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NI switch modules are capable of switching one, two and/or four wires.
In 1-wire mode, you connect the HI leads to the relays and the LO leads to a common connection. All signals are referenced to this common connection. All NI multiplexers other than the NI SCXI-1163R can operate in 1-wire mode. The following figure shows an example of 1-wire mode.
In 2-wire mode, you connect both positive and negative leads to the terminals of a channel. The following figure shows an example of 2-wire mode.
4-wire mode is usually used for 4-wire resistance measurements. One channel (two leads) is used for the current excitation and another channel (two leads) is used for measuring the voltage drop (sense) across the resistor. For example, if you are in 4-wire mode with the NI SCXI-1127 and want to make a 4-wire resistance measurement on channel 6, connect the two excitation current wires and the two measurement channel wires to the corresponding connections on channel 6. In software, select channel 6 to be scanned. The following figure shows an example of 4-wire mode.