Erik is right. Check out the current loop app note at http://www.bb-elec.com/techlibr.asp Seems like RS422/RS485 diminished current loop application in the '80s Regards, Peter Tiang > At 03:16 AM 9/15/99 +0800, you wrote: > >If I'm not wrong, RS485 uses current loop for signaling. > > > > RS 485 is a voltage based standard. It uses differential pairs to transmit > data making it quite resistant to noise. Its length limitations are > primarily based on the RC time constant of the line. At 2400 baud you > could go really far. > > The differential pair typically go to a comparator, and since any external > RFI will affect them both the same way, the comparator will still see the > proper signal. I believe the 488 part gives you +/- 9V so you can do your > noise immunity based on that. > > -Erik Reikes >