Measurement Computing PCI-DAS-TC - Manual (Page 13)

PCI-DAS-TC User's Guide Functional Details
13
CIO-STA-TC screw terminal adapter board
The CIO-STA-TC is a specially configured screw terminal adapter board designed specifically for use with the
PCI-DAS-TC. The board has screw terminals for each thermocouple channel, a cold junction sensor integrated
into an isothermal bar, and the option of installing an "open thermocouple detection" circuit.
Each thermocouple input is made through two screw terminals — one positive (+) and one negative (-). Connect
the thermocouple wires to the appropriate terminals, connect the CIO-STA-TC to the PCI-DAS-TC, and your
board is ready for use.
Note the polarity when connecting the thermocouples
Be careful to observe correct polarity when connecting thermocouple wires or extension wires.
The CIO-STA-TC is available by itself (MCC part number CIO-STA-TC), or as part of a kit that includes the
PCI-DAS-TC board and five foot shielded cable.
Open thermocouple detection
The only user configurable option in the CIO-STA-TC is the open thermocouple detection resistors. These are a
series of 20 MΩ resistors that can be connected between the + terminal of the thermocouple and a known
voltage that is larger than any allowable thermocouple output.
The 20 MΩ resistors are large enough so that they do not affect the readings from the thermocouples. If a
thermocouple junction should open, a 20 MΩ will drive the input voltage high enough so the software can
recognize that it is not a valid thermocouple reading.
Open thermocouple detection circuitry is set via dip switches on the CIO-STA-TC. DIP switches are labeled,
and each channel has a switch. To enable open thermocouple detection for a channel, set the switch to ON (up,
towards the isothermal block). To disable the function, set the switch to OFF (down, towards the outside of the
board).
0 1 2 3
O
N
4 5 6 7 8 9 10 11
O
N
13 14 15
Figure 4. PCI-DAS-TC functional block diagram
The PCI-DAS-TC is shipped with open thermocouple detection disabled.
Theory of operation
Isolated analog inputs
The analog input section of the PCI-DAS-TC consists of the following components:
 CJC (Cold Junction Compensation) sensor input
 20 (differential) channel multiplexer
 Precision 9.90 V source
 Analog ground source
 Programmable gain amplifier suitable for scaling the seven thermocouple types
 High frequency, synchronous V-F A/D converter.