DB-889D Wiring Diagram
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.

The DB-889D provides 16-channel analog input and CJC circuit for thermocouple input. 

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)