At 06:44 PM 9/8/2011, David Duffy (AVD) wrote: >I'd like to send 4 x RS485 signals over a CAT5 cable. The device at >each end of the cable is completely isolated, so I can't get a common >mode reference from ground. > >Any ideas on sending 4 differential signals over a CAT5 ? The cable >length can vary from 1 - 100 metres and the data rate is about 1mb/s. > >Magnetics like Ethernet springs to mind, but I was wondering about >somehow using the multiple pairs to generate a virtual common mode >reference. Because the end device is completely isolated, you *should* be able=20 to dispense with the ground connection. However, I *might* be wrong. In that case, split your termination=20 resistors in two, each half the normal value and connected in=20 series. Then connect the mid-point of those termination resistors to=20 your ground connection. In fact, you should need to do that for only one of the signals. The=20 rest might move around with repsect to that ground but they will=20 still be well within the common-mode range of the differential receivers. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .