USB-TC-AI User's Guide
Signal I/O Connections
12
Figure 3. Typical thermocouple connection
Connect thermocouples to the USB-TC-AI such that they are floating with respect to GND (pins 9, 19, 28, 38).
The USB-TC-AI
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
USB-TC-AI is powered on, and ±40 VDC when the USB-TC-AI 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
DIO0
to
DIO7
. You can configure
each digital bit for either input or output. All digital I/O lines are pulled up to +5V with a 47 kΩ 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-AI digital I/O terminals to detect the
state of a TTL-compatible device. Refer to the schematic shown in Figure 4. 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 4. Schematic showing switch detection by digital channel DIO0
All ground pins on the USB-TC-AI (pins 9, 19, 22, 27, 30, 33, 36, 39, 49) 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
For general information regarding digital signal connections and digital I/O techniques, refer to the
Guide to
Signal Connections
(available on our web site at
www.mccdaq.com/signals/signals.pdf
Distributed by MicroDAQ.com, Ltd. www.MicroDAQ.com (603) 746-5524
