJC volatge : Vcjc
caculate Vcjc to temperature : Tcjc = Vc *1000 / 24.4
change Tcjc to T-type thermocouple Voltage : Vtv
(refer. T-type thermocouple table T-to-V )
caculate Vt + Vtv =Vtc
Change Vtc to temperature = T
(refer. T-type thermocouple table V-to-T)
 
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