The aux socket is for a multichannel multiplexer board. I've never seen one. Don't bother with a socket, unless you want to build your own multiplexer. :) The board was a set of relays that switched the inputs to the ref and input connectors. You can program the 7081 from the front panel, via the 488 bus, or via the serial port to take measurements in various ways over various of the channels. Actually, making one wouldn't be all that hard; there was little logic on the board, basically just a one-of-8 (or was it 16?) decoder, drivers, and relays. -- Bill Ezell ---------- They said 'Windows or better' so I used Linux.