At 04:14 AM 6/18/2010, Michael Watterson wrote: > >However if both ends are isolated, you will find that a centre tapped to >local 0V termination at both ends and "earthing" screen at one end will >work. You don't inherently need a 3rd connection. The 3rd connection, if >used carries no signal, it's just a primitive (and unsafe if the gear >is on different supplies without isolated I/O) method of limiting >common mode voltage. Good point here. For what its worth, the transmitter (lighting-console or whatever) in DMX lighting systems IS normally connected to mains earth. It is only the receivers that are isolated. Same goes for any RS-422 or RS-485 systems that I design. The comms at the master station is connected to earth ground, the slaves all have isolated receivers. However, you are wrong when you say that the 3rd connection carries no signal. The reference that it provides ensures that the differential receiver is operated within its rated common-mode range. There are ways to eliminate the need for that 3rd wire but they are messy and can impact the line termination if done improperly. Its simply easier and MUCH more reliable to use shielded twisted-pair cable and to use the shield as the 3rd wire. dwayne -- 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 -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist