Chapter 3
Theory of Operation
© National Instruments Corporation
3-7
SCXI-1160 User Manual
Address Handler. The Data Register controls the state of the SCXI-1160 relays. The complete
descriptions of the register bits are given in Chapter 4, Register Descriptions. Writes to the Data
Register require the following steps:
1. SS* goes low, enabling communication with the board.
2. Write 0001 (hexadecimal) to the Address Handler. This selects the Data Register.
3. D*/A goes low, indicating that the information sent on the MOSI line is data.
4. The serial data is available on MOSI and SPICLK clocks it into the register.
5. SS* goes high and D*/A goes high, indicating the end of communication. This action latches
the Data Register bits.
At reset time or at power up, the state of the Data Register and relays is unknown. To set the
relays to a known state, perform a write to the Data Register.
Note: If a power loss or a reset occurs, the state of the relays is unchanged.
The Module ID Register connects to MISO on the SCXIbus. The Module ID Register is an 8-bit
parallel serial-in, serial-out shift register and an SPI communication adapter. The contents of the
Module ID Register are written onto MISO during the first four bytes of transfer after selecting
the Module ID Register in the Address Handler. Zeros are written to MISO thereafter until the
Module ID Register is deselected. The SCXI-1160 module ID is hex C.
The Status Register is a 1-bit register that indicates whether or not the relays are switching.
Relay Channels
The SCXI-1160 consists of 16 independent one form C relays. Each relay has three connections
at the front connector–the common (COM), the normally closed (NC), and the normally open
(NO) positions, which are available at each relay. To protect the relay contacts from inductive
loads, arc suppressors may be installed at the pads, which are at each relay. Refer to the Contact
Protection for Inductive Load Connections section of Chapter 2, Configuration and Installation,
for further details on contact protection.
All 16 relays are two-coil latching relays. One coil is for resetting and the other is for setting the
relay. The Data Register controls the relays as follows. The 16 LSBs of the Data Register
control the set coils. When the set coil is energized, the COM position connects to the NO
position. The 16 MSBs of the Data Register control the reset coils. When the reset coil is
energized, the COM connects to the NC position. Never energize both coils at the same time.
After a write to the Data Register, all relay coils are driven for a period of time (20 msec
maximum) to ensure that the relays have switched states. Then the Data Register outputs go to a
high-impedance state; this ensures that the heating effect of the relay coils is minimal, thus
reducing the thermal electromotive force (emf) that may be generated across the contacts.
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. To reduce power consumption, thermal emf, and help extend the relay life, it is