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OM-USB-TC-AI User's Guide
Thermocouple selection
The thermocouple type you select will depend on your application needs. Review the temperature ranges and
accuracies of each type to determine which is best suited for your application.
Ground terminals (GND)
The six analog ground terminals (
isolated (500 VDC) from the USB GND.
Power terminals (+5V)
The two
+5V
terminals are isolated (500 VDC) from the USB +5V.
Digital terminals (DIO0 to DIO7)
You can connect up to eight digital I/O lines to the screw terminals labeled
software configurable for input or output.
If a digital bit is set up as an alarm, the bit is configured for output on power-up, and assumes the state defined
by the alarm configuration
Counter terminal (CTR)
The
CTR
terminal (pin 40) is the input to the 32-bit event counter. The internal counter increments when the
TTL level transitions from low to high. The counter can count events at frequencies of up to 1 MHz.
Caution! All ground pins on the OM-USB-TC-AI (pins 9, 19, 22, 27, 30, 33, 36, 39, 49) are common and
are isolated from earth ground. If a connection is made to earth ground when using digital I/O and
conductive thermocouples, the thermocouples are no longer isolated. In this case, thermocouples
must not be connected to any conductive surfaces that may be referenced to earth ground.
CJC sensor
The OM-USB-TC-AI has one built-in high-resolution temperature sensor. The CJC sensor measures the
ambient temperature at the terminal block so that the cold junction voltage can be calculated.
Thermocouple connections
A thermocouple consists of two dissimilar metals that are joined together at one end. When the junction of the
metals is heated or cooled, a voltage is produced that correlates to temperature.
The OM-USB-TC-AI makes fully differential thermocouple measurements without the need of ground-
referencing resistors. A 32-bit floating point value in either a voltage or temperature format is returned by
software. An open thermocouple detection feature is available for each thermocouple input which automatically
detects an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) on one or more sensor input channels
to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the OM-USB-TC-AI using a differential configuration, as shown in Figure 3.
) provide a common ground for the input channels and DIO bits and are
GND
12
Signal I/O Connections
DIO0
to
DIO7
. Each terminal is
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