Measurement Computing DataShuttle - Manual (Page 115)

DataShuttle and DynaRes 11-12-01 Digital I/O 14-1
Digital I/O 14
Digital Input & Output
This chapter provides instruction for digital I/O applications. Please note the implementation differences
below for the various terminal panels.
The T31, terminal panel has provisions for high power optical isolation components or direct connection.
Low power optical isolation components cannot be used with this terminal panel. This terminal panel also
has open collector digital outputs ( a separate power supply must be provided).
Common Applications
Digital I/O lines have two fundamental purposes: 1) to turn something on or off; or 2) to detect the state of
an on/off device. All digital I/O applications are a variation of one of these two alternatives. The primary
difference between variations is the electrical characteristics of the digital circuit, namely voltage.
There are two different ways to implement digital I/O applications: Isolated or Non-isolated. It is
recommended to use isolation whenever possible. This protects most of the hardware from damage by
unanticipated high voltage. Isolation also enables digital I/O operation for high-voltage applications, such
as sensing the presence of a 120VAC source or switching a 50VDC circuit on and off.
Note: When making digital I/O connections without isolation, be certain there is no risk of an
unanticipated high voltage. Such a situation can damage the data acquisition hardware, the computer, and
also pose a risk of bodily injury.
Following are general instructions for implementing isolated digital I/O applications.
High Power Optically Isolated Digital Output
(For T31 terminal panels)
Devices used for high power digital switching are called solid state relays (SSRs). The devices you want to
control may be AC or DC and may have any of a large variety of voltage and current ratings. If you need
to drive a device that has a higher rating than the high power SSRs, which are installed on the terminal
panel, use the SSR to drive another solid state relay or mechanical relay with a higher voltage or current
rating.
Mechanical relays and SSRs provide electrical isolation from the controlled device as well as the ability to
switch large currents.
Connections to the screw terminals on the terminal panel are shown in Figure 14-1 for both AC and DC
loads. They are almost identical. The loads indicated in the figure might be a heater, pump, light, or other
device that you are turning on and off. Almost any type of load can be handled except inductive or
capacitive loads, such as motors and fluorescent lights. These are best controlled by driving a relay which
in turn switches the load. When the output is set low the SSR turns on.