Buffered Digital Input for Isolated or High-Voltage Applications

This tutorial helps you configure and program NI 653x high-speed digital devices for clocked (or buffered) digital input applications that require voltage isolation or acquisition of signals with voltage levels that exceed TTL-compatible voltages.

For applications involving field digital logic levels, such as TTL and CMOS, where high common-mode voltages can be present, you can use the SCXI-1162 isolated input module in conjunction with the NI 653x. For buffered input applications with high voltage levels, the SCXI-1162HV senses the presence of a wide range of AC and DC levels, from digital logic level signals up to 240 VAC/VDC signals, and relays the information to the NI 653x at TTL/CMOS levels. The isolation of the SCXI-1162 and SCXI-1162HV modules eliminate ground-loop problems and isolate the host computer from damaging voltages. Figure 1 shows the digital logic levels for the SCXI-1162 and SCXI-1162HV.

Figure 1. Digital Logic Levels for the SCXI-1162 and SCXI-1162HV

Required Hardware

• NI 653x device
• SH68-68-D1 cable
• SCXI chassis
• SCXI-1355
• SCXI-1162 or SCXI-1162HV
• SCXI-1326

Hardware Overview

The NI 653x devices include the NI PCI-6534, NI PXI-6534, NI PCI-DIO-32HS, NI PXI-6533, NI DAQCard-6533, and the NI AT-DIO-32HS. All these devices work equally well in this application.

The SH68-68-D1 is a shielded, series-terminated, twisted-pair cable, which works well for high-speed digital applications.

Signal Conditioning eXtensions for Instrumentation (SCXI) is a versatile, high performance signal conditioning platform. The SCXI chassis is made to house, power, and control SCXI modules and conditioned signals.

The SCXI-1355 is a cable assembly used to connect the back of an SCXI-1163 or SCXI-1162 module to the SH68-68-D1 cable.

The SCXI-1162 and SCXI-1162HV are 32-channel optically isolated digital input modules.

• The SCXI-1162 is suited for 4 to 10 V input signals that need up to 240 V of isolation from computer ground to eliminate ground-loop problems and protect the host computer.
• The SCXI-1162HV has current-limiting circuitry that allows up to 240 VAC/VDC. The signal must be less than +/- 1 V for a logic low.
  
• The SCXI-1326 is a high-voltage terminal block that mounts to the front of an SCXI-1163 or SCXI-1162 module.
  

To perform high-voltage clocked/buffered digital input, configure the SCXI input module for parallel mode in conjunction with the NI 653x. In parallel mode, the SCXI module can bypass the SCXI backplane, so all 32 lines are always accessible. In multiplexed mode, the SCXI backplane multiplexes the 32 lines to one serial line.

It is important to note that in multiplexed mode all modules in an SCXI chassis can be multiplexed to one module and to one NI 653x. In parallel mode, each parallel-mode module has a dedicated connection to one NI 653x.

Installing and Configuring Your Hardware

Step 1. Install the NI 653x

Before installing your NI 653x, ensure that you have installed the latest version of NI-DAQ. NI-DAQ is the driver software that controls National Instruments data acquisition (DAQ) hardware. Next, configure the NI 653x in Measurement & Automation Explorer (MAX). For more information on installing and configuring a DAQ device, see the Hardware Installation Wizard at http://www.ni.com/support/install/

Step 2. Set SCXI Module Jumpers

Before installing your SCXI-1162/1162HV in the SCXI chassis, set the jumpers to the settings shown in the following tables. Table 1 lists the jumper settings for the SCXI-1162. Table 2 lists the jumper settings for the SCXI-1162HV.

• Table 1. SCXI-1162 Jumper Settings

  Jumper	Setting	Description
  W1	32	32 or 24 bit
  W2	PAR	Parallel connection
  W3	DIO	DIO
  W4	DIO	DIO
  W5	A	Pull-up resistor
  W6	PAR	Parallel connection

  Table 2. SCXI-1162HV Jumper Settings

  Jumper	Setting	Description
  W1	no change	--
  W2	PAR	Parallel connection
  W3	P	Parallel connection
  W4	DIO	DIO
  W5	32	32 or 24 bit
  W6	P	Parallel connection
  W7	DIO	DIO

  
  
  Step 3. Assemble the System
  Connect the components of your system as shown in the following figure according the following procedure:
  
  
  
  
  1. Insert the SCXI-1162 or SCXI-1162HV module into an unused slot in the SCXI chassis.
  2. Connect the SCXI-1355 shielded cable assembly (which includes the SH6868-D1 cable) to the back of the SCXI-1162/1162HV module.
  3. Connect the SH68-68-D1 cable to the NI 653x in the computer and to the SCXI-1326.
  Step 4. Connect Signals to the SCXI-1326 Connector Block Tip: It is easier to make connections to the SCXI-1326 connector block while it is not connected to the front of your SCXI-1162/1162HV. The connections you must make to the SCX1-1326 are shown in the following figure.
  • 
    
  • SCXI-1162
  Connect the expected high voltage to V_CC. Connect the input to the corresponding channel (CHAN(x).
  
  • SCXI-1162HV
  Connect the GND to your ground reference, and connect the input to the corresponding channel (CHAN(x).
  
  Notice that there are eight different terminals with four inputs each. The V_CC and/or GND have to be connected to each bank for that bank's four inputs to work. You can connect different voltage sources to different banks, if you need to operate individual banks at different voltage levels.
  
  Note: Do not configure the SCXI-1163R in such a way that the voltage difference between any V_CC, GND, or CHAN(x) and any other V_CC or GND or CHAN(x) exceeds 240 V.
  
  For more information, refer to the SCXI-1162, SCXI-1162HV, or SCXI-1326 documentation. Click the links at the bottom of this page to get PDF copies of these manuals.
  
  Step 5. Configure Your System in MAX
• Note: The configuration in MAX that is listed below is only applicable when using one of the NI 653X boards with the Traditional DAQ driver. When using the NI 653X in conjunction with the SCXI-1162HV there is no explicit software configuration, the jumper settings shown above are all that is needed. Programming with this configuration in DAQmx is not changed because the 653X is connected to the SCXI-1162 or HV version.
  
  Launch MAX, then configure your SCXI module as shown in the following procedure.
  
  a. Right-click Devices and Interfaces and select Create New...
  
  [+] Enlarge Image
  b. Select the SCXI chassis you are using, then click Finish, as shown in the following figure.
  
  [+] Enlarge Image
  c. Set Auto-Detect modules to No, as shown below. Then, click Finish.
  
  d. Expand the SCXI Chassis folder in MAX to display all the available module slots. Right-click the slot the SCXI module is in, and select Insert... from the menu.
  
  [+] Enlarge Image
  e. Locate the SCXI module in the list, and click it to select it. Click Next to continue.
  
  
  
  f. Select your NI 653x from the Connected To drop-down list. Deselect This device will control the chassis. Choose parallel mode as the Operating Mode, then click Next.
  
  
  
  g. Choose the SCXI-1326 from the Accessory drop-down list, then click Finish to complete configuring your system in MAX.
  
  
  
  For more information about installing and configuring SCXI systems, refer to the SCXI Installation Wizard at http://www.ni.com/support/scxi/

  Transfer Rates
  

  
  You can calculate the maximum scan rate of the system by using the following formula below:
  
  
  
  
  The propagation delay of the SCXI-1162 is 0.5 µs typical, 2 µs maximum. Therefore, the scan rate should be at least
  .
  The scan rate has been benchmarked at 750 kHz. Because the NI-6534 and the NI-6533 scan rates are higher, the propagation delay of the SCXI-1162 is the limiting factor.
  
  The SCXI-1162HV has a propagation delay of 1.5 ms.
  
  The update rate is, therefore, not less than 667 Hz.

  Software Examples
  

  
  Use Buffered Pattern Input.vi or Cont Pattern Input.vi, which are included in the LabVIEW examples. Buffered Pattern Input.vi acquires a specified number of input samples at a specified clock rate, then stops. Cont Pattern Input.vi acquires input samples at a specified clock rate until the VI is stopped. For more information on these LabVIEW examples, please see the LabVIEW example documentation.

  Other System Considerations
  

  
  It is possible to have SCXI modules configured for both parallel and multiplexed modes in the same SCXI chassis. For example, you can have one module in parallel mode connected to its dedicated DAQ device and other modules in the same chassis configured for multiplexed mode and connected to a separate DAQ device.
  
  To do both input and output on the same host computer, you must use two NI 653x devices, each in parallel mode, with the respective input and output SCXI modules. For more information about digital output with SCXI signal conditioning, click the Buffered Digital Output for Isolated or High Voltage Applications link in the Related Links section.
  Related Links:
  653X User Manual
  SCXI-1162 Manual
  SCXI-1162HV Manual
  SCXI-1326 Manual
  Buffered Digital Output for Isolated or High-Voltage Applications
  

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