Chapter 4 Theory of Operation
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National Instruments Corporation 4-5 SCXI-1161 User Manual
Relay Channels
The SCXI-1161 has eight independent one form C (single-pole
double-throw) relays. Each relay has three connections at the front
screw terminals–the common (COM), the normally closed (NC), and
the normally open (NO) positions. Each relay has pads for arc
suppressors to protect the relays from inductive loads. Refer to the
Contact Protection for Inductive Load Connections section of
Chapter 3, Signal Connections, for further details on contact protection.
All eight relays are single-side stable relays. When you energize the
relay coil, the COM connects to the NO position. The Data Register
controls the relays as follows. The four LSBs of the least significant
byte of the Data Register control the coils of relays 0 through 3. The
four LSBs of the most significant byte of the Data Register control the
coils of relays 4 through 7. When you do not energize the coils, the
COM connects to the NC position.
After a write to the Data Register, the data is latched and the selected
relay coils are continuously driven.
Each relay can be set (the relay COM position connected to the NO
position) or reset (the relay COM position connected to the NC
position) without affecting the other relays, or all relays can change
states at the same time. However, each time a relay changes its state, the
Data Register is rewritten to. Refer to the SCXI-1161 Register-Level
Programmers Manual for further details.
The relays are break-before-make, but only if you do not exceed the
specified relay operation speed. In addition, each relay is fully isolated
from the other relays and from earth ground, with a working
common-mode voltage of 250 Vrms.
Note: It is important to notice that this module has no access to the analog
backplane; this protects the backplane from faults when high voltages are
available on the relay contacts.