Measurement Computing USB-TC - Manual (Page 11)

USB-TC User's Guide Sensor Connections
11
Digital I/O
You can connect up to eight digital I/O lines to the screw terminals labeled DIO0 to DIO7 . Each terminal is
software configurable for input or output.
Power output
The two +5V terminals are isolated (500 VDC) from the USB +5V.
Caution! Each +5V terminal is an output. Do not connect to an external power supply or you may damage
the USB-TC and possibly the computer.
Ground
The six analog ground terminals ( GND ) provide a common ground for the input channels and DIO bits and are
isolated (500 VDC) from the USB GND.
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, T, 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.
Figure 3. Typical thermocouple connection
Connect thermocouples to the USB-TC such that they are floating with respect to GND (pins 9, 19, 28, 38). The
USB-TC
GND pins are isolated from earth ground, so connecting thermocouple sensors to 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
device is powered on, and ±40 VDC when the device 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.