NI PXI-2720 Independent Topology

NI Switches Help

Edition Date: April 2015

Part Number: 375472H-01

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The following figure represents the NI PXI-2720 in the independent topology.

Note Note  Bank connect relays allow adjacent banks or channels to connect together internally. For example, you can connect two or more adjacent channels together to create a potentiometer, a voltage divider, or a multi-segment resistor chain.

Making a Connection

Each 8-bit channel is composed of one bank on the NI PXI-2720. For example, ch0 is composed of bank 0 and ch1 is composed of bank 1. NI has created a set of reference VIs that will programmatically open and close relays based on a user-specified resistance value or RTD temperature to simulate. To access these reference VIs, visit ni.com/info and enter the Info Code 272xOverview. NI recommends using this set of reference VIs for the easiest programming experience. If not using these reference VIs, NI recommends using the low-level relay control VIs or functions instead of the connect channel VIs or functions. The DAQmx Relay API supports closing multiple relays in a single driver call, which is faster than the channel API.

When a bank relay is closed, the corresponding resistor is placed in parallel with the low resistance of the relay, which nominally equates to a zero Ω shunt. Closing any of the 8 bank relays in a given bank decreases the resistance of that bank.

For example, the following procedure uses the NI-SWITCH Relay API to short across the largest resistor in bank 0 and join bank 0 and bank 1 in series.

  1. Close b0r7 by calling the niSwitch Relay Control VI with the inputs of b0r7 and close.
  2. Close bc01 by calling the niSwitch Relay Control VI with the inputs of bc01 and close.

You can perform the same operation using the NI-SWITCH Channel API, as shown below.

  1. Connect b0->b0r7 by calling niSwitch Connect Channels VI with the inputs of b0 and b0r7.
  2. Connect b0->b1 by calling niSwitch Connect Channels VI with the inputs of b0 and b1.

Each bank is initially in a high impedance (open) state across the bank terminals. To enable the desired output channel you must first connect the bank relay, bN.

Each bank includes a shunt relay that completely bypasses the bank's string of 8 series relays. Closing the shunt relay bN->bNshunt, for example, b0->b0shunt or low-level kb0shunt, results in a low resistance across the bank, 0 Ω nominally. This allows the NI PXI-2720 module to pass signals with minimal attenuation.

Note Note  It is not necessary to close all 8 bank relays and the shunt relay. Configuring multiple banks this way can exceed the power budget of the NI PXI-2720.

The 4-pin front panel test connector can connect to any adjacent pair of even-odd banks, allowing resistance measurements, or voltage measurements, across those two banks, for example b0 to b1, b2 to b3, b4 to b5. On 16-bit NI 272x modules, this allows channel resistance measurements using a DMM with Offset Compensated Ohms (such as the NI PXI-4070, NI PXI-4071, or NI PXI-4072). To connect a pair of banks to the test leads, close the appropriate test relays using the command testN->testout. For example, to measure the resistance across banks 0 and 1, call test0->testout and test1->testout. For banks 2 and 3, call test1->testout and test2->testout. Refer to the device's hardware diagram for valid test relay connections.

Note Note  Closing multiple pairs of test relays introduces low impedance paths between banks. NI recommends disconnecting the 37-pin DSUB connector if these low impedance paths would damage your device under test or cause the current specifications of the NI PXIe-2727 to be exceeded.
Note Note  Each channel on the NI PXI-2720 exhibits an "open" when initialized, reset, and first powered on. The niSwitch Reset and niSwitch Disconnect All Channels VIs or niSwitch_reset and niSwitch_DisconnectAll functions will disconnect all relays, resulting in an open circuit on each channel.

Pinout

The following figure identifies the pins for the NI PXI-2720.

Note Note  Do not connect to "No Connect" pins.

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