National Instruments SCXI-1160 User Manual

Also see for SCXI-1160: Getting started Getting started guide Getting started guide Instructions for safe use

Contents

National Instruments SCXI-1160 User Manual Manual pdf 48 page image

Chapter 3 Theory of Operation© National Instruments Corporation 3-7 SCXI-1160 User ManualAddress Handler. The Data Register controls the state of the SCXI-1160 relays. The completedescriptions of the register bits are given in Chapter 4, Register Descriptions. Writes to the DataRegister 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 latchesthe Data Register bits.At reset time or at power up, the state of the Data Register and relays is unknown. To set therelays 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-bitparallel serial-in, serial-out shift register and an SPI communication adapter. The contents of theModule ID Register are written onto MISO during the first four bytes of transfer after selectingthe Module ID Register in the Address Handler. Zeros are written to MISO thereafter until theModule 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 ChannelsThe SCXI-1160 consists of 16 independent one form C relays. Each relay has three connectionsat 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 inductiveloads, arc suppressors may be installed at the pads, which are at each relay. Refer to the ContactProtection 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 therelay. The Data Register controls the relays as follows. The 16 LSBs of the Data Registercontrol the set coils. When the set coil is energized, the COM position connects to the NOposition. The 16 MSBs of the Data Register control the reset coils. When the reset coil isenergized, 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 msecmaximum) to ensure that the relays have switched states. Then the Data Register outputs go to ahigh-impedance state; this ensures that the heating effect of the relay coils is minimal, thusreducing 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 relayCOM position connected to the NC position) without affecting the other relays, or all relays canchange states at the same time. However, each time a relay changes its state, the Data Register isrewritten to. To reduce power consumption, thermal emf, and help extend the relay life, it is