The ADLINK PXI 7921 is a relay multiplexer which consists of 24 2-wire relays (DPDT). It is used to connect multiple units under test to one instrument in 48x1 1-wire, 24x1 2-wire or 12x1 4-wire configurations. The 1-wire configuration routes single-ended signals to one point (ie PXI-2205), the 2-wire configuration multiplexes differential signals (ie PXI-2205, DMM, or AWG), while the 4-wire configuration is usually used to measure resistance or RTDs.