Thermocouple Connections; Wiring Configuration; Digital I/O Connections - Measurement Computing Measurement Advantage USB-TC User Manual

USB-TC User's Guide

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 USB-TC 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 analog input which automatically
detects an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, N. E, and B) and one or more sensor input
channels to connect the thermocouple.

Wiring configuration

Connect the thermocouple to the USB-TC using a differential configuration, as shown in Figure 3-2.
Connect thermocouples to the USB-TC such that they are floating with respect to GND (pins 9, 19, 28,
38). The USB-TC
GND
voltages referenced to earth ground is permissible as long as the isolation between the GND pins (9, 19,
28, 38) and earth ground is maintained.
When thermocouples are attached to conductive surfaces, the voltage differential between multiple
thermocouples must remain within ±1.4 V. For best results, we recommend the use of insulated or
ungrounded thermocouples when possible.
Maximum input voltage between analog input and ground
The absolute maximum input voltage between an analog input and the isolated GND pins is ±25 VDC
when the USB-TC is powered on, and ±40 VDC when the USB-TC is powered off.
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to
minimize the error introduced by thermal EMFs.

Digital I/O connections

You can connect up to eight digital I/O lines to the screw terminals labeled
configure each digital bit for either input or output. All digital I/O lines are pulled up to +5V with a
47 K ohm resistor (default). You can request the factory to configure the resistor for pull-down to ground
if desired.
When you configure the digital bits for input, you can use the USB-TC digital I/O terminals to detect the
state of any TTL-level input. Refer to the schematic shown in Figure 3-3. If you set the switch to the +5V
input, DIO0 reads TRUE (1). If you move the switch to GND, DIO0 reads FALSE (0).
Figure 3-3. Schematic showing switch detection by digital channel DIO0
Figure 3-2. Typical thermocouple connection
pins are isolated from earth ground, so connecting thermocouple sensors to
+GND
3-3
DIO0
DIO0
+5V
Sensor Connections
to . You can
DIO7