Keithley dual multiplexer plug in card 37xx series keithley 3721 the keithley 3721 dual 1x20 multiplexer plug in card that offers two independent banks of 1x20 two pole multiplexers that are ideal for general purpose switching including temperature measurements the two banks can automatically be connected to the series 3700a mainframe backplane and optional dmm through the analogue backplane connection relays this connection allows the mainframe to reconfigure the 3721 as a single 1x40 tw